<!-- doc/src/sgml/indexam.sgml -->
<chapter id="indexam">
<title>Index Access Method Interface Definition</title>
<indexterm>
<primary>Index Access Method</primary>
</indexterm>
<indexterm>
<primary>indexam</primary>
<secondary>Index Access Method</secondary>
</indexterm>
<para>
This chapter defines the interface between the core
<productname>PostgreSQL</productname> system and <firstterm>index access
methods</firstterm>, which manage individual index types. The core system
knows nothing about indexes beyond what is specified here, so it is
possible to develop entirely new index types by writing add-on code.
</para>
<para>
All indexes in <productname>PostgreSQL</productname> are what are known
technically as <firstterm>secondary indexes</firstterm>; that is, the index is
physically separate from the table file that it describes. Each index
is stored as its own physical <firstterm>relation</firstterm> and so is described
by an entry in the <structname>pg_class</structname> catalog. The contents of an
index are entirely under the control of its index access method. In
practice, all index access methods divide indexes into standard-size
pages so that they can use the regular storage manager and buffer manager
to access the index contents. (All the existing index access methods
furthermore use the standard page layout described in <xref
linkend="storage-page-layout"/>, and most use the same format for index
tuple headers; but these decisions are not forced on an access method.)
</para>
<para>
An index is effectively a mapping from some data key values to
<firstterm>tuple identifiers</firstterm>, or <acronym>TIDs</acronym>, of row versions
(tuples) in the index's parent table. A TID consists of a
block number and an item number within that block (see <xref
linkend="storage-page-layout"/>). This is sufficient
information to fetch a particular row version from the table.
Indexes are not directly aware that under MVCC, there might be multiple
extant versions of the same logical row; to an index, each tuple is
an independent object that needs its own index entry. Thus, an
update of a row always creates all-new index entries for the row, even if
the key values did not change. (<link linkend="storage-hot">HOT
tuples</link> are an exception to this
statement; but indexes do not deal with those, either.) Index entries for
dead tuples are reclaimed (by vacuuming) when the dead tuples themselves
are reclaimed.
</para>
<sect1 id="index-api">
<title>Basic API Structure for Indexes</title>
<para>
Each index access method is described by a row in the
<link linkend="catalog-pg-am"><structname>pg_am</structname></link>
system catalog. The <structname>pg_am</structname> entry
specifies a name and a <firstterm>handler function</firstterm> for the index
access method. These entries can be created and deleted using the
<xref linkend="sql-create-access-method"/> and
<xref linkend="sql-drop-access-method"/> SQL commands.
</para>
<para>
An index access method handler function must be declared to accept a
single argument of type <type>internal</type> and to return the
pseudo-type <type>index_am_handler</type>. The argument is a dummy value that
simply serves to prevent handler functions from being called directly from
SQL commands. The result of the function must be a palloc'd struct of
type <structname>IndexAmRoutine</structname>, which contains everything
that the core code needs to know to make use of the index access method.
The <structname>IndexAmRoutine</structname> struct, also called the access
method's <firstterm>API struct</firstterm>, includes fields specifying assorted
fixed properties of the access method, such as whether it can support
multicolumn indexes. More importantly, it contains pointers to support
functions for the access method, which do all of the real work to access
indexes. These support functions are plain C functions and are not
visible or callable at the SQL level. The support functions are described
in <xref linkend="index-functions"/>.
</para>
<para>
The structure <structname>IndexAmRoutine</structname> is defined thus:
<programlisting>
typedef struct IndexAmRoutine
{
NodeTag type;
/*
* Total number of strategies (operators) by which we can traverse/search
* this AM. Zero if AM does not have a fixed set of strategy assignments.
*/
uint16 amstrategies;
/* total number of support functions that this AM uses */
uint16 amsupport;
/* opclass options support function number or 0 */
uint16 amoptsprocnum;
/* does AM support ORDER BY indexed column's value? */
bool amcanorder;
/* does AM support ORDER BY result of an operator on indexed column? */
bool amcanorderbyop;
/* does AM support hashing using API consistent with the hash AM? */
bool amcanhash;
/* do operators within an opfamily have consistent equality semantics? */
bool amconsistentequality;
/* do operators within an opfamily have consistent ordering semantics? */
bool amconsistentordering;
/* does AM support backward scanning? */
bool amcanbackward;
/* does AM support UNIQUE indexes? */
bool amcanunique;
/* does AM support multi-column indexes? */
bool amcanmulticol;
/* does AM require scans to have a constraint on the first index column? */
bool amoptionalkey;
/* does AM handle ScalarArrayOpExpr quals? */
bool amsearcharray;
/* does AM handle IS NULL/IS NOT NULL quals? */
bool amsearchnulls;
/* can index storage data type differ from column data type? */
bool amstorage;
/* can an index of this type be clustered on? */
bool amclusterable;
/* does AM handle predicate locks? */
bool ampredlocks;
/* does AM support parallel scan? */
bool amcanparallel;
/* does AM support parallel build? */
bool amcanbuildparallel;
/* does AM support columns included with clause INCLUDE? */
bool amcaninclude;
/* does AM use maintenance_work_mem? */
bool amusemaintenanceworkmem;
/* does AM summarize tuples, with at least all tuples in the block
* summarized in one summary */
bool amsummarizing;
/* OR of parallel vacuum flags */
uint8 amparallelvacuumoptions;
/* type of data stored in index, or InvalidOid if variable */
Oid amkeytype;
/* interface functions */
ambuild_function ambuild;
ambuildempty_function ambuildempty;
aminsert_function aminsert;
aminsertcleanup_function aminsertcleanup;
ambulkdelete_function ambulkdelete;
amvacuumcleanup_function amvacuumcleanup;
amcanreturn_function amcanreturn; /* can be NULL */
amcostestimate_function amcostestimate;
amgettreeheight_function amgettreeheight; /* can be NULL */
amoptions_function amoptions;
amproperty_function amproperty; /* can be NULL */
ambuildphasename_function ambuildphasename; /* can be NULL */
amvalidate_function amvalidate;
amadjustmembers_function amadjustmembers; /* can be NULL */
ambeginscan_function ambeginscan;
amrescan_function amrescan;
amgettuple_function amgettuple; /* can be NULL */
amgetbitmap_function amgetbitmap; /* can be NULL */
amendscan_function amendscan;
ammarkpos_function ammarkpos; /* can be NULL */
amrestrpos_function amrestrpos; /* can be NULL */
/* interface functions to support parallel index scans */
amestimateparallelscan_function amestimateparallelscan; /* can be NULL */
aminitparallelscan_function aminitparallelscan; /* can be NULL */
amparallelrescan_function amparallelrescan; /* can be NULL */
/* interface functions to support planning */
amtranslate_strategy_function amtranslatestrategy; /* can be NULL */
amtranslate_cmptype_function amtranslatecmptype; /* can be NULL */
} IndexAmRoutine;
</programlisting>
</para>
<para>
To be useful, an index access method must also have one or more
<firstterm>operator families</firstterm> and
<firstterm>operator classes</firstterm> defined in
<link linkend="catalog-pg-opfamily"><structname>pg_opfamily</structname></link>,
<link linkend="catalog-pg-opclass"><structname>pg_opclass</structname></link>,
<link linkend="catalog-pg-amop"><structname>pg_amop</structname></link>, and
<link linkend="catalog-pg-amproc"><structname>pg_amproc</structname></link>.
These entries allow the planner
to determine what kinds of query qualifications can be used with
indexes of this access method. Operator families and classes are described
in <xref linkend="xindex"/>, which is prerequisite material for reading
this chapter.
</para>
<para>
An individual index is defined by a
<link linkend="catalog-pg-class"><structname>pg_class</structname></link>
entry that describes it as a physical relation, plus a
<link linkend="catalog-pg-index"><structname>pg_index</structname></link>
entry that shows the logical content of the index &mdash; that is, the set
of index columns it has and the semantics of those columns, as captured by
the associated operator classes. The index columns (key values) can be
either simple columns of the underlying table or expressions over the table
rows. The index access method normally has no interest in where the index
key values come from (it is always handed precomputed key values) but it
will be very interested in the operator class information in
<structname>pg_index</structname>. Both of these catalog entries can be
accessed as part of the <structname>Relation</structname> data structure that is
passed to all operations on the index.
</para>
<para>
Some of the flag fields of <structname>IndexAmRoutine</structname> have nonobvious
implications. The requirements of <structfield>amcanunique</structfield>
are discussed in <xref linkend="index-unique-checks"/>.
The <structfield>amcanmulticol</structfield> flag asserts that the
access method supports multi-key-column indexes, while
<structfield>amoptionalkey</structfield> asserts that it allows scans
where no indexable restriction clause is given for the first index column.
When <structfield>amcanmulticol</structfield> is false,
<structfield>amoptionalkey</structfield> essentially says whether the
access method supports full-index scans without any restriction clause.
Access methods that support multiple index columns <emphasis>must</emphasis>
support scans that omit restrictions on any or all of the columns after
the first; however they are permitted to require some restriction to
appear for the first index column, and this is signaled by setting
<structfield>amoptionalkey</structfield> false.
One reason that an index <acronym>AM</acronym> might set
<structfield>amoptionalkey</structfield> false is if it doesn't index
null values. Since most indexable operators are
strict and hence cannot return true for null inputs,
it is at first sight attractive to not store index entries for null values:
they could never be returned by an index scan anyway. However, this
argument fails when an index scan has no restriction clause for a given
index column. In practice this means that
indexes that have <structfield>amoptionalkey</structfield> true must
index nulls, since the planner might decide to use such an index
with no scan keys at all. A related restriction is that an index
access method that supports multiple index columns <emphasis>must</emphasis>
support indexing null values in columns after the first, because the planner
will assume the index can be used for queries that do not restrict
these columns. For example, consider an index on (a,b) and a query with
<literal>WHERE a = 4</literal>. The system will assume the index can be
used to scan for rows with <literal>a = 4</literal>, which is wrong if the
index omits rows where <literal>b</literal> is null.
It is, however, OK to omit rows where the first indexed column is null.
An index access method that does index nulls may also set
<structfield>amsearchnulls</structfield>, indicating that it supports
<literal>IS NULL</literal> and <literal>IS NOT NULL</literal> clauses as search
conditions.
</para>
<para>
The <structfield>amcaninclude</structfield> flag indicates whether the
access method supports <quote>included</quote> columns, that is it can
store (without processing) additional columns beyond the key column(s).
The requirements of the preceding paragraph apply only to the key
columns. In particular, the combination
of <structfield>amcanmulticol</structfield>=<literal>false</literal>
and <structfield>amcaninclude</structfield>=<literal>true</literal> is
sensible: it means that there can only be one key column, but there can
also be included column(s). Also, included columns must be allowed to be
null, independently of <structfield>amoptionalkey</structfield>.
</para>
<para>
The <structfield>amsummarizing</structfield> flag indicates whether the
access method summarizes the indexed tuples, with summarizing granularity
of at least per block.
Access methods that do not point to individual tuples, but to block ranges
(like <acronym>BRIN</acronym>), may allow the <acronym>HOT</acronym> optimization
to continue. This does not apply to attributes referenced in index
predicates, an update of such an attribute always disables <acronym>HOT</acronym>.
</para>
</sect1>
<sect1 id="index-functions">
<title>Index Access Method Functions</title>
<para>
The index construction and maintenance functions that an index access
method must provide in <structname>IndexAmRoutine</structname> are:
</para>
<para>
<programlisting>
IndexBuildResult *
ambuild (Relation heapRelation,
Relation indexRelation,
IndexInfo *indexInfo);
</programlisting>
Build a new index. The index relation has been physically created,
but is empty. It must be filled in with whatever fixed data the
access method requires, plus entries for all tuples already existing
in the table. Ordinarily the <function>ambuild</function> function will call
<function>table_index_build_scan()</function> to scan the table for existing tuples
and compute the keys that need to be inserted into the index.
The function must return a palloc'd struct containing statistics about
the new index.
The <structfield>amcanbuildparallel</structfield> flag indicates whether
the access method supports parallel index builds. When set to <literal>true</literal>,
the system will attempt to allocate parallel workers for the build.
Access methods supporting only non-parallel index builds should leave
this flag set to <literal>false</literal>.
</para>
<para>
<programlisting>
void
ambuildempty (Relation indexRelation);
</programlisting>
Build an empty index, and write it to the initialization fork (<symbol>INIT_FORKNUM</symbol>)
of the given relation. This method is called only for unlogged indexes; the
empty index written to the initialization fork will be copied over the main
relation fork on each server restart.
</para>
<para>
<programlisting>
bool
aminsert (Relation indexRelation,
Datum *values,
bool *isnull,
ItemPointer heap_tid,
Relation heapRelation,
IndexUniqueCheck checkUnique,
bool indexUnchanged,
IndexInfo *indexInfo);
</programlisting>
Insert a new tuple into an existing index. The <literal>values</literal> and
<literal>isnull</literal> arrays give the key values to be indexed, and
<literal>heap_tid</literal> is the TID to be indexed.
If the access method supports unique indexes (its
<structfield>amcanunique</structfield> flag is true) then
<literal>checkUnique</literal> indicates the type of uniqueness check to
perform. This varies depending on whether the unique constraint is
deferrable; see <xref linkend="index-unique-checks"/> for details.
Normally the access method only needs the <literal>heapRelation</literal>
parameter when performing uniqueness checking (since then it will have to
look into the heap to verify tuple liveness).
</para>
<para>
The <literal>indexUnchanged</literal> Boolean value gives a hint
about the nature of the tuple to be indexed. When it is true,
the tuple is a duplicate of some existing tuple in the index. The
new tuple is a logically unchanged successor MVCC tuple version. This
happens when an <command>UPDATE</command> takes place that does not
modify any columns covered by the index, but nevertheless requires a
new version in the index. The index AM may use this hint to decide
to apply bottom-up index deletion in parts of the index where many
versions of the same logical row accumulate. Note that updating a non-key
column or a column that only appears in a partial index predicate does not
affect the value of <literal>indexUnchanged</literal>. The core code
determines each tuple's <literal>indexUnchanged</literal> value using a low
overhead approach that allows both false positives and false negatives.
Index AMs must not treat <literal>indexUnchanged</literal> as an
authoritative source of information about tuple visibility or versioning.
</para>
<para>
The function's Boolean result value is significant only when
<literal>checkUnique</literal> is <literal>UNIQUE_CHECK_PARTIAL</literal>.
In this case a true result means the new entry is known unique, whereas
false means it might be non-unique (and a deferred uniqueness check must
be scheduled). For other cases a constant false result is recommended.
</para>
<para>
Some indexes might not index all tuples. If the tuple is not to be
indexed, <function>aminsert</function> should just return without doing anything.
</para>
<para>
If the index AM wishes to cache data across successive index insertions
within an SQL statement, it can allocate space
in <literal>indexInfo-&gt;ii_Context</literal> and store a pointer to the
data in <literal>indexInfo-&gt;ii_AmCache</literal> (which will be NULL
initially). If resources other than memory have to be released after
index insertions, <function>aminsertcleanup</function> may be provided,
which will be called before the memory is released.
</para>
<para>
<programlisting>
void
aminsertcleanup (Relation indexRelation,
IndexInfo *indexInfo);
</programlisting>
Clean up state that was maintained across successive inserts in
<literal>indexInfo-&gt;ii_AmCache</literal>. This is useful if the data
requires additional cleanup steps (e.g., releasing pinned buffers), and
simply releasing the memory is not sufficient.
</para>
<para>
<programlisting>
IndexBulkDeleteResult *
ambulkdelete (IndexVacuumInfo *info,
IndexBulkDeleteResult *stats,
IndexBulkDeleteCallback callback,
void *callback_state);
</programlisting>
Delete tuple(s) from the index. This is a <quote>bulk delete</quote> operation
that is intended to be implemented by scanning the whole index and checking
each entry to see if it should be deleted.
The passed-in <literal>callback</literal> function must be called, in the style
<literal>callback(<replaceable>TID</replaceable>, callback_state) returns bool</literal>,
to determine whether any particular index entry, as identified by its
referenced TID, is to be deleted. Must return either NULL or a palloc'd
struct containing statistics about the effects of the deletion operation.
It is OK to return NULL if no information needs to be passed on to
<function>amvacuumcleanup</function>.
</para>
<para>
Because of limited <varname>maintenance_work_mem</varname>,
<function>ambulkdelete</function> might need to be called more than once when many
tuples are to be deleted. The <literal>stats</literal> argument is the result
of the previous call for this index (it is NULL for the first call within a
<command>VACUUM</command> operation). This allows the AM to accumulate statistics
across the whole operation. Typically, <function>ambulkdelete</function> will
modify and return the same struct if the passed <literal>stats</literal> is not
null.
</para>
<para>
<programlisting>
IndexBulkDeleteResult *
amvacuumcleanup (IndexVacuumInfo *info,
IndexBulkDeleteResult *stats);
</programlisting>
Clean up after a <command>VACUUM</command> operation (zero or more
<function>ambulkdelete</function> calls). This does not have to do anything
beyond returning index statistics, but it might perform bulk cleanup
such as reclaiming empty index pages. <literal>stats</literal> is whatever the
last <function>ambulkdelete</function> call returned, or NULL if
<function>ambulkdelete</function> was not called because no tuples needed to be
deleted. If the result is not NULL it must be a palloc'd struct.
The statistics it contains will be used to update <structname>pg_class</structname>,
and will be reported by <command>VACUUM</command> if <literal>VERBOSE</literal> is given.
It is OK to return NULL if the index was not changed at all during the
<command>VACUUM</command> operation, but otherwise correct stats should
be returned.
</para>
<para>
<function>amvacuumcleanup</function> will also be called at completion of an
<command>ANALYZE</command> operation. In this case <literal>stats</literal> is always
NULL and any return value will be ignored. This case can be distinguished
by checking <literal>info-&gt;analyze_only</literal>. It is recommended
that the access method do nothing except post-insert cleanup in such a
call, and that only in an autovacuum worker process.
</para>
<para>
<programlisting>
bool
amcanreturn (Relation indexRelation, int attno);
</programlisting>
Check whether the index can support <link
linkend="indexes-index-only-scans"><firstterm>index-only scans</firstterm></link> on
the given column, by returning the column's original indexed value.
The attribute number is 1-based, i.e., the first column's attno is 1.
Returns true if supported, else false.
This function should always return true for included columns
(if those are supported), since there's little point in an included
column that can't be retrieved.
If the access method does not support index-only scans at all,
the <structfield>amcanreturn</structfield> field in its <structname>IndexAmRoutine</structname>
struct can be set to NULL.
</para>
<para>
<programlisting>
void
amcostestimate (PlannerInfo *root,
IndexPath *path,
double loop_count,
Cost *indexStartupCost,
Cost *indexTotalCost,
Selectivity *indexSelectivity,
double *indexCorrelation,
double *indexPages);
</programlisting>
Estimate the costs of an index scan. This function is described fully
in <xref linkend="index-cost-estimation"/>, below.
</para>
<para>
<programlisting>
int
amgettreeheight (Relation rel);
</programlisting>
Compute the height of a tree-shaped index. This information is supplied to
the <function>amcostestimate</function> function in
<literal>path->indexinfo->tree_height</literal> and can be used to support
the cost estimation. The result is not used anywhere else, so this
function can actually be used to compute any kind of data (that fits into
an integer) about the index that the cost estimation function might want to
know. If the computation is expensive, it could be useful to cache the
result as part of <literal>RelationData.rd_amcache</literal>.
</para>
<para>
<programlisting>
bytea *
amoptions (ArrayType *reloptions,
bool validate);
</programlisting>
Parse and validate the reloptions array for an index. This is called only
when a non-null reloptions array exists for the index.
<parameter>reloptions</parameter> is a <type>text</type> array containing entries of the
form <replaceable>name</replaceable><literal>=</literal><replaceable>value</replaceable>.
The function should construct a <type>bytea</type> value, which will be copied
into the <structfield>rd_options</structfield> field of the index's relcache entry.
The data contents of the <type>bytea</type> value are open for the access
method to define; most of the standard access methods use struct
<structname>StdRdOptions</structname>.
When <parameter>validate</parameter> is true, the function should report a suitable
error message if any of the options are unrecognized or have invalid
values; when <parameter>validate</parameter> is false, invalid entries should be
silently ignored. (<parameter>validate</parameter> is false when loading options
already stored in <structname>pg_catalog</structname>; an invalid entry could only
be found if the access method has changed its rules for options, and in
that case ignoring obsolete entries is appropriate.)
It is OK to return NULL if default behavior is wanted.
</para>
<para>
<programlisting>
bool
amproperty (Oid index_oid, int attno,
IndexAMProperty prop, const char *propname,
bool *res, bool *isnull);
</programlisting>
The <function>amproperty</function> method allows index access methods to override
the default behavior of <function>pg_index_column_has_property</function>
and related functions.
If the access method does not have any special behavior for index property
inquiries, the <structfield>amproperty</structfield> field in
its <structname>IndexAmRoutine</structname> struct can be set to NULL.
Otherwise, the <function>amproperty</function> method will be called with
<parameter>index_oid</parameter> and <parameter>attno</parameter> both zero for
<function>pg_indexam_has_property</function> calls,
or with <parameter>index_oid</parameter> valid and <parameter>attno</parameter> zero for
<function>pg_index_has_property</function> calls,
or with <parameter>index_oid</parameter> valid and <parameter>attno</parameter> greater than
zero for <function>pg_index_column_has_property</function> calls.
<parameter>prop</parameter> is an enum value identifying the property being tested,
while <parameter>propname</parameter> is the original property name string.
If the core code does not recognize the property name
then <parameter>prop</parameter> is <literal>AMPROP_UNKNOWN</literal>.
Access methods can define custom property names by
checking <parameter>propname</parameter> for a match (use <function>pg_strcasecmp</function>
to match, for consistency with the core code); for names known to the core
code, it's better to inspect <parameter>prop</parameter>.
If the <structfield>amproperty</structfield> method returns <literal>true</literal> then
it has determined the property test result: it must set <literal>*res</literal>
to the Boolean value to return, or set <literal>*isnull</literal>
to <literal>true</literal> to return a NULL. (Both of the referenced variables
are initialized to <literal>false</literal> before the call.)
If the <structfield>amproperty</structfield> method returns <literal>false</literal> then
the core code will proceed with its normal logic for determining the
property test result.
</para>
<para>
Access methods that support ordering operators should
implement <literal>AMPROP_DISTANCE_ORDERABLE</literal> property testing, as the
core code does not know how to do that and will return NULL. It may
also be advantageous to implement <literal>AMPROP_RETURNABLE</literal> testing,
if that can be done more cheaply than by opening the index and calling
<function>amcanreturn</function>, which is the core code's default behavior.
The default behavior should be satisfactory for all other standard
properties.
</para>
<para>
<programlisting>
char *
ambuildphasename (int64 phasenum);
</programlisting>
Return the textual name of the given build phase number.
The phase numbers are those reported during an index build via the
<function>pgstat_progress_update_param</function> interface.
The phase names are then exposed in the
<structname>pg_stat_progress_create_index</structname> view.
</para>
<para>
<programlisting>
bool
amvalidate (Oid opclassoid);
</programlisting>
Validate the catalog entries for the specified operator class, so far as
the access method can reasonably do that. For example, this might include
testing that all required support functions are provided.
The <function>amvalidate</function> function must return false if the opclass is
invalid. Problems should be reported with <function>ereport</function>
messages, typically at <literal>INFO</literal> level.
</para>
<para>
<programlisting>
void
amadjustmembers (Oid opfamilyoid,
Oid opclassoid,
List *operators,
List *functions);
</programlisting>
Validate proposed new operator and function members of an operator family,
so far as the access method can reasonably do that, and set their
dependency types if the default is not satisfactory. This is called
during <command>CREATE OPERATOR CLASS</command> and during
<command>ALTER OPERATOR FAMILY ADD</command>; in the latter
case <parameter>opclassoid</parameter> is <literal>InvalidOid</literal>.
The <type>List</type> arguments are lists
of <structname>OpFamilyMember</structname> structs, as defined
in <filename>amapi.h</filename>.
Tests done by this function will typically be a subset of those
performed by <function>amvalidate</function>,
since <function>amadjustmembers</function> cannot assume that it is
seeing a complete set of members. For example, it would be reasonable
to check the signature of a support function, but not to check whether
all required support functions are provided. Any problems can be
reported by throwing an error.
The dependency-related fields of
the <structname>OpFamilyMember</structname> structs are initialized by
the core code to create hard dependencies on the opclass if this
is <command>CREATE OPERATOR CLASS</command>, or soft dependencies on the
opfamily if this is <command>ALTER OPERATOR FAMILY ADD</command>.
<function>amadjustmembers</function> can adjust these fields if some other
behavior is more appropriate. For example, GIN, GiST, and SP-GiST
always set operator members to have soft dependencies on the opfamily,
since the connection between an operator and an opclass is relatively
weak in these index types; so it is reasonable to allow operator members
to be added and removed freely. Optional support functions are typically
also given soft dependencies, so that they can be removed if necessary.
</para>
<para>
The purpose of an index, of course, is to support scans for tuples matching
an indexable <literal>WHERE</literal> condition, often called a
<firstterm>qualifier</firstterm> or <firstterm>scan key</firstterm>. The semantics of
index scanning are described more fully in <xref linkend="index-scanning"/>,
below. An index access method can support <quote>plain</quote> index scans,
<quote>bitmap</quote> index scans, or both. The scan-related functions that an
index access method must or may provide are:
</para>
<para>
<programlisting>
IndexScanDesc
ambeginscan (Relation indexRelation,
int nkeys,
int norderbys);
</programlisting>
Prepare for an index scan. The <literal>nkeys</literal> and <literal>norderbys</literal>
parameters indicate the number of quals and ordering operators that will be
used in the scan; these may be useful for space allocation purposes.
Note that the actual values of the scan keys aren't provided yet.
The result must be a palloc'd struct.
For implementation reasons the index access method
<emphasis>must</emphasis> create this struct by calling
<function>RelationGetIndexScan()</function>. In most cases
<function>ambeginscan</function> does little beyond making that call and perhaps
acquiring locks;
the interesting parts of index-scan startup are in <function>amrescan</function>.
</para>
<para>
<programlisting>
void
amrescan (IndexScanDesc scan,
ScanKey keys,
int nkeys,
ScanKey orderbys,
int norderbys);
</programlisting>
Start or restart an index scan, possibly with new scan keys. (To restart
using previously-passed keys, NULL is passed for <literal>keys</literal> and/or
<literal>orderbys</literal>.) Note that it is not allowed for
the number of keys or order-by operators to be larger than
what was passed to <function>ambeginscan</function>. In practice the restart
feature is used when a new outer tuple is selected by a nested-loop join
and so a new key comparison value is needed, but the scan key structure
remains the same.
</para>
<para>
<programlisting>
bool
amgettuple (IndexScanDesc scan,
ScanDirection direction);
</programlisting>
Fetch the next tuple in the given scan, moving in the given
direction (forward or backward in the index). Returns true if a tuple was
obtained, false if no matching tuples remain. In the true case the tuple
TID is stored into the <literal>scan</literal> structure. Note that
<quote>success</quote> means only that the index contains an entry that matches
the scan keys, not that the tuple necessarily still exists in the heap or
will pass the caller's snapshot test. On success, <function>amgettuple</function>
must also set <literal>scan-&gt;xs_recheck</literal> to true or false.
False means it is certain that the index entry matches the scan keys.
True means this is not certain, and the conditions represented by the
scan keys must be rechecked against the heap tuple after fetching it.
This provision supports <quote>lossy</quote> index operators.
Note that rechecking will extend only to the scan conditions; a partial
index predicate (if any) is never rechecked by <function>amgettuple</function>
callers.
</para>
<para>
If the index supports <link linkend="indexes-index-only-scans">index-only
scans</link> (i.e., <function>amcanreturn</function> returns true for any
of its columns),
then on success the AM must also check <literal>scan-&gt;xs_want_itup</literal>,
and if that is true it must return the originally indexed data for the
index entry. Columns for which <function>amcanreturn</function> returns
false can be returned as nulls.
The data can be returned in the form of an
<structname>IndexTuple</structname> pointer stored at <literal>scan-&gt;xs_itup</literal>,
with tuple descriptor <literal>scan-&gt;xs_itupdesc</literal>; or in the form of
a <structname>HeapTuple</structname> pointer stored at <literal>scan-&gt;xs_hitup</literal>,
with tuple descriptor <literal>scan-&gt;xs_hitupdesc</literal>. (The latter
format should be used when reconstructing data that might possibly not fit
into an <structname>IndexTuple</structname>.) In either case,
management of the data referenced by the pointer is the access method's
responsibility. The data must remain good at least until the next
<function>amgettuple</function>, <function>amrescan</function>, or <function>amendscan</function>
call for the scan.
</para>
<para>
The <function>amgettuple</function> function need only be provided if the access
method supports <quote>plain</quote> index scans. If it doesn't, the
<structfield>amgettuple</structfield> field in its <structname>IndexAmRoutine</structname>
struct must be set to NULL.
</para>
<para>
<programlisting>
int64
amgetbitmap (IndexScanDesc scan,
TIDBitmap *tbm);
</programlisting>
Fetch all tuples in the given scan and add them to the caller-supplied
<type>TIDBitmap</type> (that is, OR the set of tuple IDs into whatever set is already
in the bitmap). The number of tuples fetched is returned (this might be
just an approximate count, for instance some AMs do not detect duplicates).
While inserting tuple IDs into the bitmap, <function>amgetbitmap</function> can
indicate that rechecking of the scan conditions is required for specific
tuple IDs. This is analogous to the <literal>xs_recheck</literal> output parameter
of <function>amgettuple</function>. Note: in the current implementation, support
for this feature is conflated with support for lossy storage of the bitmap
itself, and therefore callers recheck both the scan conditions and the
partial index predicate (if any) for recheckable tuples. That might not
always be true, however.
<function>amgetbitmap</function> and
<function>amgettuple</function> cannot be used in the same index scan; there
are other restrictions too when using <function>amgetbitmap</function>, as explained
in <xref linkend="index-scanning"/>.
</para>
<para>
The <function>amgetbitmap</function> function need only be provided if the access
method supports <quote>bitmap</quote> index scans. If it doesn't, the
<structfield>amgetbitmap</structfield> field in its <structname>IndexAmRoutine</structname>
struct must be set to NULL.
</para>
<para>
<programlisting>
void
amendscan (IndexScanDesc scan);
</programlisting>
End a scan and release resources. The <literal>scan</literal> struct itself
should not be freed, but any locks or pins taken internally by the
access method must be released, as well as any other memory allocated
by <function>ambeginscan</function> and other scan-related functions.
</para>
<para>
<programlisting>
void
ammarkpos (IndexScanDesc scan);
</programlisting>
Mark current scan position. The access method need only support one
remembered scan position per scan.
</para>
<para>
The <function>ammarkpos</function> function need only be provided if the access
method supports ordered scans. If it doesn't,
the <structfield>ammarkpos</structfield> field in its <structname>IndexAmRoutine</structname>
struct may be set to NULL.
</para>
<para>
<programlisting>
void
amrestrpos (IndexScanDesc scan);
</programlisting>
Restore the scan to the most recently marked position.
</para>
<para>
The <function>amrestrpos</function> function need only be provided if the access
method supports ordered scans. If it doesn't,
the <structfield>amrestrpos</structfield> field in its <structname>IndexAmRoutine</structname>
struct may be set to NULL.
</para>
<para>
In addition to supporting ordinary index scans, some types of index
may wish to support <firstterm>parallel index scans</firstterm>, which allow
multiple backends to cooperate in performing an index scan. The
index access method should arrange things so that each cooperating
process returns a subset of the tuples that would be performed by
an ordinary, non-parallel index scan, but in such a way that the
union of those subsets is equal to the set of tuples that would be
returned by an ordinary, non-parallel index scan. Furthermore, while
there need not be any global ordering of tuples returned by a parallel
scan, the ordering of that subset of tuples returned within each
cooperating backend must match the requested ordering. The following
functions may be implemented to support parallel index scans:
</para>
<para>
<programlisting>
Size
amestimateparallelscan (Relation indexRelation,
int nkeys,
int norderbys);
</programlisting>
Estimate and return the number of bytes of dynamic shared memory which
the access method will be needed to perform a parallel scan. (This number
is in addition to, not in lieu of, the amount of space needed for
AM-independent data in <structname>ParallelIndexScanDescData</structname>.)
</para>
<para>
The <literal>nkeys</literal> and <literal>norderbys</literal>
parameters indicate the number of quals and ordering operators that will be
used in the scan; the same values will be passed to <function>amrescan</function>.
Note that the actual values of the scan keys aren't provided yet.
</para>
<para>
It is not necessary to implement this function for access methods which
do not support parallel scans or for which the number of additional bytes
of storage required is zero.
</para>
<para>
<programlisting>
void
aminitparallelscan (void *target);
</programlisting>
This function will be called to initialize dynamic shared memory at the
beginning of a parallel scan. <parameter>target</parameter> will point to at least
the number of bytes previously returned by
<function>amestimateparallelscan</function>, and this function may use that
amount of space to store whatever data it wishes.
</para>
<para>
It is not necessary to implement this function for access methods which
do not support parallel scans or in cases where the shared memory space
required needs no initialization.
</para>
<para>
<programlisting>
void
amparallelrescan (IndexScanDesc scan);
</programlisting>
This function, if implemented, will be called when a parallel index scan
must be restarted. It should reset any shared state set up by
<function>aminitparallelscan</function> such that the scan will be restarted from
the beginning.
</para>
<para>
<programlisting>
CompareType
amtranslatestrategy (StrategyNumber strategy, Oid opfamily, Oid opcintype);
StrategyNumber
amtranslatecmptype (CompareType cmptype, Oid opfamily, Oid opcintype);
</programlisting>
These functions, if implemented, will be called by the planner and executor
to convert between fixed <type>CompareType</type> values and the specific
strategy numbers used by the access method. These functions can be
implemented by access methods that implement functionality similar to the
built-in btree or hash access methods, and by implementing these
translations, the system can learn about the semantics of the access
method's operations and can use them in place of btree or hash indexes in
various places. If the functionality of the access method is not similar
to those built-in access methods, these functions do not need to be
implemented. If the functions are not implemented, the access method will
be ignored for certain planner and executor decisions, but is otherwise
fully functional.
</para>
</sect1>
<sect1 id="index-scanning">
<title>Index Scanning</title>
<para>
In an index scan, the index access method is responsible for regurgitating
the TIDs of all the tuples it has been told about that match the
<firstterm>scan keys</firstterm>. The access method is <emphasis>not</emphasis> involved in
actually fetching those tuples from the index's parent table, nor in
determining whether they pass the scan's visibility test or other
conditions.
</para>
<para>
A scan key is the internal representation of a <literal>WHERE</literal> clause of
the form <replaceable>index_key</replaceable> <replaceable>operator</replaceable>
<replaceable>constant</replaceable>, where the index key is one of the columns of the
index and the operator is one of the members of the operator family
associated with that index column. An index scan has zero or more scan
keys, which are implicitly ANDed &mdash; the returned tuples are expected
to satisfy all the indicated conditions.
</para>
<para>
The access method can report that the index is <firstterm>lossy</firstterm>, or
requires rechecks, for a particular query. This implies that the index
scan will return all the entries that pass the scan key, plus possibly
additional entries that do not. The core system's index-scan machinery
will then apply the index conditions again to the heap tuple to verify
whether or not it really should be selected. If the recheck option is not
specified, the index scan must return exactly the set of matching entries.
</para>
<para>
Note that it is entirely up to the access method to ensure that it
correctly finds all and only the entries passing all the given scan keys.
Also, the core system will simply hand off all the <literal>WHERE</literal>
clauses that match the index keys and operator families, without any
semantic analysis to determine whether they are redundant or
contradictory. As an example, given
<literal>WHERE x &gt; 4 AND x &gt; 14</literal> where <literal>x</literal> is a b-tree
indexed column, it is left to the b-tree <function>amrescan</function> function
to realize that the first scan key is redundant and can be discarded.
The extent of preprocessing needed during <function>amrescan</function> will
depend on the extent to which the index access method needs to reduce
the scan keys to a <quote>normalized</quote> form.
</para>
<para>
Some access methods return index entries in a well-defined order, others
do not. There are actually two different ways that an access method can
support sorted output:
<itemizedlist>
<listitem>
<para>
Access methods that always return entries in the natural ordering
of their data (such as btree) should set
<structfield>amcanorder</structfield> to true.
Currently, such access methods must use btree-compatible strategy
numbers for their equality and ordering operators.
</para>
</listitem>
<listitem>
<para>
Access methods that support ordering operators should set
<structfield>amcanorderbyop</structfield> to true.
This indicates that the index is capable of returning entries in
an order satisfying <literal>ORDER BY</literal> <replaceable>index_key</replaceable>
<replaceable>operator</replaceable> <replaceable>constant</replaceable>. Scan modifiers
of that form can be passed to <function>amrescan</function> as described
previously.
</para>
</listitem>
</itemizedlist>
</para>
<para>
The <function>amgettuple</function> function has a <literal>direction</literal> argument,
which can be either <literal>ForwardScanDirection</literal> (the normal case)
or <literal>BackwardScanDirection</literal>. If the first call after
<function>amrescan</function> specifies <literal>BackwardScanDirection</literal>, then the
set of matching index entries is to be scanned back-to-front rather than in
the normal front-to-back direction, so <function>amgettuple</function> must return
the last matching tuple in the index, rather than the first one as it
normally would. (This will only occur for access
methods that set <structfield>amcanorder</structfield> to true.) After the
first call, <function>amgettuple</function> must be prepared to advance the scan in
either direction from the most recently returned entry. (But if
<structfield>amcanbackward</structfield> is false, all subsequent
calls will have the same direction as the first one.)
</para>
<para>
Access methods that support ordered scans must support <quote>marking</quote> a
position in a scan and later returning to the marked position. The same
position might be restored multiple times. However, only one position need
be remembered per scan; a new <function>ammarkpos</function> call overrides the
previously marked position. An access method that does not support ordered
scans need not provide <function>ammarkpos</function> and <function>amrestrpos</function>
functions in <structname>IndexAmRoutine</structname>; set those pointers to NULL
instead.
</para>
<para>
Both the scan position and the mark position (if any) must be maintained
consistently in the face of concurrent insertions or deletions in the
index. It is OK if a freshly-inserted entry is not returned by a scan that
would have found the entry if it had existed when the scan started, or for
the scan to return such an entry upon rescanning or backing
up even though it had not been returned the first time through. Similarly,
a concurrent delete might or might not be reflected in the results of a scan.
What is important is that insertions or deletions not cause the scan to
miss or multiply return entries that were not themselves being inserted or
deleted.
</para>
<para>
If the index stores the original indexed data values (and not some lossy
representation of them), it is useful to
support <link linkend="indexes-index-only-scans">index-only scans</link>, in
which the index returns the actual data not just the TID of the heap tuple.
This will only avoid I/O if the visibility map shows that the TID is on an
all-visible page; else the heap tuple must be visited anyway to check
MVCC visibility. But that is no concern of the access method's.
</para>
<para>
Instead of using <function>amgettuple</function>, an index scan can be done with
<function>amgetbitmap</function> to fetch all tuples in one call. This can be
noticeably more efficient than <function>amgettuple</function> because it allows
avoiding lock/unlock cycles within the access method. In principle
<function>amgetbitmap</function> should have the same effects as repeated
<function>amgettuple</function> calls, but we impose several restrictions to
simplify matters. First of all, <function>amgetbitmap</function> returns all
tuples at once and marking or restoring scan positions isn't
supported. Secondly, the tuples are returned in a bitmap which doesn't
have any specific ordering, which is why <function>amgetbitmap</function> doesn't
take a <literal>direction</literal> argument. (Ordering operators will never be
supplied for such a scan, either.)
Also, there is no provision for index-only scans with
<function>amgetbitmap</function>, since there is no way to return the contents of
index tuples.
Finally, <function>amgetbitmap</function>
does not guarantee any locking of the returned tuples, with implications
spelled out in <xref linkend="index-locking"/>.
</para>
<para>
Note that it is permitted for an access method to implement only
<function>amgetbitmap</function> and not <function>amgettuple</function>, or vice versa,
if its internal implementation is unsuited to one API or the other.
</para>
</sect1>
<sect1 id="index-locking">
<title>Index Locking Considerations</title>
<para>
Index access methods must handle concurrent updates
of the index by multiple processes.
The core <productname>PostgreSQL</productname> system obtains
<literal>AccessShareLock</literal> on the index during an index scan, and
<literal>RowExclusiveLock</literal> when updating the index (including plain
<command>VACUUM</command>). Since these lock types do not conflict, the access
method is responsible for handling any fine-grained locking it might need.
An <literal>ACCESS EXCLUSIVE</literal> lock on the index as a whole will be
taken only during index creation, destruction, or <command>REINDEX</command>
(<literal>SHARE UPDATE EXCLUSIVE</literal> is taken instead with
<literal>CONCURRENTLY</literal>).
</para>
<para>
Building an index type that supports concurrent updates usually requires
extensive and subtle analysis of the required behavior. For the b-tree
and hash index types, you can read about the design decisions involved in
<filename>src/backend/access/nbtree/README</filename> and
<filename>src/backend/access/hash/README</filename>.
</para>
<para>
Aside from the index's own internal consistency requirements, concurrent
updates create issues about consistency between the parent table (the
<firstterm>heap</firstterm>) and the index. Because
<productname>PostgreSQL</productname> separates accesses
and updates of the heap from those of the index, there are windows in
which the index might be inconsistent with the heap. We handle this problem
with the following rules:
<itemizedlist>
<listitem>
<para>
A new heap entry is made before making its index entries. (Therefore
a concurrent index scan is likely to fail to see the heap entry.
This is okay because the index reader would be uninterested in an
uncommitted row anyway. But see <xref linkend="index-unique-checks"/>.)
</para>
</listitem>
<listitem>
<para>
When a heap entry is to be deleted (by <command>VACUUM</command>), all its
index entries must be removed first.
</para>
</listitem>
<listitem>
<para>
An index scan must maintain a pin
on the index page holding the item last returned by
<function>amgettuple</function>, and <function>ambulkdelete</function> cannot delete
entries from pages that are pinned by other backends. The need
for this rule is explained below.
</para>
</listitem>
</itemizedlist>
Without the third rule, it is possible for an index reader to
see an index entry just before it is removed by <command>VACUUM</command>, and
then to arrive at the corresponding heap entry after that was removed by
<command>VACUUM</command>.
This creates no serious problems if that item
number is still unused when the reader reaches it, since an empty
item slot will be ignored by <function>heap_fetch()</function>. But what if a
third backend has already re-used the item slot for something else?
When using an MVCC-compliant snapshot, there is no problem because
the new occupant of the slot is certain to be too new to pass the
snapshot test. However, with a non-MVCC-compliant snapshot (such as
<literal>SnapshotAny</literal>), it would be possible to accept and return
a row that does not in fact match the scan keys. We could defend
against this scenario by requiring the scan keys to be rechecked
against the heap row in all cases, but that is too expensive. Instead,
we use a pin on an index page as a proxy to indicate that the reader
might still be <quote>in flight</quote> from the index entry to the matching
heap entry. Making <function>ambulkdelete</function> block on such a pin ensures
that <command>VACUUM</command> cannot delete the heap entry before the reader
is done with it. This solution costs little in run time, and adds blocking
overhead only in the rare cases where there actually is a conflict.
</para>
<para>
This solution requires that index scans be <quote>synchronous</quote>: we have
to fetch each heap tuple immediately after scanning the corresponding index
entry. This is expensive for a number of reasons. An
<quote>asynchronous</quote> scan in which we collect many TIDs from the index,
and only visit the heap tuples sometime later, requires much less index
locking overhead and can allow a more efficient heap access pattern.
Per the above analysis, we must use the synchronous approach for
non-MVCC-compliant snapshots, but an asynchronous scan is workable
for a query using an MVCC snapshot.
</para>
<para>
In an <function>amgetbitmap</function> index scan, the access method does not
keep an index pin on any of the returned tuples. Therefore
it is only safe to use such scans with MVCC-compliant snapshots.
</para>
<para>
When the <structfield>ampredlocks</structfield> flag is not set, any scan using that
index access method within a serializable transaction will acquire a
nonblocking predicate lock on the full index. This will generate a
read-write conflict with the insert of any tuple into that index by a
concurrent serializable transaction. If certain patterns of read-write
conflicts are detected among a set of concurrent serializable
transactions, one of those transactions may be canceled to protect data
integrity. When the flag is set, it indicates that the index access
method implements finer-grained predicate locking, which will tend to
reduce the frequency of such transaction cancellations.
</para>
</sect1>
<sect1 id="index-unique-checks">
<title>Index Uniqueness Checks</title>
<para>
<productname>PostgreSQL</productname> enforces SQL uniqueness constraints
using <firstterm>unique indexes</firstterm>, which are indexes that disallow
multiple entries with identical keys. An access method that supports this
feature sets <structfield>amcanunique</structfield> true.
(At present, only b-tree supports it.) Columns listed in the
<literal>INCLUDE</literal> clause are not considered when enforcing
uniqueness.
</para>
<para>
Because of MVCC, it is always necessary to allow duplicate entries to
exist physically in an index: the entries might refer to successive
versions of a single logical row. The behavior we actually want to
enforce is that no MVCC snapshot could include two rows with equal
index keys. This breaks down into the following cases that must be
checked when inserting a new row into a unique index:
<itemizedlist>
<listitem>
<para>
If a conflicting valid row has been deleted by the current transaction,
it's okay. (In particular, since an UPDATE always deletes the old row
version before inserting the new version, this will allow an UPDATE on
a row without changing the key.)
</para>
</listitem>
<listitem>
<para>
If a conflicting row has been inserted by an as-yet-uncommitted
transaction, the would-be inserter must wait to see if that transaction
commits. If it rolls back then there is no conflict. If it commits
without deleting the conflicting row again, there is a uniqueness
violation. (In practice we just wait for the other transaction to
end and then redo the visibility check in toto.)
</para>
</listitem>
<listitem>
<para>
Similarly, if a conflicting valid row has been deleted by an
as-yet-uncommitted transaction, the would-be inserter must wait
for that transaction to commit or abort, and then repeat the test.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Furthermore, immediately before reporting a uniqueness violation
according to the above rules, the access method must recheck the
liveness of the row being inserted. If it is committed dead then
no violation should be reported. (This case cannot occur during the
ordinary scenario of inserting a row that's just been created by
the current transaction. It can happen during
<command>CREATE UNIQUE INDEX CONCURRENTLY</command>, however.)
</para>
<para>
We require the index access method to apply these tests itself, which
means that it must reach into the heap to check the commit status of
any row that is shown to have a duplicate key according to the index
contents. This is without a doubt ugly and non-modular, but it saves
redundant work: if we did a separate probe then the index lookup for
a conflicting row would be essentially repeated while finding the place to
insert the new row's index entry. What's more, there is no obvious way
to avoid race conditions unless the conflict check is an integral part
of insertion of the new index entry.
</para>
<para>
If the unique constraint is deferrable, there is additional complexity:
we need to be able to insert an index entry for a new row, but defer any
uniqueness-violation error until end of statement or even later. To
avoid unnecessary repeat searches of the index, the index access method
should do a preliminary uniqueness check during the initial insertion.
If this shows that there is definitely no conflicting live tuple, we
are done. Otherwise, we schedule a recheck to occur when it is time to
enforce the constraint. If, at the time of the recheck, both the inserted
tuple and some other tuple with the same key are live, then the error
must be reported. (Note that for this purpose, <quote>live</quote> actually
means <quote>any tuple in the index entry's HOT chain is live</quote>.)
To implement this, the <function>aminsert</function> function is passed a
<literal>checkUnique</literal> parameter having one of the following values:
<itemizedlist>
<listitem>
<para>
<literal>UNIQUE_CHECK_NO</literal> indicates that no uniqueness checking
should be done (this is not a unique index).
</para>
</listitem>
<listitem>
<para>
<literal>UNIQUE_CHECK_YES</literal> indicates that this is a non-deferrable
unique index, and the uniqueness check must be done immediately, as
described above.
</para>
</listitem>
<listitem>
<para>
<literal>UNIQUE_CHECK_PARTIAL</literal> indicates that the unique
constraint is deferrable. <productname>PostgreSQL</productname>
will use this mode to insert each row's index entry. The access
method must allow duplicate entries into the index, and report any
potential duplicates by returning false from <function>aminsert</function>.
For each row for which false is returned, a deferred recheck will
be scheduled.
</para>
<para>
The access method must identify any rows which might violate the
unique constraint, but it is not an error for it to report false
positives. This allows the check to be done without waiting for other
transactions to finish; conflicts reported here are not treated as
errors and will be rechecked later, by which time they may no longer
be conflicts.
</para>
</listitem>
<listitem>
<para>
<literal>UNIQUE_CHECK_EXISTING</literal> indicates that this is a deferred
recheck of a row that was reported as a potential uniqueness violation.
Although this is implemented by calling <function>aminsert</function>, the
access method must <emphasis>not</emphasis> insert a new index entry in this
case. The index entry is already present. Rather, the access method
must check to see if there is another live index entry. If so, and
if the target row is also still live, report error.
</para>
<para>
It is recommended that in a <literal>UNIQUE_CHECK_EXISTING</literal> call,
the access method further verify that the target row actually does
have an existing entry in the index, and report error if not. This
is a good idea because the index tuple values passed to
<function>aminsert</function> will have been recomputed. If the index
definition involves functions that are not really immutable, we
might be checking the wrong area of the index. Checking that the
target row is found in the recheck verifies that we are scanning
for the same tuple values as were used in the original insertion.
</para>
</listitem>
</itemizedlist>
</para>
</sect1>
<sect1 id="index-cost-estimation">
<title>Index Cost Estimation Functions</title>
<para>
The <function>amcostestimate</function> function is given information describing
a possible index scan, including lists of WHERE and ORDER BY clauses that
have been determined to be usable with the index. It must return estimates
of the cost of accessing the index and the selectivity of the WHERE
clauses (that is, the fraction of parent-table rows that will be
retrieved during the index scan). For simple cases, nearly all the
work of the cost estimator can be done by calling standard routines
in the optimizer; the point of having an <function>amcostestimate</function> function is
to allow index access methods to provide index-type-specific knowledge,
in case it is possible to improve on the standard estimates.
</para>
<para>
Each <function>amcostestimate</function> function must have the signature:
<programlisting>
void
amcostestimate (PlannerInfo *root,
IndexPath *path,
double loop_count,
Cost *indexStartupCost,
Cost *indexTotalCost,
Selectivity *indexSelectivity,
double *indexCorrelation,
double *indexPages);
</programlisting>
The first three parameters are inputs:
<variablelist>
<varlistentry>
<term><parameter>root</parameter></term>
<listitem>
<para>
The planner's information about the query being processed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>path</parameter></term>
<listitem>
<para>
The index access path being considered. All fields except cost and
selectivity values are valid.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>loop_count</parameter></term>
<listitem>
<para>
The number of repetitions of the index scan that should be factored
into the cost estimates. This will typically be greater than one when
considering a parameterized scan for use in the inside of a nestloop
join. Note that the cost estimates should still be for just one scan;
a larger <parameter>loop_count</parameter> means that it may be appropriate
to allow for some caching effects across multiple scans.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
The last five parameters are pass-by-reference outputs:
<variablelist>
<varlistentry>
<term><parameter>*indexStartupCost</parameter></term>
<listitem>
<para>
Set to cost of index start-up processing
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>*indexTotalCost</parameter></term>
<listitem>
<para>
Set to total cost of index processing
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>*indexSelectivity</parameter></term>
<listitem>
<para>
Set to index selectivity
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>*indexCorrelation</parameter></term>
<listitem>
<para>
Set to correlation coefficient between index scan order and
underlying table's order
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><parameter>*indexPages</parameter></term>
<listitem>
<para>
Set to number of index leaf pages
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
Note that cost estimate functions must be written in C, not in SQL or
any available procedural language, because they must access internal
data structures of the planner/optimizer.
</para>
<para>
The index access costs should be computed using the parameters used by
<filename>src/backend/optimizer/path/costsize.c</filename>: a sequential
disk block fetch has cost <varname>seq_page_cost</varname>, a nonsequential fetch
has cost <varname>random_page_cost</varname>, and the cost of processing one index
row should usually be taken as <varname>cpu_index_tuple_cost</varname>. In
addition, an appropriate multiple of <varname>cpu_operator_cost</varname> should
be charged for any comparison operators invoked during index processing
(especially evaluation of the indexquals themselves).
</para>
<para>
The access costs should include all disk and CPU costs associated with
scanning the index itself, but <emphasis>not</emphasis> the costs of retrieving or
processing the parent-table rows that are identified by the index.
</para>
<para>
The <quote>start-up cost</quote> is the part of the total scan cost that
must be expended before we can begin to fetch the first row. For most
indexes this can be taken as zero, but an index type with a high start-up
cost might want to set it nonzero.
</para>
<para>
The <parameter>indexSelectivity</parameter> should be set to the estimated fraction of the parent
table rows that will be retrieved during the index scan. In the case
of a lossy query, this will typically be higher than the fraction of
rows that actually pass the given qual conditions.
</para>
<para>
The <parameter>indexCorrelation</parameter> should be set to the correlation (ranging between
-1.0 and 1.0) between the index order and the table order. This is used
to adjust the estimate for the cost of fetching rows from the parent
table.
</para>
<para>
The <parameter>indexPages</parameter> should be set to the number of leaf pages.
This is used to estimate the number of workers for parallel index scan.
</para>
<para>
When <parameter>loop_count</parameter> is greater than one, the returned numbers
should be averages expected for any one scan of the index.
</para>
<procedure>
<title>Cost Estimation</title>
<para>
A typical cost estimator will proceed as follows:
</para>
<step>
<para>
Estimate and return the fraction of parent-table rows that will be visited
based on the given qual conditions. In the absence of any index-type-specific
knowledge, use the standard optimizer function <function>clauselist_selectivity()</function>:
<programlisting>
*indexSelectivity = clauselist_selectivity(root, path-&gt;indexquals,
path-&gt;indexinfo-&gt;rel-&gt;relid,
JOIN_INNER, NULL);
</programlisting>
</para>
</step>
<step>
<para>
Estimate the number of index rows that will be visited during the
scan. For many index types this is the same as <parameter>indexSelectivity</parameter> times
the number of rows in the index, but it might be more. (Note that the
index's size in pages and rows is available from the
<literal>path-&gt;indexinfo</literal> struct.)
</para>
</step>
<step>
<para>
Estimate the number of index pages that will be retrieved during the scan.
This might be just <parameter>indexSelectivity</parameter> times the index's size in pages.
</para>
</step>
<step>
<para>
Compute the index access cost. A generic estimator might do this:
<programlisting>
/*
* Our generic assumption is that the index pages will be read
* sequentially, so they cost seq_page_cost each, not random_page_cost.
* Also, we charge for evaluation of the indexquals at each index row.
* All the costs are assumed to be paid incrementally during the scan.
*/
cost_qual_eval(&amp;index_qual_cost, path-&gt;indexquals, root);
*indexStartupCost = index_qual_cost.startup;
*indexTotalCost = seq_page_cost * numIndexPages +
(cpu_index_tuple_cost + index_qual_cost.per_tuple) * numIndexTuples;
</programlisting>
However, the above does not account for amortization of index reads
across repeated index scans.
</para>
</step>
<step>
<para>
Estimate the index correlation. For a simple ordered index on a single
field, this can be retrieved from pg_statistic. If the correlation
is not known, the conservative estimate is zero (no correlation).
</para>
</step>
</procedure>
<para>
Examples of cost estimator functions can be found in
<filename>src/backend/utils/adt/selfuncs.c</filename>.
</para>
</sect1>
</chapter>