we need to
scan through all of the overflow pages. Thus an unbalanced hash index
might actually be worse than a B-tree in terms of number of block
accesses required, for some data.
</para>
<para>
As a result of the overflow cases, we can say that hash indexes are
most suitable for unique, nearly unique data or data with a low number
of rows per hash bucket.
One possible way to avoid problems is to exclude highly non-unique
values from the index using a partial index condition, but this may
not be suitable in many cases.
</para>
<para>
Like B-Trees, hash indexes perform simple index tuple deletion. This
is a deferred maintenance operation that deletes index tuples that are
known to be safe to delete (those whose item identifier's LP_DEAD bit
is already set). If an insert finds no space is available on a page we
try to avoid creating a new overflow page by attempting to remove dead
index tuples. Removal cannot occur if the page is pinned at that time.
Deletion of dead index pointers also occurs during VACUUM.
</para>
<para>
If it can, VACUUM will also try to squeeze the index tuples onto as
few overflow pages as possible, minimizing the overflow chain. If an
overflow page becomes empty, overflow pages can be recycled for reuse
in other buckets, though we never return them to the operating system.
There is currently no provision to shrink a hash index, other than by
rebuilding it with REINDEX.
There is no provision for reducing the number of buckets, either.
</para>
<para>
Hash indexes may expand the number of bucket pages as the number of
rows indexed grows. The hash key-to-bucket-number mapping is chosen so that
the index can be incrementally expanded. When a new bucket is to be added to
the index, exactly one existing bucket will need to be "split", with some of
its tuples being transferred to the new bucket according to the updated
key-to-bucket-number mapping.
</para>
<para>
The expansion occurs in the foreground, which could increase execution
time for user inserts. Thus, hash indexes may not be suitable for tables
with rapidly increasing number of rows.
</para>
</sect2>