<replaceable>offset</replaceable> of type <type>interval</type>.
A family can provide <function>in_range</function> functions for
any of its supported types and one or more
<replaceable>offset</replaceable> types. Each
<function>in_range</function> function should be entered in
<structname>pg_amproc</structname> with
<structfield>amproclefttype</structfield> equal to
<type>type1</type> and <structfield>amprocrighttype</structfield>
equal to <type>type2</type>.
</para>
<para>
The essential semantics of an <function>in_range</function>
function depend on the two Boolean flag parameters. It should
add or subtract <replaceable>base</replaceable> and
<replaceable>offset</replaceable>, then compare
<replaceable>val</replaceable> to the result, as follows:
<itemizedlist>
<listitem>
<para>
if <literal>!</literal><replaceable>sub</replaceable> and
<literal>!</literal><replaceable>less</replaceable>, return
<replaceable>val</replaceable> <literal>>=</literal>
(<replaceable>base</replaceable> <literal>+</literal>
<replaceable>offset</replaceable>)
</para>
</listitem>
<listitem>
<para>
if <literal>!</literal><replaceable>sub</replaceable> and
<replaceable>less</replaceable>, return
<replaceable>val</replaceable> <literal><=</literal>
(<replaceable>base</replaceable> <literal>+</literal>
<replaceable>offset</replaceable>)
</para>
</listitem>
<listitem>
<para>
if <replaceable>sub</replaceable> and
<literal>!</literal><replaceable>less</replaceable>, return
<replaceable>val</replaceable> <literal>>=</literal>
(<replaceable>base</replaceable> <literal>-</literal>
<replaceable>offset</replaceable>)
</para>
</listitem>
<listitem>
<para>
if <replaceable>sub</replaceable> and
<replaceable>less</replaceable>, return
<replaceable>val</replaceable> <literal><=</literal>
(<replaceable>base</replaceable> <literal>-</literal>
<replaceable>offset</replaceable>)
</para>
</listitem>
</itemizedlist>
Before doing so, the function should check the sign of
<replaceable>offset</replaceable>: if it is less than zero, raise
error
<literal>ERRCODE_INVALID_PRECEDING_OR_FOLLOWING_SIZE</literal>
(22013) with error text like <quote>invalid preceding or
following size in window function</quote>. (This is required by
the SQL standard, although nonstandard operator families might
perhaps choose to ignore this restriction, since there seems to
be little semantic necessity for it.) This requirement is
delegated to the <function>in_range</function> function so that
the core code needn't understand what <quote>less than
zero</quote> means for a particular data type.
</para>
<para>
An additional expectation is that <function>in_range</function>
functions should, if practical, avoid throwing an error if
<replaceable>base</replaceable> <literal>+</literal>
<replaceable>offset</replaceable> or
<replaceable>base</replaceable> <literal>-</literal>
<replaceable>offset</replaceable> would overflow. The correct
comparison result can be determined even if that value would be
out of the data type's range. Note that if the data type
includes concepts such as <quote>infinity</quote> or
<quote>NaN</quote>, extra care may be needed to ensure that
<function>in_range</function>'s results agree with the normal
sort order of the operator family.
</para>
<para>
The results of the <function>in_range</function> function must be
consistent with the sort ordering imposed by the operator family.
To be precise, given any fixed values of
<replaceable>offset</replaceable> and
<replaceable>sub</replaceable>,