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Diffstat (limited to 'doc/src/sgml/xaggr.sgml')
| -rw-r--r-- | doc/src/sgml/xaggr.sgml | 16 |
1 files changed, 15 insertions, 1 deletions
diff --git a/doc/src/sgml/xaggr.sgml b/doc/src/sgml/xaggr.sgml index fa98572ed6..79a9f288b2 100644 --- a/doc/src/sgml/xaggr.sgml +++ b/doc/src/sgml/xaggr.sgml @@ -626,7 +626,7 @@ if (AggCheckCallContext(fcinfo, NULL)) function, the first input must be a temporary state value and can therefore safely be modified in-place rather than allocating a new copy. - See <literal>int8inc()</> for an example. + See <function>int8inc()</> for an example. (This is the <emphasis>only</> case where it is safe for a function to modify a pass-by-reference input. In particular, final functions for normal aggregates must not @@ -635,6 +635,20 @@ if (AggCheckCallContext(fcinfo, NULL)) </para> <para> + The second argument of <function>AggCheckCallContext</> can be used to + retrieve the memory context in which aggregate state values are being kept. + This is useful for transition functions that wish to use <quote>expanded</> + objects (see <xref linkend="xtypes-toast">) as their state values. + On first call, the transition function should return an expanded object + whose memory context is a child of the aggregate state context, and then + keep returning the same expanded object on subsequent calls. See + <function>array_append()</> for an example. (<function>array_append()</> + is not the transition function of any built-in aggregate, but it is written + to behave efficiently when used as transition function of a custom + aggregate.) + </para> + + <para> Another support routine available to aggregate functions written in C is <function>AggGetAggref</>, which returns the <literal>Aggref</> parse node that defines the aggregate call. This is mainly useful |