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-rw-r--r--doc/src/sgml/xfunc.sgml21
1 files changed, 21 insertions, 0 deletions
diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index d70aa6eed78..b486ef391ee 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -3439,4 +3439,25 @@ supportfn(internal) returns internal
simplify. Ensure rigorous equivalence between the simplified
expression and an actual execution of the target function.
</para>
+
+ <para>
+ For target functions that return boolean, it is often useful to estimate
+ the fraction of rows that will be selected by a WHERE clause using that
+ function. This can be done by a support function that implements
+ the <literal>SupportRequestSelectivity</literal> request type.
+ </para>
+
+ <para>
+ If the target function's runtime is highly dependent on its inputs,
+ it may be useful to provide a non-constant cost estimate for it.
+ This can be done by a support function that implements
+ the <literal>SupportRequestCost</literal> request type.
+ </para>
+
+ <para>
+ For target functions that return sets, it is often useful to provide
+ a non-constant estimate for the number of rows that will be returned.
+ This can be done by a support function that implements
+ the <literal>SupportRequestRows</literal> request type.
+ </para>
</sect1>