diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/xfunc.sgml | 21 |
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> |