diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/catalogs.sgml | 3 | ||||
| -rw-r--r-- | doc/src/sgml/xfunc.sgml | 34 |
2 files changed, 36 insertions, 1 deletions
diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml index 9564e012e66..dcaeda413d3 100644 --- a/doc/src/sgml/catalogs.sgml +++ b/doc/src/sgml/catalogs.sgml @@ -4398,7 +4398,8 @@ <entry><structfield>protransform</structfield></entry> <entry><type>regproc</type></entry> <entry><literal><link linkend="catalog-pg-proc"><structname>pg_proc</structname></link>.oid</literal></entry> - <entry>Calls to function can be simplified by this other function</entry> + <entry>Calls to this function can be simplified by this other function + (see <xref linkend="xfunc-transform-functions">)</entry> </row> <row> diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml index e8620ef0738..58df90e30fe 100644 --- a/doc/src/sgml/xfunc.sgml +++ b/doc/src/sgml/xfunc.sgml @@ -3177,6 +3177,40 @@ CREATE FUNCTION make_array(anyelement) RETURNS anyarray </para> </sect2> + <sect2 id="xfunc-transform-functions"> + <title>Transform Functions</title> + + <para> + Some function calls can be simplified during planning based on + properties specific to the function. For example, + <literal>int4mul(n, 1)</> could be simplified to just <literal>n</>. + To define such function-specific optimizations, write a + <firstterm>transform function</> and place its OID in the + <structfield>protransform</> field of the primary function's + <structname>pg_proc</> entry. The transform function must have the SQL + signature <literal>protransform(internal) RETURNS internal</>. The + argument, actually <type>FuncExpr *</>, is a dummy node representing a + call to the primary function. If the transform function's study of the + expression tree proves that a simplified expression tree can substitute + for all possible concrete calls represented thereby, build and return + that simplified expression. Otherwise, return a <literal>NULL</> + pointer (<emphasis>not</> a SQL null). + </para> + + <para> + We make no guarantee that <productname>PostgreSQL</> will never call the + primary function in cases that the transform function could simplify. + Ensure rigorous equivalence between the simplified expression and an + actual call to the primary function. + </para> + + <para> + Currently, this facility is not exposed to users at the SQL level + because of security concerns, so it is only practical to use for + optimizing built-in functions. + </para> + </sect2> + <sect2> <title>Shared Memory and LWLocks</title> |