diff options
Diffstat (limited to 'doc/src/sgml/rowtypes.sgml')
| -rw-r--r-- | doc/src/sgml/rowtypes.sgml | 13 |
1 files changed, 9 insertions, 4 deletions
diff --git a/doc/src/sgml/rowtypes.sgml b/doc/src/sgml/rowtypes.sgml index bc2fc9b8850..7e436a46065 100644 --- a/doc/src/sgml/rowtypes.sgml +++ b/doc/src/sgml/rowtypes.sgml @@ -328,11 +328,16 @@ SELECT (myfunc(x)).a, (myfunc(x)).b, (myfunc(x)).c FROM some_table; with either syntax. If it's an expensive function you may wish to avoid that, which you can do with a query like: <programlisting> -SELECT (m).* FROM (SELECT myfunc(x) AS m FROM some_table OFFSET 0) ss; +SELECT m.* FROM some_table, LATERAL myfunc(x) AS m; </programlisting> - The <literal>OFFSET 0</literal> clause keeps the optimizer - from <quote>flattening</quote> the sub-select to arrive at the form with - multiple calls of <function>myfunc()</function>. + Placing the function in + a <literal>LATERAL</literal> <literal>FROM</literal> item keeps it from + being invoked more than once per row. <literal>m.*</literal> is still + expanded into <literal>m.a, m.b, m.c</literal>, but now those variables + are just references to the output of the <literal>FROM</literal> item. + (The <literal>LATERAL</literal> keyword is optional here, but we show it + to clarify that the function is getting <structfield>x</structfield> + from <structname>some_table</structname>.) </para> </tip> |