diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/plpgsql.sgml | 32 |
1 files changed, 18 insertions, 14 deletions
diff --git a/doc/src/sgml/plpgsql.sgml b/doc/src/sgml/plpgsql.sgml index 45d3e43ed14..9d41967ad3a 100644 --- a/doc/src/sgml/plpgsql.sgml +++ b/doc/src/sgml/plpgsql.sgml @@ -917,6 +917,24 @@ PREPARE <replaceable>statement_name</replaceable>(integer, integer) AS SELECT $1 they are useful to know when trying to diagnose a problem. More information appears in <xref linkend="plpgsql-plan-caching"/>. </para> + + <para> + Since an <replaceable>expression</replaceable> is converted to a + <literal>SELECT</literal> command, it can contain the same clauses + that an ordinary <literal>SELECT</literal> would, except that it + cannot include a top-level <literal>UNION</literal>, + <literal>INTERSECT</literal>, or <literal>EXCEPT</literal> clause. + Thus for example one could test whether a table is non-empty with +<programlisting> +IF count(*) > 0 FROM my_table THEN ... +</programlisting> + since the <replaceable>expression</replaceable> + between <literal>IF</literal> and <literal>THEN</literal> is parsed as + though it were <literal>SELECT count(*) > 0 FROM my_table</literal>. + The <literal>SELECT</literal> must produce a single column, and not + more than one row. (If it produces no rows, the result is taken as + NULL.) + </para> </sect1> <sect1 id="plpgsql-statements"> @@ -973,20 +991,6 @@ my_array[1:3] := array[1,2,3]; complex_array[n].realpart = 12.3; </programlisting> </para> - - <para> - It's useful to know that what follows the assignment operator is - essentially treated as a <literal>SELECT</literal> command; as long - as it returns a single row and column, it will work. Thus for example - one can write something like -<programlisting> -total_sales := sum(quantity) from sales; -</programlisting> - This provides an effect similar to the single-row <literal>SELECT - ... INTO</literal> syntax described in - <xref linkend="plpgsql-statements-sql-onerow"/>. However, that syntax - is more portable. - </para> </sect2> <sect2 id="plpgsql-statements-sql-noresult"> |