diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/xfunc.sgml | 52 |
1 files changed, 39 insertions, 13 deletions
diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml index f2f379870fa..09427bbed2b 100644 --- a/doc/src/sgml/xfunc.sgml +++ b/doc/src/sgml/xfunc.sgml @@ -962,12 +962,11 @@ SELECT name, child FROM nodes, LATERAL listchildren(name) AS child; </para> <para> - Currently, functions returning sets can also be called in the select list + Functions returning sets can also be called in the select list of a query. For each row that the query - generates by itself, the function returning set is invoked, and an output - row is generated for each element of the function's result set. Note, - however, that this capability is deprecated and might be removed in future - releases. The previous example could also be done with queries like + generates by itself, the set-returning function is invoked, and an output + row is generated for each element of the function's result set. + The previous example could also be done with queries like these: <screen> @@ -998,6 +997,33 @@ SELECT name, listchildren(name) FROM nodes; the <literal>LATERAL</> syntax. </para> + <para> + If there is more than one set-returning function in the same select + list, the behavior is similar to what you get from putting the functions + into a single <literal>LATERAL ROWS FROM( ... )</> <literal>FROM</>-clause + item. For each row from the underlying query, there is an output row + using the first result from each function, then an output row using the + second result, and so on. If some of the set-returning functions + produce fewer outputs than others, null values are substituted for the + missing data, so that the total number of rows emitted for one + underlying row is the same as for the set-returning function that + produced the most outputs. + </para> + + <para> + Set-returning functions can be nested in a select list, although that is + not allowed in <literal>FROM</>-clause items. In such cases, each level + of nesting is treated separately, as though it were + another <literal>LATERAL ROWS FROM( ... )</> item. For example, in +<programlisting> +SELECT srf1(srf2(x), srf3(y)), srf4(srf5(z)) FROM ... +</programlisting> + the set-returning functions <function>srf2</>, <function>srf3</>, + and <function>srf5</> would be run in lockstep for each row of the + underlying query, and then <function>srf1</> and <function>srf4</> would + be applied in lockstep to each row produced by the lower functions. + </para> + <note> <para> If a function's last command is <command>INSERT</>, <command>UPDATE</>, @@ -1012,14 +1038,14 @@ SELECT name, listchildren(name) FROM nodes; <note> <para> - The key problem with using set-returning functions in the select list, - rather than the <literal>FROM</> clause, is that putting more than one - set-returning function in the same select list does not behave very - sensibly. (What you actually get if you do so is a number of output - rows equal to the least common multiple of the numbers of rows produced - by each set-returning function.) The <literal>LATERAL</> syntax - produces less surprising results when calling multiple set-returning - functions, and should usually be used instead. + Before <productname>PostgreSQL</> 10, putting more than one + set-returning function in the same select list did not behave very + sensibly unless they always produced equal numbers of rows. Otherwise, + what you got was a number of output rows equal to the least common + multiple of the numbers of rows produced by the set-returning + functions. Furthermore, nested set-returning functions did not work at + all. Use of the <literal>LATERAL</> syntax is recommended when writing + queries that need to work in older <productname>PostgreSQL</> versions. </para> </note> </sect2> |