> This executes query and discards the result. Write the query the same way you would write an SQL SELECT command, but replace the initial keyword SELECT with PERFORM. For WITH queries, use PERFORM and then place the query in parentheses. (In this case, the query can only return one row.)
But that only works for a single returned value (one column of one row). Else we need to treat the WITH query like a subquery with alias. There was a related question on Stackoverflow:
For WITH queries, use PERFORM and place the query in parentheses. If the query returns more than a single value (one column of one row) you must treat it as subquery, writing PERFORM * FROM (query) my_alias;
We define the term "Scalar Subquery" in the documentation, we should not be avoiding it here and simply telling the user to "use parentheses". You are using parentheses because you are writing a scalar subquery and placing it in the target list of the PERFORM command.
So, I'd suggest the following wording:
Since WITH queries do not start with the SELECT keyword you must instead write your query independently of the PERFORM top-level query. This wrapping query will have a FROM clause just like any other query and you can place your WITH query there as a normal subquery. An alternative is to use a scalar subquery (provide xref to syntax), in which case you can simply place it after the PERFORM keyword.
Combining that with examples (or, as below, adapting the syntax example already provided) of both forms should suffice. We don't need to interleave an example in the prose.
PERFORM select_query; -- must begin with the SELECT keyword
PERFORM * FROM (with_query) AS from_alias; -- normal subquery form
