Re: Aggregate functions with FROM clause and ROW_COUNT diagnostics

On Mon, May 21, 2018 at 5:54 AM, Alexey Dokuchaev <danfe(at)nsu(dot)ru> wrote:

> result := json_agg(_) FROM (
> SELECT foo, bar, baz ...
> FROM t1, t2, t3 WHERE ...) AS _; -- this works fine
>
> GET DIAGNOSTICS retcode = ROW_COUNT; -- always returns 1
>
> I'd expected `retcode' to contain the number of SELECT'ed rows, but it
> is something else (always 1). Apparently, aggregation functions like
> json_agg()/array_agg() mangle the ROW_COUNT from the inner SELECT (the
> one I'm interested in).
>
> Is this expected and correct behavior?

​Yes, the query returned only one row, with a single json column. You
wrote the equivalent of:

SELECT json_agg(...) FROM ... INTO result;

And you are getting the count of the top-most select (which is implied in
the syntax that you used).

> Is it possible to obtain the
> first ROW_COUNT (after SELECT) without performing it twice?
>

​Not directly, no. You should execute the inner query to a temporary table
than perform your counting and json_agg from that.

David J.