"union all" query consumes all memory

Hello,

one my customer reported a "out of memory" issue. After investigation he
found a main problem in large query that uses a lot of union all queries.
He wrote a self test:

do $$

declare i integer; str text='';

begin

for i in 1..1000 loop

str := str || 'union all select i,i,i from generate_series(1,50000) g(i) ';

end loop;

execute 'select 1,2,3 ' || str;

end;
$$

is it expected behave?

Tested on PostgreSQL 9.1, 9.2, 9.3

It looks so all generated data are saved in memory only.

Regards

Pavel Stehule

Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> one my customer reported a "out of memory" issue. After investigation he
> found a main problem in large query that uses a lot of union all queries.
> He wrote a self test:

> do $$

> declare i integer; str text='';

> begin

> for i in 1..1000 loop

> str := str || 'union all select i,i,i from generate_series(1,50000) g(i) ';

> end loop;

> execute 'select 1,2,3 ' || str;

> end;
> $$

> is it expected behave?

Well, that query is entitled to use 1000 * work_mem for the
generate_series output tuplestores. What's your work_mem setting?
Perhaps more to the point, what was your customer doing?

regards, tom lane