Multirow subselect as parameter to `execute using` - postgresql

Question

The multirow subselect will be used in the right hand side of the in operator in the where clause:
create table t (a integer);
insert into t (a) values (1), (9);
drop function if exists f();
create function f()
returns void as $$
begin
execute '
select a
from t
where a in $1
' using (select 1 union select 2);
end;$$
language plpgsql;
select f();
ERROR: more than one row returned by a subquery used as an expression
CONTEXT: SQL statement "SELECT (select 1 union select 2)"
PL/pgSQL function "f" line 3 at EXECUTE statement
How to achieve what the above function would if it worked?

Answer 1

I see nothing in your question that couldn't more easily be solved with:
SELECT a
FROM t
JOIN (VALUES (1), (2)) x(a) USING (a); -- any query returning multiple int
Can you clarify the necessity in your example?
As a proof of concept, this would be simpler / faster:
CREATE OR REPLACE FUNCTION x.f1()
RETURNS SETOF integer AS
$BODY$
BEGIN
RETURN QUERY EXECUTE '
SELECT a
FROM t
WHERE a = ANY($1)'
USING ARRAY(VALUES (1), (2)); -- any query here
END;
$BODY$
LANGUAGE plpgsql;
Performance of IN () and = ANY()
Your observation is spot on. And there is reason for it. Try:
EXPLAIN ANALYZE SELECT * FROM tbl WHERE id IN (1,2,3);
The query plan will reveal:
Index Cond: (id = ANY ('{1,2,3}'::integer[]))
PostgreSQL transforms the id IN (..) construct into id = ANY(..) internally. These two perform identically - except for a negligible overhead penalty.
My code is only faster in building the statement - exactly as you diagnosed.

Answer 2

create function f()
returns setof integer as $$
begin
return query execute format('
select a
from t
where a in %s
', replace(replace(array(select 1 union select 2)::text, '{', '('), '}', ')'));
end$$
language plpgsql;