PostgreSQL spread array values as input parameters for using clause - postgresql

Question

CREATE OR REPLACE FUNCTION demo(vsql text, vals text[])
RETURNS void AS
$BODY$
BEGIN
execute vsql using vals;
END;
$BODY$
LANGUAGE plpgsql VOLATILE;
select demo('select $1 || $2', ARRAY['Barrett', ' Gilmour'] );
ERROR: there is no parameter $2
LINE 1: select $1 || $2
The error is that postgres does not understand that the two items in the array must be spread to the input parameters $1 and $2. It understand the entire array as value for $1

Answer 1

As discussed in SO question https://dba.stackexchange.com/questions/83278/postgressql-dynamic-execute-with-argument-values-in-array, the EXECUTE ... USING will treat an array as a single parameter.
You could try this hack, which edits the passed SQL statement so that $n becomes $1[n]
vsql := regexp_replace( vsql, '[$]([0-9]+)', '$1[\1]', 'g' );
The array vals is treated as a single parameter but the parameterised select can index into it to get the elements.
a_horse_with_no_name mentioned your other problem which is that your function doesn't actually return anything. If you want to see what is being executed:
-- anyelement is a polymorphic pseudo-type
-- problem with returning 'setof record' is that select * from so_demov3() gives error
-- ERROR: a column definition list is required for functions returning "record"
drop function so_demov3(anyelement, text, text[]);
create function so_demov3(rec_type anyelement, p_sql text, p_vals text[]) returns setof anyelement as
$$
declare v_sql text;
begin
-- edit the executable SQL stmt so that $n -> $1[n]
v_sql := regexp_replace( p_sql, '[$]([0-9]+)', '$1[\1]', 'g' );
return query execute v_sql using p_vals;
end;
$$
language plpgsql volatile;
And call it like this:
select * from so_demov3(null::text, 'select $1 || $2', array['Barrett', 'Gilmour'] );