Select from dynamic table in function without using EXECUTE - function

Question

Can we select inside a PostgreSQL function without using EXECUTE?
I'm trying to use quote_ident() to create dynamic SQL but it doesn't work.
CREATE OR REPLACE FUNCTION select_server(p_id text)
RETURNS integer AS $$
DECLARE
serialnum_value INTEGER;
STATEMENT TEXT;
BEGIN
STATEMENT := 'tbl' || substr($1, 1, 4);
SELECT serialnum INTO serialnum_value FROM quote_ident(STATEMENT ) WHERE id = $1;
RETURN serialnum;
END;
Does anybody have idea how to select from dynamic table in a PostgreSQL function without using EXECUTE?

Answer 1

You can't avoid execute here, per igor's comment. The best alternative is to create a function that creates a function, which you can then call as needed. Example:
create function gen_select_server(p_id text)
returns boolean
as $gen$
begin
execute $exec$
create function $exec$ || quote_ident('select_server_' || p_id) || $exec$
returns integer
as $body$
begin
return serialnum
from $exec$ || quote_ident('tbl_' || p_id) || $exec$
where id = $exec$ || quote_literal('id_' || p_id) || $exec$;
end;
$body$ language plpgsql;
$exec$;
return true;
end;
$gen$ language plpgsql;
-- usage:
select gen_select_server('1234'); -- call this only once; uses execute
select * from select_server_1234(); -- no execute here
As the above highlights, it can get messy with string delimiters. Also note that the above isn't your original function -- it's primarily to illustrate how to quote things properly within the big $exec$ "string" block.
I'd recommend to stick to using execute, that being said. Or using an ORM, for that matter. Maintaining these micro-optimizations is potentially a pain worse than the superficial gain in performance.

Answer 2

You cannot execute dynamic SQL without EXECUTE in PL/pgSQL. That's what makes it dynamic in the first place.
But you can streamline your function quite a bit:
CREATE OR REPLACE FUNCTION select_server(p_id text, OUT serial_value integer) AS
$func$
BEGIN
EXECUTE 'SELECT serialnum FROM tbl' || left($1::text, 4)) || ' WHERE id = $1'
USING $1
INTO serial_value;
END
$func$ LANGUAGE plpgsql STRICT;
Normally you would have to sanitize any identifier.
But you do not need quote_ident() in this special case, since the only dynamic component are 4 digits from an integer. Neither SQL injection nor illegal identifiers are possible this way.
Pass the value of p_id as value using the USING clause.
Reduce the number of assignments. Those are comparatively expensive in plpgsql. You only need a single SQL statement to do everything.
The OUT parameter helps to shorten the syntax.
I also made the function STRICT (RETURNS NULL ON NULL INPUT), since it would not make sense with NULL as input.
left() is slightly faster than substr().