Question

I am attempting to write a PL/PgSQL function which will execute a query stored within a PostgreSQL table like so:

CREATE OR REPLACE FUNCTION evaluate_scenario(scenario_id int)
RETURNS TABLE(line_item_id int, organization_id int, data_element_id int, value varchar) AS $$
BEGIN
  RETURN QUERY
  SELECT
    li.id,
    li.organization_id,
    de.id,
    (EXECUTE 'SELECT ' || de.query)::varchar
  FROM
    line_items AS li INNER JOIN
    summary_files AS sf ON li.summary_file_id = sf.id INNER JOIN
    scenarios AS s ON s.summary_file_id = sf.id CROSS JOIN
    data_elements AS de
  WHERE
    s.id = 1 AND
    de.scope = 3 AND (
      de.model_id = s.model_id OR
      de.scenario_id = s.id OR
      de.organization_id = s.organization_id
    );
END;
$$ LANGUAGE plpgsql;

When I try to run this query:

select line_item_id, data_element_id, value from evaluate_scenario(1)

I receive the following error:

********** Error **********

ERROR: type "execute" does not exist
SQL state: 42704
Context: PL/pgSQL function "evaluate_scenario" line 3 at RETURN QUERY

Any help would be greatly appreciated. Note that if I remove the “EXECUTE” and just do SELECT de.query inside those parens, I receive the value for that column as expected, but I can’t figure out how to make de.query an acceptable query string for “EXECUTE”.

Answer 1

EXECUTE takes a query string as parameter and executes it, but you cannot nest EXECUTE in a query.

There are a couple of variants for EXECUTE in plpgsql. For your case, RETURN QUERY EXECUTE query might be best. Read the manual.

This could work – I am not trying to fix all the mess, just demonstrate a working example.

CREATE OR REPLACE FUNCTION evaluate_scenario(_scenario_id int)
RETURNS TABLE(line_item_id int, organization_id int, data_element_id int, value varchar) AS
$func$

BEGIN
  RETURN QUERY EXECUTE '
  SELECT li.id,
         li.organization_id,
         de.id,
        (SELECT '
|| (SELECT query
    FROM   data_elements
    WHERE  scenario_id = _scenario_id)
|| ')
  FROM   line_items AS li
  JOIN   summary_files AS sf ON li.summary_file_id = sf.id
  JOIN   scenarios AS s ON s.summary_file_id = sf.id
  CROSS  JOIN data_elements AS de
  WHERE  s.id = 1
  AND    de.scope = 3
  AND   (de.model_id = s.model_id OR
         de.scenario_id = s.id OR
         de.organization_id = s.organization_id
        )';

END;
$func$ LANGUAGE plpgsql;

Note that it is inherently unsafe to execute text literals as code. If someone can smuggle a DROP * FROM tbl or some such into the table data_elements, you are in big trouble. I am speaking of SQL injection.
Read more about SQLi at bobby-tables.com.