Variables for identifiers inside IF EXISTS in a plpgsql function

Question

CREATE OR REPLACE FUNCTION drop_now()
  RETURNS void AS
$BODY$
DECLARE
    row     record;
BEGIN
    RAISE INFO 'in';
    FOR row IN 
        select relname from pg_stat_user_tables
        WHERE schemaname='public' AND relname LIKE '%test%'
    LOOP    
    IF EXISTS(SELECT row.relname.tm FROM row.relname
              WHERE row.relname.tm < current_timestamp - INTERVAL '90 minutes'
              LIMIT 1)              
    THEN
    -- EXECUTE 'DROP TABLE ' || quote_ident(row.relname);
    RAISE INFO 'Dropped table: %', quote_ident(row.relname);

    END IF;

    END LOOP;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE;

Could you tell me how to use variables in SELECT which is inside IF EXISTS? At the present moment, row.relname.tm and row.relname are treated literally which is not I want.

Answer 1

CREATE OR REPLACE FUNCTION drop_now()
  RETURNS void AS
$func$
DECLARE
   _tbl   regclass;
   _found int;
BEGIN
   FOR _tbl IN 
      SELECT relid
      FROM   pg_stat_user_tables
      WHERE  schemaname = 'public'
      AND    relname LIKE '%test%'
   LOOP
      EXECUTE format($f$SELECT 1 FROM %s
                        WHERE  tm < now() - interval '90 min'$f$, _tbl);
      GET DIAGNOSTICS _found = ROW_COUNT;
      IF _found > 0 THEN
         -- EXECUTE 'DROP TABLE ' || _tbl;
         RAISE NOTICE 'Dropped table: %', _tbl;
      END IF;
   END LOOP;
END
$func$ LANGUAGE plpgsql;

Major points

• row is a reserved word in the SQL standard. It's use is allowed in Postgres, but it's still unwise. I make it a habbit to prepend psql variable with an underscore _ to avoid any naming conflicts.

• You don't don't select the whole row anyway, just the table name in this example. Best use a variable of type regclass, thereby avoiding SQL injection by way of illegal table names automatically. Details in this related answer:
  Table name as a PostgreSQL function parameter

• You don't need LIMIT in an EXISTS expression, which only checks for the existence of any rows. And you don't need meaningful target columns for the same reason. Just write SELECT 1 or SELECT * or something.

• You need dynamic SQL for queries with variable identifiers. Plain SQL does not allow for that. I.e.: build a query string and EXECUTE it. Details in this closely related answer:
  Dynamic SQL (EXECUTE) as condition for IF statement

• The same is true for a DROP statement, should you want to run it. I added a comment.

Answer 2

You'll need to build your query as a string then execute that - see the section on executing dynamic commands in the plpgsql section of the manual.