Cursor based records in PostgreSQL

问题:

I'm trying to use cursors for a query that joins multiple tables. I've seen that for oracle there is a cursor based record. When I try the same for Postgres, it throws some error. How can I do the same in Postgres?

CREATE OR REPLACE FUNCTION avoidable_states()
RETURNS SETOF varchar AS
$BODY$
DECLARE
    xyz CURSOR FOR select * from address ad
                            join city ct on ad.city_id = ct.city_id;    
    xyz_row RECORD;
BEGIN   
    open xyz;

    LOOP
    fetch xyz into xyz_row;
        exit when xyz_row = null;
        if xyz_row.city like '%hi%' then
            return next xyz_row.city;               
        end if;
    END LOOP;
    close xyz;  
END;
$BODY$
  LANGUAGE plpgsql VOLATILE;

Error I get is:

ERROR:  relation "xyz" does not exist
CONTEXT:  compilation of PL/pgSQL function "avoidable_states" near line 4

回答1:

Just use the RECORD type:

DECLARE
    ...
    cur_row RECORD;
BEGIN
    ...
    FETCH xyz INTO cur_row;
    EXIT WHEN NOT FOUND;
    IF cur_row.city LIKE 'CH%' THEN
        ...

回答2:

1. Implicit cursor

It's almost always better to use the implicit cursor of a FOR loop than to resort to a somewhat slower and unwieldy explicit cursor. I have written thousands of plpgsql functions and only a hand full of times explicit cursors made any sense.

CREATE OR REPLACE FUNCTION avoidable_states()
  RETURNS SETOF varchar AS
$func$
DECLARE
    rec record;
BEGIN   
   FOR rec IN
      SELECT *
      FROM   address ad
      JOIN   city    ct USING (city_id)
   LOOP
      IF rec.city LIKE '%hi%' THEN
          RETURN NEXT rec.city;               
      END IF;
   END LOOP;
END
$func$  LANGUAGE plpgsql STABLE;

Aside: there is nothing in the function that would need volatility VOLATILE. Use STABLE.

2. Set-based approach

It's almost always better to use a set-based approach if possible. Use RETURN QUERY to return as set from a query directly.

CREATE OR REPLACE FUNCTION avoidable_states()
  RETURNS SETOF varchar AS
$func$
BEGIN   
   RETURN QUERY
   SELECT ct.city
   FROM   address ad
   JOIN   city    ct USING (city_id)
   WHERE  ct.city LIKE '%hi%';
END
$func$  LANGUAGE plpgsql STABLE;

3. SQL function

For the simple case (probably a simplification), you might also use a simple SQL function or even just the query:

CREATE OR REPLACE FUNCTION avoidable_states()
  RETURNS SETOF varchar AS
$func$
   SELECT ct.city
   FROM   address ad
   JOIN   city    ct USING (city_id)
   WHERE  ct.city LIKE '%hi%';
$func$  LANGUAGE sql STABLE;