> I am trying to find the difference between the size column. So the > desired output would be > > ts | size| Diff > -------------------+-----+------ > 2002-03-16 | 11 | 0 > > 2002-03-17 | 15 | 4 > 2002-03-18 | 18 | 3 > 2002-03-19 | 12 | -6 > > > I need the first column to be 0, since it will be 11-11. The second > colum is 15-11. The third column is 18-15. The fourth column is 12-18. > > Any thoughts about this?
Here's one way to do this with PL/PgSQL. It's probably not the most efficient, but it does work. For this code to be safe `size' must never be NULL and `ts' must be unique across all records in the input set.
CREATE OR REPLACE FUNCTION x_diff( OUT ts TIMESTAMP, OUT size INTEGER, OUT diff INTEGER) RETURNS SETOF record AS $$ DECLARE cur_x x; last_size INTEGER := null; BEGIN FOR cur_x IN SELECT * FROM x ORDER BY ts ASC LOOP ts := cur_x.ts; size := cur_x.size; IF last_size IS NULL THEN -- First record in set has diff `0' because the differences -- are defined against the previous, rather than next, -- record. diff := 0; ELSE diff := cur_x.size - last_size; END IF; last_size := cur_x.size; RETURN NEXT; END LOOP; RETURN; END; $$ LANGUAGE 'plpgsql' STRICT;
If you need to constrain the range of values processed that's not too tricky - either feed the function a refcursor for a query result set to iterate over, or pass it parameters to constrain the query with a WHERE clause. The former is more flexible, the latter is easier to use.
