Question

i have the following select statement :

(
  SELECT CAL_DATE,BUS_DAY
  FROM risk_mart.vw_calendar_t5
  WHERE type = 'RPC'
  AND BUS_DAY BETWEEN 0 AND 1
) CAL
 WHERE t.cal_date         = cal.cal_date

It selects all the dates and then filters them between 0 and 1 (displays only two dates e.g. ’22-OCT-12′,’19-OCT-12′).

Now, i can re-write that manually as

WHERE t.cal_date in (’22-OCT-12′,’19-OCT-12′)

The difference is performance. The first example i get a return back within 90 seconds and the second example, i get it within 10 seconds. Why?.

How can i improve the first statement/rewrite it?

Answer 1

FROM
(
  SELECT CAL_DATE,BUS_DAY
  FROM risk_mart.vw_calendar_t5
  WHERE type = 'RPC'
  AND BUS_DAY BETWEEN 0 AND 1
) CAL --inner view
 WHERE t.cal_date         = cal.cal_date --join of table t and CAL

Now ,this is your first query .In this you are first retrieving the records into an inner view CAL which takes some amount of time .And after that you join t.cal_date = cal.cal_date which will again take some amount of time (TOTAL 90Sec).

Now ,here is the second query .

FROM T
WHERE t.cal_date      IN ('22-OCT-12','19-OCT-12');

In this case ,you dint use the inner view ,so you have somehow managed to get the dates directly and use that in your where condition(HARDCODED) .This was you have reduce the processing time of the inner view (CAL) and the join with the CAL table with table t.