Question

Here is my query,

   SELECT ID As Col1,
   (
         SELECT VID FROM TABLE2 t 
         WHERE (a.ID=t.ID or a.ID=t.ID2) 
         AND t.STARTDTE =
         (
               SELECT MAX(tt.STARTDTE) 
               FROM TABLE2 tt 
               WHERE (a.ID=tt.ID or a.ID=tt.ID2)  AND tt.STARTDTE < SYSDATE
         )
   ) As Col2
   FROM TABLE1 a

Table1 has 48850 records and Table2 has 15944098 records.

I have separate indexes in TABLE2 on ID,ID & STARTDTE, STARTDTE, ID, ID2 & STARTDTE.

The query is still too slow. How can this be improved? Please help.

Answer 1

I’m guessing that the OR in inner queries is messing up with the optimizer’s ability to use indexes. Also I wouldn’t recommend a solution that would scan all of TABLE2 given its size.

This is why in this case I would suggest using a function that will efficiently retrieve the information you are looking for (2 index scan per call):

CREATE OR REPLACE FUNCTION getvid(p_id table1.id%TYPE) 
   RETURN table2.vid%TYPE IS
   l_result table2.vid%TYPE;
BEGIN
   SELECT vid
     INTO l_result
     FROM (SELECT vid, startdte
             FROM (SELECT vid, startdte
                     FROM table2 t
                    WHERE t.id = p_id
                      AND t.startdte < SYSDATE
                    ORDER BY t.startdte DESC)
            WHERE rownum = 1
           UNION ALL
           SELECT vid, startdte
             FROM (SELECT vid, startdte
                     FROM table2 t
                    WHERE t.id2 = p_id
                      AND t.startdte < SYSDATE
                    ORDER BY t.startdte DESC)
            WHERE rownum = 1
            ORDER BY startdte DESC)
    WHERE rownum = 1;
   RETURN l_result;
END;

Your SQL would become:

SELECT ID As Col1,
       getvid(a.id) vid
  FROM TABLE1 a

Make sure you have indexes on both table2(id, startdte DESC) and table2(id2, startdte DESC). The order of the index is very important.