Question

I have the following query :

update tab1.abc,
tab1.sbd = (select tab2.abc, tab2.sbd from tab2,tab1 where --some conditions)
where exists(select tab2.abc, tab2.sbd from tab2,tab1 where --some conditions)

Now my task is to optmize this query.

I believe removing the exists clause or combining both the where clauses would help a lot.
But how to do it ?

PS : exists clause is in place because I want the number of rows updated to be zero if the select clause returns zero rows.

Answer 1

JOIN the two tables instead of EXISTS. Something like the following:

UPDATE tab1
INNER JOIN tab2 ON --some join condition
SET sbd = --something
AND abc = --other something
WHERE --some conditions