avoid unnecessary updates
I do update t set x=:a;
If I do it twice, I am doing a lot of unnecessary updates. This is true in an update, and also in the update clause of a MERGE.
I need to take care of null, I can update null with something, or something with null, but update null with null is also unnecessary.
SQL> update t set x=:a;
4977 rows updated.
Elapsed: 00:00:00.34
SQL> update t set x=:a;
4977 rows updated.
Elapsed: 00:00:00.32
SQL> update t set x=:a
2 where x!=:a
3 or (x is null and :a is not null)
4 or (x is not null and :a is null);
0 rows updated.
Elapsed: 00:00:00.04
Laurent,
I would rather use this:
update t set x=:a
where decode(x,:a,’Y',’N') = ‘N’;
Paweł
Not only is the query faster (as it updates fewer rows), but it also places write-locks on fewer rows. This is normally a good thing too!
In my opinion decode is better because:
1. it’s shorter
2. it’s easier to maintain ‘and’ & ‘or’ conditions in complex queries
3. it runs at the same speed (assuming full table scan situation)
I don’t assume full table scan situation
Pawel,
and probably 3. I have not found a case where decode would be slower, but it may exists.
I agree with 1
I am not convinced about maintanability. Decode is a trick after all