by Russ » Thu, 05 Feb 1998 04:00:00
I've been evaluating RDO in Visual Basic accessing a SQLAnywhere v5.0
database. I'm getting very odd performance metrics:
select note_id, note_type, note_text from mynotes 12 seconds
select * from mynotes 70 seconds.
There are a total of twelve columns in mynotes and none of them are
"large" except note_text. Any idea what's going on?
-Russ
by ke.. » Fri, 06 Feb 1998 04:00:00
Well..
SELECT * will always do table scan..
I guess your table may have non-clustered index on note_id, note_type and note_text.. If this is the case, the optimizer will just scan the index leaf pages to get the result, which can be much faster than table scan.
Hope this help.
Ken
by Richard Freedma » Fri, 06 Feb 1998 04:00:00
This should be a non-problem as far as any software that
you are writing. Due to the fact that a column could be
added to the table (as well as the performance problem),
IMHO, "SELECT *" should never be used in production code.
> Well..
> SELECT * will always do table scan..
> I guess your table may have non-clustered index on note_id, note_type and note_text.. If this is the case, the optimizer will just scan the index leaf pages to get the result, which can be much faster than table scan.
> Hope this help.
> Ken
by Russ » Sat, 07 Feb 1998 04:00:00
> Well..
> SELECT * will always do table scan..
> I guess your table may have non-clustered index on note_id, note_type and note_text.. If this is the case, the optimizer will just scan the index leaf pages to get the result, which can be much faster than table scan.
> Hope this help.
> Ken
-Russ
by ke.. » Tue, 10 Feb 1998 04:00:00
Yes absolutely.. as a rule of thumb.. you should avoid using SELECT * anyway.. because it force SQL server to return un-necessary data columns back to client application..
Ken
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