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This is actually a common case with columns like "status" on task-tracking or ticket-tracking systems.
I wanted to see how Sybase handles this case, so I did the following little test with the text_events table, which is used by Sybase to track what rows in source tables need to be updated in a Verity Full-Text index.
select count(*), event_status from text_events group by event_status; event_status ----------- ------------ 26 0 2352103 2 81921 3
Rows with event_status == 0 have to be processed, and there's an index on event_status.

I first ran a SQL snippet like this:

declare @status int select @status = 0 select * from text_events where status = @status
And this did indeed do a table scan.

Then I tried a short DBI script, using placeholders:

my $sth = $dbh->prepare("select * from payment_db..text_events where e +vent_status = ?"); $sth->execute(1); fetch_all($sth); $sth->execute(0); fetch_all($sth);
In this case the index is used every time. Then I wrote an ad-hoc stored procedure to do the same thing, and I get a table scan. Which just goes to show that this sort of thing is a bit of a black art :-)

Michael


In reply to Re: Re: Re: Re: Use Placeholders. For SECURITY and (sometimes) for PERFORMANCE by mpeppler
in thread Use Placeholders. For SECURITY and (sometimes) for PERFORMANCE by jZed

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