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
[download]

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
[download]

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);
[download]

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

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