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Re: Where the advice to use DBI bind parameters can go wrong (long)

by mpeppler (Vicar)
on Jun 14, 2002 at 22:58 UTC ( #174741=note: print w/ replies, xml ) Need Help??


in reply to Where the advice to use DBI bind parameters can go wrong (long)

Interesting observation.

For Sybase the optimizer determines index usage when the query is run the first time. So if the first request only has a trailing '%' and the column has an index it will be used. A subsequent query with a leading '%' won't be incorrect - the optimizer detects the leading '%' and while it uses the index it will look at all the values for a match.

However, if the first query has a leading '%' then you will get a table scan each time.

I ran the following:

my $sth = $dbh->prepare("select * from market where commodity like ?") +; $sth->execute("SPM%"); while(my $d = $sth->fetch) { ; } $sth->execute("%XXX%"); while(my $d = $sth->fetch) { ; }
The showplan output shows that the index is used, but the stats IO shows that the second execute() call does a lot more work:
Table: market scan count 1, logical reads: (regular=111 apf=0 total=11 +1), physical reads: (regular=0 apf=0 total=0), apf IOs used=0
vs.
Table: market scan count 1, logical reads: (regular=51889 apf=0 total= +51889), physical reads: (regular=0 apf=0 total=0), apf IOs used=0

FWIW :-)

Michael


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