Message-ID: <42B9BB20.7050109@student.kuleuven.ac.be>
Date: Wed, 22 Jun 2005 21:25:20 +0200
From: Kurt De Grave <Kurt.DeGrave@student.kuleuven.ac.be>
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To: pgsql-performance@postgresql.org
Subject: parameterized LIKE does not use index
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Hi,

Consider the where-clauses:

WHERE lower(col) LIKE 'abc';
WHERE lower(col) LIKE 'abc%';

these will both use a b-tree functional index in lower(col) if one exists.
The clause

WHERE lower(col) LIKE '%abc';

can't use the index as you would expect, because of the wildcard at the
front.  Thus, it has to do a seqscan, on what
in my case is a very large table.  But still that's not too bad, because I
expect an overwhelming amount of the simple cases, and only very few that
start with a percentage sign. Now, what's problematic is if I replace the
literal with a parameter, like this:

WHERE lower(col) LIKE ?

It seems that the parameterized query gets compiled once, and because the
parameter is not yet known, one cannot be sure it doesn't start with a
percentage sign.  Using the parameterized version causes ALL cases to use
a seqscan.

Of course, I could modify the application and send different SQL depending
on which case we're in or just constructing a query with a literal each
time, but is there a way to add a hint to the SQL that would cause the
query to be re-planned if it's a case that could use the index?  Or can I
convince the (Perl) driver to do so?


kurt.