Question

I have a Postgres table with more than 8 million rows. Given the following two ways of doing the same query via DBD::Pg, I get wildly different results.

$q .= '%';

## query 1
my $sql = qq{
    SELECT a, b, c
    FROM t 
    WHERE Lower( a ) LIKE '$q'
};
my $sth1 = $dbh->prepare($sql);
$sth1->execute();

## query 2
my $sth2 = $dbh->prepare(qq{
    SELECT a, b, c
    FROM t  
    WHERE Lower( a ) LIKE ?
});
$sth2->execute($q);

query 2 is at least an order of magnitude slower than query 1… seems like it is not using the indexes, while query 1 is using the index.

Would love hear why.

Answer 1

With LIKE expressions, b-tree indexes can only be used if the search pattern is left-anchored, i.e. terminated with %. More details in the manual.
_{Thanks to @evil otto for the link. This link to the current version.}

Your first query provides this essential information at prepare time, so the query planner can use a matching index.

Your second query does not provide any information about the pattern at prepare time, so the query planner cannot use any indexes.