From: "Sam Wong" <sam@hellosam.net>
To: "'Martin Kjeldsen'" <martin@martinkjeldsen.dk>,
	"'Marti Raudsepp'" <marti@juffo.org>
Cc: <pgsql-performance@postgresql.org>
References: <019101cbd498$7192ba70$54b82f50$@hellosam.net>
	<AANLkTi=ZUN+T3h+QT1ff1krbaxj62qVpQda4ECEjo1Ou@mail.gmail.com>
	<E520CD71-04E2-4B07-B28E-50255585EF8B@martinkjeldsen.dk>
In-Reply-To: <E520CD71-04E2-4B07-B28E-50255585EF8B@martinkjeldsen.dk>
Subject: Re: Perl Binding affects speed?
Date: Fri, 25 Feb 2011 21:24:16 +0800
Message-ID: <01e601cbd4ef$4eb702c0$ec250840$@hellosam.net>
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From: Martin Kjeldsen, Sent: 2011/2/25, 20:59 
> 
> On 25/02/2011, at 13.25, Marti Raudsepp wrote:
> 
> > On Fri, Feb 25, 2011 at 05:02, Sam Wong <sam@hellosam.net> wrote:
> >> * But if I do this - using binding:
> >> $dbh->selectall_arrayref("SELECT * from shipment_lookup WHERE
> >> (UPPER(lookup) LIKE ?)", undef, '0GURG5YGVQA9%'); It took 10 seconds
> >> to finish the query, just like it was using full table scan instead!
> >> Even though the 'explain' shows the same query plan.
> >
> > This is a pretty common shortcoming with placeholders. Since planning
> > of parameterized queries is done *before* binding parameters, the
> > planner has no knowledge of what the "?" placeholder actually is. Thus
> > it often gets the selectivity statistics wrong and produces worse
> > plans for your values.
> >
> > AFAIK the only workaround is to not use variable binding in these
> > cases, but escape and insert your variables straight it into the SQL
> > query.
> 
> Instead of not using the placeholder syntax you can use:
> 
> local $dbh->{pg_server_prepare} = 0;
> 
> which disables prepared queries serverside in the current scope and
> therefore doesn't have the late variable binding issue, but allows you to
> avoid SQL injection attacks.
> 

Thanks, I will look into that.

Sam