Re: Perl Binding affects speed?

On 25/02/2011, at 13.25, Marti Raudsepp wrote:

> On Fri, Feb 25, 2011 at 05:02, Sam Wong <sam(at)hellosam(dot)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.

Regards,
Martin