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Re: Poor plan choice in prepared statement

From: Scott Carey <scott(at)richrelevance(dot)com>
To: bricklen <bricklen(at)gmail(dot)com>, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
Cc: Merlin Moncure <mmoncure(at)gmail(dot)com>, "pgsql-performance(at)postgresql(dot)org"<pgsql-performance(at)postgresql(dot)org>
Subject: Re: Poor plan choice in prepared statement
Date: 2008-12-30 21:09:24
Message-ID: BDFBB77C9E07BE4A984DAAE981D19F961ACD5F388E@EXVMBX018-1.exch018.msoutlookonline.net (view raw or flat)
Thread:
Lists: pgsql-performance
There is no way to force Postgres to re-plan a prepared statement.  In many cases, this would be a hugely beneficial feature (perhaps part of the definition of the statement?).

I have had similar issues, and had to code the application to prevent SQL injection (Postgres $ quotes and other stuff is helpful, but not always adequate or easy).  With the current state of things, you'll have to often do your SQL injection defense in your application due to this weakness in prepared statements.

I have also had a case where one query would take a  couple hundred ms to parse, but was fairly fast to plan and execute (1/3 the parse cost) -- yet another case where a prepared statement that re-plans each execution would be helpful.  At least you can prevent SQL injection and cut the parse cost.  Its not all about the cost of planning the query.

________________________________________
From: pgsql-performance-owner(at)postgresql(dot)org [pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of bricklen [bricklen(at)gmail(dot)com]
Sent: Tuesday, December 30, 2008 12:14 PM
To: Scott Marlowe
Cc: Merlin Moncure; pgsql-performance(at)postgresql(dot)org
Subject: Re: [PERFORM] Poor plan choice in prepared statement

Hi Scott,

On Tue, Dec 30, 2008 at 12:09 PM, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com> wrote:
> On Tue, Dec 30, 2008 at 12:42 PM, Merlin Moncure <mmoncure(at)gmail(dot)com> wrote:
>> On Tue, Dec 30, 2008 at 1:59 PM, bricklen <bricklen(at)gmail(dot)com> wrote:
>>> Hi, I am re-posting my question here after trying to find a solution
>>> in the PHP pgsql list with no luck.
>>>
>>> I am experiencing some performance issues that I think are stemming
>>> from prepared statements. I have a pretty simple query:
>>> -- bad plan, from prepared statement
>>> --
>>> dev=# prepare fooplan (date,date,int,int) as
>>> dev-# SELECT cl.idOffer AS campaign, cl.idAffiliate AS affiliate,
>>> cl.idCreative AS creative, cl.subid, cl.datetime
>>> dev-# FROM click AS cl LEFT JOIN conversion AS co ON cl.clickGenerated
>>> = co.clickGenerated
>>> dev-# WHERE cl."date" >= $1
>>> dev-# AND cl."date" <= $2
>>> dev-# AND cl.idAffiliate = $3
>>> dev-# LIMIT $4;
>>
>> Your problem is that the query as written is hard to plan.  The
>> database has no idea what you pass in, it has to guess.  (IMO, It
>> almost always guesses wrong...I think it should assume 1 row
>> returned).  Also, the db has no idea what you want to pass in at plan
>> time for date.
>
> One of the things you can try here is to build your query then execute
> it so it has to be planned each time.
>

Yeah, I've tested that in the application itself and it worked
correctly. I am trying to discover a way to use bind variables in PHP
without using the prepare function (to block sql injection), or if I
must use the prepare function, then force it to replan each time
somehow. That's part of where I'm stuck (and I'm no php guy).

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