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Re: Slow deletes in 8.1 when FKs are involved

From: Will Reese <wreese(at)rackspace(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Slow deletes in 8.1 when FKs are involved
Date: 2006-04-27 02:13:18
Message-ID: 1D2B84DA-3FE5-4583-9414-5A7CD9D48E70@rackspace.com (view raw or flat)
Thread:
Lists: pgsql-performance
Stef:

There is already a post explaining the solution.  All the proper  
indexes were there, and it works great on 7.4.  The problem lies with  
leftover 7.4 RI triggers being carried over to an 8.1 database.  The  
solution is to drop the triggers and add the constraint.  Hopefully  
this will not cause as many locking issues with FKs on 8.1 as it did  
in 7.4 (which is why one of the RI triggers was removed in the first  
place).

Will Reese  -- http://blog.rezra.com

On Apr 26, 2006, at 6:43 PM, Stef T wrote:

>
> Hey there Will,
>     I would assume that, perhaps, jst perhaps, the FK doesn't have an
> index on the field on both sides, so, your seeing a potential  
> sequential
> scan happening. Can you fling up an explain anaylze for everyone  
> please
> ? Anything more will be merely shooting in the dark, and, tracer  
> bullets
> aside, I have heard that -that- can be dangerous ;p
>
>     Regards
>     Stef
>
> Will Reese wrote:
>> I'm preparing for an upgrade from PostgreSQL 7.4.5 to 8.1.3, and I
>> noticed a potential performance issue.
>>
>> I have two servers, a dual proc Dell with raid 5 running PostgreSQL
>> 7.4, and a quad proc Dell with a storage array running PostgreSQL  
>> 8.1.
>> Both servers have identical postgresql.conf settings and were  
>> restored
>> from the same 7.4 backup. Almost everything is faster on the 8.1
>> server (mostly due to hardware), except one thing...deletes from
>> tables with foreign keys.
>>
>> I have table A with around 100,000 rows, that has foreign keys to
>> around 50 other tables.  Some of these other tables (table B, for
>> example) have around 10 million rows.
>>
>> On the 7.4 server, I can delete a single row from a table A in well
>> under a second (as expected).  On the 8.1 server, it takes over a
>> minute to delete.  I tried all the usual stuff, recreating indexes,
>> vacuum analyzing, explain analyze.  Everything is identical between
>> the systems.  If I hit ctrl-c while the delete was running on 8.1, I
>> repeatedly got the following message...
>>
>> db=# delete from "A" where "ID" in ('6');
>> Cancel request sent
>> ERROR:  canceling statement due to user request
>> CONTEXT:  SQL statement "SELECT 1 FROM ONLY "public"."B" x WHERE
>> "A_ID" = $1 FOR SHARE OF x"
>>
>> It looks to me like the "SELECT ... FOR SHARE" functionality in  
>> 8.1 is
>> the culprit. Has anyone else run into this issue?
>>
>>
>> Will Reese -- http://blog.rezra.com
>>
>>
>>
>>
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