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Re: Query-plan for partitioned UPDATE/DELETE slow and swaps vmem compared to SELECT

From: John Papandriopoulos <dr(dot)jpap(at)gmail(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Query-plan for partitioned UPDATE/DELETE slow and swaps vmem compared to SELECT
Date: 2010-12-05 11:06:39
Message-ID: idfrnv$2bc$1@news.hub.org (view raw or flat)
Thread:
Lists: pgsql-performance
On 12/4/10 2:40 PM, Tom Lane wrote:
> [ pokes at that for a bit ... ]  Ah, I had forgotten that UPDATE/DELETE
> go through inheritance_planner() while SELECT doesn't.  And
> inheritance_planner() makes a copy of the querytree, including the
> already-expanded range table, for each target relation.  So the memory
> usage is O(N^2) in the number of child tables.

Thanks for the pointer to the code and explanation.

In inheritance_planner(...) I see the memcpy of the input query tree, but for my example constraint exclusion would only result in one child being included.  Or is the O(N^2) memory usage from elsewhere?

> It's difficult to do much better than that in the general case where the
> children might have different rowtypes from the parent: you need a
> distinct targetlist for each target relation.  I expect that we can be a
> lot smarter when we have true partitioning support (which among other
> things is going to have to enforce that all the children have identical
> column sets).

Is this the same as saying that the inheritance_planner(...) can be avoided if it were known that the children have the same rowtype as the parent?  Is it easy to check?

> But the inheritance mechanism was never intended to scale to anything like
> this number of children.

Unfortunately so. :(

When I push the number of child tables up to 10k, the SELECT planning starts to slow down (~1 sec), though no swapping.
 
> I remain of the opinion that you're using far too many child tables.
> Please note the statement at the bottom of
> http://www.postgresql.org/docs/9.0/static/ddl-partitioning.html:
> 
> 	Partitioning using these techniques will work well with up to
> 	perhaps a hundred partitions; don't try to use many thousands of
> 	partitions.

Thanks for the reference---I'm well aware of it, but it was not clear to me why: the reason I was structuring my partition inheritance as a tree, because I thought it was simply a case of time-to-scan the CHECK constraints at any level in the inheritance hierarchy.  You've been a great help in helping my understanding PostgreSQL inheritance.

Best,
John

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Subject: Re: Query-plan for partitioned UPDATE/DELETE slow and swaps vmem compared to SELECT
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