Re: partitioning - changing a slot's descriptor is expensive

From: Andres Freund <andres(at)anarazel(dot)de>
To: Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com>
Cc: Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, Amit Langote <amitlangote09(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Robert Haas <robertmhaas(at)gmail(dot)com>
Subject: Re: partitioning - changing a slot's descriptor is expensive
Date: 2018-06-29 05:59:36
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On 2018-06-29 11:20:53 +0530, Amit Khandekar wrote:
> On 27 June 2018 at 18:33, Ashutosh Bapat
> <ashutosh(dot)bapat(at)enterprisedb(dot)com> wrote:
> > On Wed, Jun 27, 2018 at 10:39 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >> Unfortunately calling ExecSetSlotDescriptor() is far from cheap, it has
> >> to reallocate the values/isnull arrays (and potentially do desc pinning
> >> etc).
> >
> > I bumped into this code yesterday while looking at ExecFetchSlotTuple
> > and ExecMaterializeSlot usages. I was wondering the same.
> >
> > ExecSetSlotDescriptor() always frees tts_values and tts_isnull array
> > even if the tuple descriptor being set has same number of attributes
> > as previous one. Most of the times that will be the case. I think we
> > should optimize that case.
> +1

This doesn't strike me as a great optimization. Any place where change
descriptors with any regularity, we're doing something wrong or at least
decidedly suboptimal. We shouldn't react to that by optimizing the wrong
thing, we should do the wrong thing less often.

> >> I think it'd be good to rewrite the code so there's an input and an
> >> output slot that each will keep their slot descriptors set.
> >
> > +1 for that.
> >
> > But I am worried that the code will have to create thousand slots if
> > there are thousand partitions. I think we will need to see how much
> > effect that has.
> I agree that it does not make sense to create as many slots, if at all
> we go by this approach. Suppose the partitioned table is the only one
> having different tuple descriptor, and rest of the partitions have
> same tuple descriptor. In that case, keep track of unique descriptors,
> and allocate a slot per unique descriptor.

Why? Compared to the rest of the structures created, a slot is not
particularly expensive? I don't see what you're optimizing here.


Andres Freund

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