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Re: Bad query optimizer misestimation because of TOAST

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Markus Schaber <schabios(at)logi-track(dot)com>
Cc: PostgreSQL Performance List <pgsql-performance(at)postgresql(dot)org>,PostGIS Users Discussion <postgis-users(at)postgis(dot)refractions(dot)net>
Subject: Re: Bad query optimizer misestimation because of TOAST
Date: 2005-02-02 23:49:13
Message-ID: 9501.1107388153@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Markus Schaber <schabios(at)logi-track(dot)com> writes:
> Tom Lane schrieb:
>> I don't buy this analysis at all.  The toasted columns are not those in
>> the index (because we don't support out-of-line-toasted index entries),
>> so a WHERE clause that only touches indexed columns isn't going to need
>> to fetch anything from the toast table.

> The column is a PostGIS column, and the index was created using GIST.
> Those are lossy indices that do not store the whole geometry, but only
> the bounding box  corners of the Geometry (2 Points).
> Without using the index, the && Operator (which tests for bbox
> overlapping) has to load the whole geometry from disk, and extract the
> bbox therein (as it cannot make use of partial fetch).

Ah, I see; I forgot to consider the GIST "storage" option, which allows
the index contents to be something different from the represented column.
Hmm ...

What I would be inclined to do is to extend ANALYZE to make an estimate
of the extent of toasting of every toastable column, and then modify
cost_qual_eval to charge a nonzero cost for evaluation of Vars that are
potentially toasted.

This implies an initdb-forcing change in pg_statistic, which might or
might not be allowed for 8.1 ... we are still a bit up in the air on
what our release policy will be for 8.1.

My first thought about what stat ANALYZE ought to collect is "average
number of out-of-line TOAST chunks per value".  Armed with that number
and size information about the TOAST table, it'd be relatively simple
for costsize.c to estimate the average cost of fetching such values.

I'm not sure if it's worth trying to model the cost of decompression of
compressed values.  Surely that's a lot cheaper than fetching
out-of-line values, so maybe we can just ignore it.  If we did want to
model it then we'd also need to make ANALYZE note the fraction of values
that require decompression, and maybe something about their sizes.

This approach would overcharge for operations that are able to work with
partially fetched values, but it's probably not reasonable to expect the
planner to account for that with any accuracy.

Given this we'd have a pretty accurate computation of the true cost of
the seqscan alternative, but what of indexscans?  The current
implementation charges one evaluation of the index qual(s) per
indexscan, which is not really right because actually the index
component is never evaluated at all.  This didn't matter when the index
component was a Var with zero eval cost, but if we're charging some eval
cost it might.  But ... since it's charging only one eval per scan
... the error is probably down in the noise in practice, and it may not
be worth trying to get it exactly right.

A bigger concern is "what about lossy indexes"?  We currently ignore the
costs of rechecking qual expressions for fetched rows, but this might be
too inaccurate for situations like yours.  I'm hesitant to mess with it
though.  For one thing, to get it right we'd need to understand how many
rows will be returned by the raw index search (which is the number of
times we'd need to recheck).  At the moment the only info we have is the
number that will pass the recheck, which could be a lot less ... and of
course, even that is probably a really crude estimate when we are
dealing with this sort of operator.

Seems like a bit of a can of worms ...

			regards, tom lane

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