Re: cost_sort() improvements

From: Teodor Sigaev <teodor(at)sigaev(dot)ru>
To: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>
Subject: Re: cost_sort() improvements
Date: 2018-07-12 15:00:27
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> One more thought about estimating the worst case - I wonder if simply
> multiplying the per-tuple cost by 1.5 is the right approach. It does not
> seem particularly principled, and it's trivial simple to construct
> counter-examples defeating it (imagine columns with 99% of the rows
> having the same value, and then many values in exactly one row - that
> will defeat the ndistinct-based group size estimates).

Agree. But right now that is best what we have. and constant 1.5 easily could be
changed to 1 or 10 - it is just arbitrary value, intuitively chosen. As I
mentioned in v7 patch description, new estimation function provides ~10% bigger
estimation and this constant should not be very large, because we could easily
get overestimation.

> The reason why constructing such counter-examples is so simple is that
> this relies entirely on the ndistinct stats, ignoring the other stats we
> already have. I think this might leverage the per-column MCV lists (and
> eventually multi-column MCV lists, if that ever gets committed).
> We're estimating the number of tuples in group (after fixing values in
> the preceding columns), because that's what determines the number of
> comparisons we need to make at a given stage. The patch does this by
> estimating the average group size, by estimating ndistinct of the
> preceding columns G(i-1), and computing ntuples/G(i-1).
> But consider that we also have MCV lists for each column - if there is a
> very common value, it should be there. So let's say M(i) is a frequency
> of the most common value in i-th column, determined either from the MCV
> list or as 1/ndistinct for that column. Then if m(i) is minimum of M(i)
> for the fist i columns, then the largest possible group of values when
> comparing values in i-th column is
> N * m(i-1)
> This seems like a better way to estimate the worst case. I don't know if
> this should be used instead of NG(i), or if those two estimates should
> be combined in some way.
Agree, definitely we need an estimation of larger group size to use it in
cost_sort. But I don't feel power to do that, pls, could you attack a computing
group size issue? Thank you.

> I think estimating the largest group we need to sort should be helpful
> for the incremental sort patch, so I'm adding Alexander to this thread.Agree
I think so. suggested estimation algorithm should be modified to support leading
preordered keys and this form could be directly used in GROUP BY and incremental
sort patches.
Teodor Sigaev E-mail: teodor(at)sigaev(dot)ru

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