Re: multivariate statistics (v25)

On 04/04/2017 09:55 AM, David Rowley wrote:
> On 1 April 2017 at 04:25, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> wrote:
>> I've attached an updated patch.
>
> I've made another pass at this and ended up removing the tryextstats
> variable. We now only try to use extended statistics when
> clauselist_selectivity() is given a valid RelOptInfo with rtekind ==
> RTE_RELATION, and of course, it must also have some extended stats
> defined too.
>
> I've also cleaned up a few more comments, many of which I managed to
> omit updating when I refactored how the selectivity estimates ties
> into clauselist_selectivity()
>
> I'm quite happy with all of this now, and would also be happy for
> other people to take a look and comment.
>
> As a reviewer, I'd be marking this ready for committer, but I've moved
> a little way from just reviewing this now, having spent two weeks
> hacking at it.
>
> The latest patch is attached.
>

Thanks David, I agree the reworked patch is much cleaner that the last
version I posted. Thanks for spending your time on it.

Two minor comments:

1) DEPENDENCY_MIN_GROUP_SIZE

I'm not sure we still need the min_group_size, when evaluating
dependencies. It was meant to deal with 'noisy' data, but I think it
after switching to the 'degree' it might actually be a bad idea.

Consider this:

create table t (a int, b int);
insert into t select 1, 1 from generate_series(1, 10000) s(i);
insert into t select i, i from generate_series(2, 20000) s(i);
create statistics s with (dependencies) on (a,b) from t;
analyze t;

select stadependencies from pg_statistic_ext ;
stadependencies
--------------------------------------------
[{1 => 2 : 0.333344}, {2 => 1 : 0.333344}]
(1 row)

So the degree of the dependency is just ~0.333 although it's obviously a
perfect dependency, i.e. a knowledge of 'a' determines 'b'. The reason
is that we discard 2/3 of rows, because those groups are only a single
row each, except for the one large group (1/3 of rows).

Without the mininum group size limitation, the dependencies are:

test=# select stadependencies from pg_statistic_ext ;
stadependencies
--------------------------------------------
[{1 => 2 : 1.000000}, {2 => 1 : 1.000000}]
(1 row)

which seems way more reasonable, I think.

2) A minor detail is that instead of this

if (estimatedclauses != NULL &&
bms_is_member(listidx, estimatedclauses))
continue;

perhaps we should do just this:

if (bms_is_member(listidx, estimatedclauses))
continue;

bms_is_member does the same NULL check right at the beginning, so I
don't think this might make a measurable difference.

kind regards

--
Tomas Vondra http://www.2ndQuadrant.com
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