Re: [HACKERS] PATCH: multivariate histograms and MCV lists

On 27 March 2018 at 01:36, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> 4) handling of NOT clauses in MCV lists (and in histograms)
>
> The query you posted does not fail anymore...
>

Ah, it turns out the previous query wasn't actually failing for the
reason I thought it was -- it was failing because it had a
ScalarArrayOpExpr that was being passed to
mcv_clauselist_selectivity() because of the wrong list being passed to
it. I could see from the code that a NOT clause would have tripped it
up, but most NOT clauses actually get rewritten by negate_clause() so
they end up not being NOT clauses.

One way to get a NOT clause, is with a boolean column, and this
reveals another couple of problems:

drop table if exists foo;
create table foo(a int, b boolean);
insert into foo values(1,true);
insert into foo values(1,true);
insert into foo values(1,false);
create statistics foo_mcv_ab (mcv) on a,b from foo;
analyse foo;

select * from foo where a=1 and b;
ERROR: unknown clause type: 99

This fails because the clause is now a Var, which
statext_is_compatible_clause() lets through, but
mcv_clauselist_selectivity() doesn't support. So it's important to
keep those 2 functions in sync, and it might be worth having comments
in each to emphasise that.

And, if a NOT clause is used:

select * from foo where a=1 and not b;
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.

This is an Assert failure in mcv_update_match_bitmap()'s BoolExpr
handling block:

Assert(bool_clauses != NIL);
Assert(list_length(bool_clauses) >= 2);

The first of those Asserts is actually redundant with the second, but
the second fails because a NOT clause always only has one argument.

Regards,
Dean