Re: Planner, check if can use consider HASH for groupings (src/backend/optimizer/plan/planner.c)

Em seg., 21 de set. de 2020 às 13:37, Tomas Vondra <
tomas(dot)vondra(at)2ndquadrant(dot)com> escreveu:

> On Mon, Sep 21, 2020 at 08:32:35AM -0300, Ranier Vilela wrote:
> >Em dom., 20 de set. de 2020 às 21:10, Tomas Vondra <
> >tomas(dot)vondra(at)2ndquadrant(dot)com> escreveu:
> >
> >> On Sun, Sep 20, 2020 at 08:09:56PM -0300, Ranier Vilela wrote:
> >> >Em sex., 18 de set. de 2020 às 10:37, Tomas Vondra <
> >> >tomas(dot)vondra(at)2ndquadrant(dot)com> escreveu:
> >> >
> >> >> On Thu, Sep 17, 2020 at 06:31:12PM -0300, Ranier Vilela wrote:
> >> >> >Em qui., 17 de set. de 2020 às 17:09, Tomas Vondra <
> >> >> >tomas(dot)vondra(at)2ndquadrant(dot)com> escreveu:
> >> >> >
> >> >> >> On Thu, Sep 17, 2020 at 02:12:12PM -0300, Ranier Vilela wrote:
> >> >> >> >Hi,
> >> >> >> >
> >> >> >> >In case gd->any_hashable is FALSE, grouping_is_hashable is never
> >> >> called.
> >> >> >> >In this case, the planner could use HASH for groupings, but will
> >> never
> >> >> >> know.
> >> >> >> >
> >> >> >>
> >> >> >> The condition is pretty simple - if the query has grouping sets,
> >> look at
> >> >> >> grouping sets, otherwise look at groupClause. For this to be an
> issue
> >> >> >> the query would need to have both grouping sets and (independent)
> >> group
> >> >> >> clause - which AFAIK is not possible.
> >> >> >>
> >> >> >Uh?
> >> >> >(parse->groupClause != NIL) If different from NIL we have
> >> ((independent)
> >> >> >group clause), grouping_is_hashable should check?
> >> >> >(gd ? gd->any_hashable : grouping_is_hashable(parse->groupClause))))
> >> >> >If gd is not NIL and gd->any_hashtable is FALSE?
> >> >> >
> >> >>
> >> >> Sorry, I'm not quite sure I understand what this is meant to say :-(
> >> >>
> >> >> Anyway, (groupClause != NIL) does not mean the groupClause is somehow
> >> >> independent (of what?). Add some debugging to create_grouping_paths
> and
> >> >> you'll see that e.g. this query ends up with groupClause with 3
> items:
> >> >>
> >> >> select 1 from t group by grouping sets ((a), (b), (c));
> >> >>
> >> >> and so does this one:
> >> >>
> >> >> select 1 from t group by grouping sets ((a,c), (a,b));
> >> >>
> >> >> I'm no expert in grouping sets, but I'd bet this means we transform
> the
> >> >> grouping sets into a groupClause by extracting the keys. I haven't
> >> >> investigated why exactly we do this, but I'm sure there's a reason
> (e.g.
> >> >> it gives us SortGroupClause).
> >> >>
> >> >> You seem to believe a query can have both grouping sets and regular
> >> >> grouping at the same level - but how would such query look like?
> Surely
> >> >> you can't have two GROuP BY clauses. You can do
> >> >>
> >> >Translating into code:
> >> >gd is grouping sets and
> >> >parse->groupClause is regular grouping
> >> >and we cannot have both at the same time.
> >> >
> >>
> >> Have you verified the claim that we can't have both at the same time? As
> >> I already explained, we build groupClause from grouping sets. I suggest
> >> you do some debugging using the queries I used as examples, and you'll
> >> see the claim is not true.
> >>
> >I think we already agreed that we cannot have both at the same time.
> >
>
> No, we haven't. I think we've agreed that you can't have both a group by
> clause with grouping sets and then another group by clause with "plain"
> grouping. But even with grouping sets you'll have groupClause generated
> from the grouping sets. See preprocess_grouping_sets.
>
> >
> >>
> >> >
> >> >> select 1 from t group by a, grouping sets ((b), (c));
> >> >>
> >> >> which is however mostly equivalent to (AFAICS)
> >> >>
> >> >> select 1 from t group by grouping sets ((a,b), (a,c))
> >> >>
> >> >> so it's not like we have an independent groupClause either I think.
> >> >>
> >> >> The whole point of the original condition is - if there are grouping
> >> >> sets, check if at least one can be executed using hashing (i.e. all
> keys
> >> >> are hashable). Otherwise (without grouping sets) look at the
> grouping as
> >> >> a whole.
> >> >>
> >> >Or if gd is NULL check parse->groupClause.
> >> >
> >>
> >> Which is what I'm saying.
> >>
> >> >
> >> >> I don't see how your change improves this - if there are grouping
> sets,
> >> >> it's futile to look at the whole groupClause if at least one grouping
> >> >> set can't be hashed.
> >> >>
> >> >> But there's a simple way to disprove this - show us a case (table
> and a
> >> >> query) where your code correctly allows hashing while the current one
> >> >> does not.
> >> >
> >> >I'm not an expert in grouping either.
> >> >The question I have here is whether gd is populated and has gd->
> >> >any_hashable as FALSE,
> >> >Its mean no use checking parse-> groupClause, it's a waste of time, Ok.
> >> >
> >>
> >> I'm sorry, I don't follow your logic. Those are two separate cases. If
> >> we have grouping sets, we have to check if at least one can be hashed.
> >> If we don't have grouping sets, we have to check groupClause directly.
> >> Why would that be a waste of time is unclear to me.
> >>
> >This is clear to me.
> >The problem is how it was implemented in create_grouping_paths.
> >
>
> You haven't demonstrated what the problem is, though. Showing us a query
> that fails would make it very clear.
>
> >
> >>
> >> >
> >> >>
> >> >> >
> >> >> >> For hashing to be worth considering, at least one grouping set has
> >> to be
> >> >> >> hashable - otherwise it's pointless.
> >> >> >>
> >> >> >> Granted, you could have something like
> >> >> >>
> >> >> >> GROUP BY GROUPING SETS ((a), (b)), c
> >> >> >>
> >> >> >> which I think essentially says "add c to every grouping set" and
> that
> >> >> >> will be covered by the any_hashable check.
> >> >> >>
> >> >> >I am not going into the merit of whether or not it is worth hashing.
> >> >> >grouping_is_hashable as a last resort, must decide.
> >> >> >
> >> >>
> >> >> I don't know what this is supposed to say either. The whole point of
> >> >> this check is to simply skip construction of hash-based paths in
> cases
> >> >> when it's obviously futile (i.e. when some of the keys don't support
> >> >> hashing). We do this as early as possible, because the whole point
> is to
> >> >> save precious CPU cycles during query planning.
> >> >>
> >> >
> >> >> >
> >> >> >> >Apparently gd pointer, will never be NULL there, verified with
> >> >> Assert(gd
> >> >> >> !=
> >> >> >> >NULL).
> >> >> >> >
> >> >> >>
> >> >> >> Um, what? If you add the assert right before the if condition, you
> >> won't
> >> >> >> even be able to do initdb. It's pretty clear it'll crash for any
> >> query
> >> >> >> without grouping sets.
> >> >> >>
> >> >> >Here not:
> >> >> >Assert(gd != NULL);
> >> >> >create_ordinary_grouping_paths(root, input_rel, grouped_rel,
> >> >> > agg_costs, gd, &extra,
> >> >> > &partially_grouped_rel);
> >> >> >
> >> >>
> >> >> I have no idea where you're adding this assert. But simply adding it
> to
> >> >> create_grouping_paths (right before the if condition changed by your
> >> >> patch) will trigger a failure during initdb. Simply because for
> queries
> >> >> without grouping sets (and with regular grouping) we pass gs=NULL.
> >> >>
> >> >> Try applying the attached patch and do "pg_ctl -D ... init" - you'll
> get
> >> >> a failure proving that gd=NULL.
> >> >>
> >> >Well, I did a test right now, downloaded the latest HEAD and added the
> >> >Assertive.
> >> >I compiled everything, ran the regression tests, installed the
> binaries,
> >> >loaded the server and installed a client database.
> >> >Everything is working.
> >> >Maybe in all these steps, there is no grouping that would trigger the
> code
> >> >in question, but I doubt it.
> >> >
> >>
> >> For me doing this leads to reliable crashes when pg_regress does initdb
> >> (so before the actual checks):
> >>
> >> patch -p1 < ~/grouping-assert.patch
> >> ./configure --enable-debug --enable-cassert
> >> make -s clean && make -s -j4 && make check
> >>
> >> And the "make check" it immediately crashes like this:
> >>
> >> ============== creating temporary instance ==============
> >> ============== initializing database system ==============
> >>
> >> pg_regress: initdb failed
> >> Examine /home/user/work/postgres/src/test/regress/log/initdb.log for
> >> the reason.
> >>
> >> on the assert. So yeah, I'd guess there's something wrong with your
> >> build. What does pg_config say?
> >>
> >Default.
> >I never change anything. I simply clone the last head:
> >git clone --single-branch https://github.com/postgres/postgres/
> >and have it compiled.
> >
>
> Compiled how?
>
build Debug

regards,
Ranier Vilela