Re: Tid scan improvements

From: Andres Freund <andres(at)anarazel(dot)de>
To: Edmund Horner <ejrh00(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Tid scan improvements
Date: 2019-02-03 10:34:36
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On 2019-01-19 17:04:13 +1300, Edmund Horner wrote:
> On Sat, 19 Jan 2019 at 05:35, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > Edmund Horner <ejrh00(at)gmail(dot)com> writes:
> > > My patch uses the same path type and executor for all extractable
> > tidquals.
> >
> > > This worked pretty well, but I am finding it difficult to reimplement it
> > in
> > > the new tidpath.c code.
> >
> > I didn't like that approach to begin with, and would suggest that you go
> > over to using a separate path type and executor node. I don't think the
> > amount of commonality for the two cases is all that large, and doing it
> > as you had it required some ugly ad-hoc conventions about the semantics
> > of the tidquals list. Where I think this should go is that the tidquals
> > list still has OR semantics in the existing path type, but you use AND
> > semantics in the new path type, so that "ctid > ? AND ctid < ?" is just
> > represented as an implicit-AND list of two simple RestrictInfos.
> >
> Thanks for the advice. This approach resembles my first draft, which had a
> separate executor type. However, it did have a combined path type, with an
> enum TidPathMethod to determine how tidquals was interpreted. At this
> point, I think a different path type is clearer, though generation of both
> types can live in tidpath.c (just as indxpath.c generates different index
> path types).
> > Now admittedly, this wouldn't give us an efficient way to execute
> > queries with conditions like "WHERE ctid = X OR (ctid > Y AND ctid < Z)",
> > but I find myself quite unable to get excited about supporting that.
> > I see no reason for the new code to worry about any cases more complex
> > than one or two TID inequalities at top level of the restriction list.
> >
> I'm a bit sad to see support for multiple ranges go, though I never saw
> such queries as ever being particularly common. (And there was always a
> nagging feeling that tidpath.c was beginning to perform feats of boolean
> acrobatics out of proportion to its importance. Perhaps in some distant
> future, TID quals will become another way of supplying TIDs to a bitmap
> heap scan, which would enable complicated boolean queries using both
> indexes and TID scans. But that's just musing, not a proposal.)
> > In the query information given to the path generator, there is no existing
> > > RestrictInfo relating to the whole expression "ctid > ? AND ctid < ?". I
> > > am still learning about RestrictInfos, but my understanding is it doesn't
> > > make sense to have a RestrictInfo for an AND clause, anyway; you're
> > > supposed to have them for the sub-expressions of it.
> >
> > FWIW, the actual data structure for cases like that is that there's
> > a RestrictInfo for the whole clause ctid = X OR (ctid > Y AND ctid < Z),
> > and if you look into its "orclause" field, you will find RestrictInfos
> > attached to the primitive clauses ctid = X, ctid > Y, ctid < Z. (The
> > old code in tidpath.c didn't know that, because it'd never been rewritten
> > since RestrictInfos were invented.) However, I think this new code should
> > not worry about OR cases at all, but just pull out top-level TID
> > comparison clauses.
> >
> Thanks for the explanation.
> > And it doesn't seem a good idea to try to create new RestrictInfos in the
> > > path generation just to pass the tidquals back to plan creation.
> >
> > No, you should avoid that. There are places that assume there's only
> > one RestrictInfo for any given original clause (or sub-clause).

The commitfest has ended, and you've not updated the patch to address
the feedback yet. Are you planning to do so soon? Otherwise I think we
ought to mark the patch as returned with feedback?


Andres Freund

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