Re: Tid scan improvements

Hi,

On 2019-01-19 17:04:13 +1300, Edmund Horner wrote:
> On Sat, 19 Jan 2019 at 05:35, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> > Edmund Horner <ejrh00@gmail.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?

Greetings,

Andres Freund