Re: Tid scan improvements - Mailing list pgsql-hackers

Thanks for the edits and fixing that pretty glaring copy-paste bug.

Regarding enable_tidscan, I couldn't decide whether we really need it,
and erred on the side of not adding yet another setting.

The current patch only creates a tid range path if there's at least
one ctid qual.  But during development of earlier patches I was a bit
concerned about the possibility of tid range scan being picked instead
of seq scan when the whole table is scanned, perhaps due to a tiny
discrepency in costing.  Both scans will scan the whole table, but seq
scan is preferred since it can be parallellised, synchronised with
other scans, and has a bit less overhead with tuple checking.  If a
future change creates tid range paths for more queries, for instance
for MIN/MAX(ctid) or ORDER BY ctid, then it might be more important to
have a separate setting for it.

On Wed, 17 Jul 2019 at 23:11, David Rowley <david.rowley@2ndquadrant.com> wrote:
>
> On Mon, 15 Jul 2019 at 17:54, Edmund Horner <ejrh00@gmail.com> wrote:
> > Summary of changes compared to last time:
> >   - I've added the additional "scan_setlimits" table AM method.  To
> > check whether it's implemented in the planner, I have added an
> > additional "has_scan_setlimits" flag to RelOptInfo.  It seems to work.
> >   - I've also changed nodeTidrangescan to not require anything from heapam now.
> >   - To simply the patch and avoid changing heapam, I've removed the
> > backward scan support (which was needed for FETCH LAST/PRIOR) and made
> > ExecSupportsBackwardScan return false for this plan type.
> >   - I've removed the vestigial passing of "direction" through
> > nodeTidrangescan.  If my understanding is correct, the direction
> > passed to TidRangeNext will always be forward.  But I did leave FETCH
> > LAST/PRIOR in the regression tests (after adding SCROLL to the
> > cursor).
>
> I spent some time today hacking at this.  I fixed a bug in how
> has_scan_setlimits set, rewrite a few comments and simplified some of
> the code.
>
> When I mentioned up-thread about the optional scan_setlimits table AM
> callback, I'd forgotten that you'd not have access to check that
> directly during planning. As you mention above, you've added
> RelOptInfo has_scan_setlimits so the planner knows if it can use TID
> Range scans or not. It would be nice to not have to add this flag, but
> that would require either:
>
> 1. Making scan_setlimits a non-optional callback function in table AM, or;
> 2. Allowing the planner to have access to the opened Relation.
>
> #2 is not for this patch, but there has been some talk about it. It
> was done for the executor last year in d73f4c74dd3.
>
> I wonder if Andres has any thoughts on #1?
>
> The other thing I was thinking about was if enable_tidscan should be
> in charge of TID Range scans too. I see you have it that way, but
> should we be adding enable_tidrangescan?  The docs claim that
> enable_tidscan: "Enables or disables the query planner's use of TID
> scan plan types.". Note: "types" is  plural.  Maybe we could call that
> fate and keep it the way the patch has it already.  Does anyone have
> another idea about that?
>
> I've attached a delta of the changes I made and also a complete v9 patch.
>
> --
>  David Rowley                   http://www.2ndQuadrant.com/
>  PostgreSQL Development, 24x7 Support, Training & Services