Re: Tid scan improvements

On Mon, 15 Jul 2019 at 17:54, Edmund Horner <ejrh00(at)gmail(dot)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/
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