Re: Tid scan improvements

Hi all,

I have managed to split my changes into 4 patches:

v3-0001-Add-selectivity-and-nullness-estimates-for-the-ItemP.patch
v3-0002-Support-range-quals-in-Tid-Scan.patch
v3-0003-Support-backward-scans-over-restricted-ranges-in-hea.patch
v3-0004-Tid-Scan-results-are-ordered.patch

(1) is basically independent, and usefully improves estimates for ctid quals.
(2) is the main patch, adding basic range scan support to TidPath and TidScan.
(3) is a small change to properly support backward scans over a
restricted range in heapam.c, and is needed for (4).
(4) adds Backward Tid Scans, and adds path keys to Tid Paths so that
the planner doesn't have to add a sort for certain queries.

I have tried to apply David's suggestions.

In (1), I've included the offset part of a CTID constant in the
selectivity calculation. I've not included "allvisfrac" in the
calculation; I'm not sure it's worth it as it would only affect the
offset part.
I have tried to use iseq to differentiate between <=,>= versus <,>,
but I'm not sure I've got this right. I am also not entirely sure
it's worth it; the changes are already an improvement over the current
behaviour of using hardcoded selectivity constants.

In (2), the planner now picks up a greater variety of TID quals,
including AND-clauses with arbitrary children instead of the original
lower bound/upper bound pair. These are resolved in the executor into
a list of ranges to scan.

(3) is the same code, but I've added a couple of comments to explain the change.

(4) is basically the same pathkey/direction code as before (but as a
separate patch).

I hope the separation will make it easier to review. Item (2) is
still quite big, but a third of it is tests.

Cheers.
Edmund