pgsql: Improve planner's selectivity estimates for inequalities onCTID

Improve planner's selectivity estimates for inequalities on CTID.

We were getting just DEFAULT_INEQ_SEL for comparisons such as
"ctid >= constant", but it's possible to do a lot better if we don't
mind some assumptions about the table's tuple density being reasonably
uniform.  There are already assumptions much like that elsewhere in
the planner, so that hardly seems like much of an objection.

Extracted from a patch set that also proposes to introduce a special
executor node type for such queries.  Not sure if that's going to make
it into v12, but improving the selectivity estimate is useful
independently of that.

Edmund Horner, reviewed by David Rowley

Discussion: https://postgr.es/m/CAMyN-kB-nFTkF=VA_JPwFNo08S0d-Yk0F741S2B7LDmYAi8eyA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f7111f72d2fd31979c6d473cdde56ba8e7aba26a

Modified Files
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src/backend/utils/adt/selfuncs.c | 75 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 75 insertions(+)