pgsql: tableam: Add helper for indexes to check if a correspondingtabl

tableam: Add helper for indexes to check if a corresponding table tuples exist.

This is, likely exclusively, useful to verify that conflicts detected
in a unique index are with live tuples, rather than dead ones.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/71bdc99d0d7c3b2d73fa04fb2ff80870ce1305f7

Modified Files
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src/backend/access/nbtree/nbtinsert.c | 17 ++++++++++-------
src/backend/access/nbtree/nbtsort.c   |  2 +-
src/backend/access/table/tableam.c    | 34 ++++++++++++++++++++++++++++++++++
src/include/access/tableam.h          | 25 +++++++++++++++++++------
4 files changed, 64 insertions(+), 14 deletions(-)