pgsql: tableam: Perform CheckXidAlive check once per scan

tableam: Perform CheckXidAlive check once per scan

Previously, the CheckXidAlive check was performed within the table_scan*next*
functions. This caused the check to be executed for every fetched tuple, an
unnecessary overhead.

To fix, move the check to table_beginscan* so it is performed once per scan
rather than once per row.

Note: table_tuple_fetch_row_version() does not use a scan descriptor;
therefore, the CheckXidAlive check is retained in that function. The overhead
is unlikely to be relevant for the existing callers.

Reported-by: Andres Freund <andres(at)anarazel(dot)de>
Author: Dilip Kumar <dilipbalaut(at)gmail(dot)com>
Suggested-by: Andres Freund <andres(at)anarazel(dot)de>
Suggested-by: Amit Kapila <akapila(at)postgresql(dot)org>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://www.postgresql.org/message-id/tlpltqm5jjwj7mp66dtebwwhppe4ri36vdypux2zoczrc2i3mp%40dhv4v4nikyfg

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/87f7b824f20c1c06884ef0711b4d32dbf4461436

Modified Files
--------------
src/backend/access/heap/heapam.c | 10 -----
src/backend/access/index/genam.c | 30 +++++++-------
src/backend/access/table/tableam.c | 20 +++-------
src/include/access/tableam.h | 82 +++++++++++++++++---------------------
4 files changed, 57 insertions(+), 85 deletions(-)