amcheck: Optimize speed of checking for unique constraint violation
Currently, when amcheck validates a unique constraint, it visits the heap for
each index tuple. This commit implements skipping keys, which have only one
non-dedeuplicated index tuple (quite common case for unique indexes). That
gives substantial economy on index checking time.
Reported-by: Noah Misch
Discussion: https://postgr.es/m/
20240325020323.fd.nmisch%40google.com
Author: Alexander Korotkov, Pavel Borisov