pgsql: Optimize TransactionIdIsCurrentTransactionId().

Optimize TransactionIdIsCurrentTransactionId().

If the passed in xid is the current top transaction, we can do a fast
check and exit early.  This should work well for the current heap but
also works very well for proposed AMs that don't use a separate xid
for subtransactions.

Author: Ashwin Agrawal, based on a suggestion from Andres Freund
Reviewed-by: Thomas Munro
Discussion: https://postgr.es/m/CALfoeiv0k3hkEb3Oqk%3DziWqtyk2Jys1UOK5hwRBNeANT_yX%2Bng%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/695c5977c8bc115029a85dcc1821d7b0136b4e4c

Modified Files
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src/backend/access/transam/xact.c | 3 +++
1 file changed, 3 insertions(+)