pgsql: Cache the result of converting now() to a struct pg_tm.

Cache the result of converting now() to a struct pg_tm.

SQL operations such as CURRENT_DATE, CURRENT_TIME, LOCALTIME, and
conversion of "now" in a datetime input string have to obtain the
transaction start timestamp ("now()") as a broken-down struct pg_tm.
This is a remarkably expensive conversion, and since now() does not
change intra-transaction, it doesn't really need to be done more than
once per transaction. Introducing a simple cache provides visible
speedups in queries that compute these values many times, for example
insertion of many rows that use a default value of CURRENT_DATE.

Peter Smith, with a bit of kibitzing by me

Discussion: https://postgr.es/m/CAHut+Pu89TWjq530V2gY5O6SWi=OEJMQ_VHMt8bdZB_9JFna5A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0a87ddff5c83589e90de236bd55e6a19b017fe9a

Modified Files
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src/backend/utils/adt/date.c | 49 ++++++++++++++---------------
src/backend/utils/adt/datetime.c | 67 +++++++++++++++++++++++++++++++++-------
2 files changed, 80 insertions(+), 36 deletions(-)