pgsql: Support for FF1-FF6 datetime format patterns

Support for FF1-FF6 datetime format patterns

SQL Standard 2016 defines FF1-FF9 format patters for fractions of seconds in
jsonpath .datetime() method and CAST (... FORMAT ...) SQL clause.  Parsing
engine of upcoming .datetime() method will be shared with to_date()/
to_timestamp().

This patch implements FF1-FF6 format patterns for upcoming jsonpath .datetime()
method.  to_date()/to_timestamp() functions will also get support of this
format patterns as positive side effect.  FF7-FF9 are not supported due to
lack of precision in our internal timestamp representation.

Extracted from original patch by Nikita Glukhov, Teodor Sigaev, Oleg Bartunov.
Heavily revised by me.

Discussion: https://postgr.es/m/fcc6fc6a-b497-f39a-923d-aa34d0c588e8%402ndQuadrant.com
Discussion: https://postgr.es/m/CAPpHfdsZgYEra_PeCLGNoXOWYx6iU-S3wF8aX0ObQUcZU%2B4XTw%40mail.gmail.com
Author: Nikita Glukhov, Teodor Sigaev, Oleg Bartunov, Alexander Korotkov
Reviewed-by: Anastasia Lubennikova, Peter Eisentraut

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/d589f94460c24d9b7ac21887d031818d6e3f354d

Modified Files
--------------
doc/src/sgml/func.sgml                    | 24 ++++++++
src/backend/utils/adt/formatting.c        | 99 ++++++++++++++++++++++++-------
src/backend/utils/adt/timestamp.c         |  3 +-
src/include/utils/datetime.h              |  2 +
src/test/regress/expected/horology.out    | 79 ++++++++++++++++++++++++
src/test/regress/expected/timestamp.out   | 15 +++++
src/test/regress/expected/timestamptz.out | 15 +++++
src/test/regress/sql/horology.sql         |  9 +++
src/test/regress/sql/timestamp.sql        |  8 +++
src/test/regress/sql/timestamptz.sql      |  8 +++
10 files changed, 239 insertions(+), 23 deletions(-)