Push down time-related SQLValue functions to foreign server

Hi.

The attached patches allow pushing down
current_timestamp/localtimestamp/current_time/localtime and now() to
remote PostgreSQL server as locally computed parameters.
The idea is based on oracle_fdw behavior.

Examples.

\d test
Foreign table "public.test"
Column | Type | Collation | Nullable | Default |
FDW options
--------+--------------------------+-----------+----------+---------+-------------------
i | integer | | | |
(column_name 'i')
t | timestamp with time zone | | | |
(column_name 't')
Server: loopback
FDW options: (schema_name 'data', table_name 'test')

Prior the patch:

explain verbose select * from test where t=current_timestamp;
QUERY PLAN
---------------------------------------------------------------------
Foreign Scan on public.test (cost=100.00..188.12 rows=11 width=12)
Output: i, t
Filter: (test.t = CURRENT_TIMESTAMP)
Remote SQL: SELECT i, t FROM data.test

explain verbose update test set t=current_timestamp where t<now();
QUERY PLAN
----------------------------------------------------------------------------
Update on public.test (cost=100.00..154.47 rows=0 width=0)
Remote SQL: UPDATE data.test SET t = $2 WHERE ctid = $1
-> Foreign Scan on public.test (cost=100.00..154.47 rows=414
width=50)
Output: CURRENT_TIMESTAMP, ctid, test.*
Filter: (test.t < now())
Remote SQL: SELECT i, t, ctid FROM data.test FOR UPDATE

After patch:
explain verbose select * from test where t=current_timestamp;
QUERY PLAN
-------------------------------------------------------------------------------------
Foreign Scan on public.test (cost=100.00..144.35 rows=11 width=12)
Output: i, t
Remote SQL: SELECT i, t FROM data.test WHERE ((t = $1::timestamp with
time zone))

explain verbose update test set t=current_timestamp where t<now();
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------
Update on public.test (cost=100.00..137.93 rows=0 width=0)
-> Foreign Update on public.test (cost=100.00..137.93 rows=414
width=50)
Remote SQL: UPDATE data.test SET t = $1::timestamp with time
zone WHERE ((t < $1::timestamp with time zone))

--
Best regards,
Alexander Pyhalov,
Postgres Professional