Deprecating postfix and factorial operators in PostgreSQL 13

Hackers,

Over in [1] we have been discussing the deprecation of postfix operators, with the general consensus that deprecation
warningsshould be included in this upcoming release and postfix operator support should be removed in PostgreSQL 14.
Sincenot all people who follow -hackers will necessarily have been following that thread, I am creating this new
thread,with a patch which adds the deprecation notices, for your consideration. 

The only postfix operator we ship in the standard catalogs is the factorial operator (!), which is deprecated by this
patch.

The standard catalogs also include a prefix factorial operator (!!) which has been deprecated since 2011.

The deprecation warnings included in this patch warn that postfix operator support, along with both postfix ! and
prefix!! factorial operators, will be removed in PostgreSQL 14. 

Some of the deprecation verbiage supplied by John Naylor, some by me:



[1]
https://www.postgresql.org/message-id/flat/20200519145449.GN13712%40tamriel.snowman.net#34294baed8757c8e078550b9ba80081d

—
Mark Dilger
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company