pgsql: Make subquery aliases optional in the FROM clause.

Make subquery aliases optional in the FROM clause.

This allows aliases for sub-SELECTs and VALUES clauses in the FROM
clause to be omitted.

This is an extension of the SQL standard, supported by some other
database systems, and so eases the transition from such systems, as
well as removing the minor inconvenience caused by requiring these
aliases.

Patch by me, reviewed by Tom Lane.

Discussion: https://postgr.es/m/CAEZATCUCGCf82=hxd9N5n6xGHPyYpQnxW8HneeH+uP7yNALkWA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bcedd8f5fce0b69970cf0cee7bca560833d05869

Modified Files
--------------
doc/src/sgml/ref/select.sgml | 16 ++++++--
src/backend/parser/analyze.c | 19 ++++++---
src/backend/parser/gram.y | 44 ---------------------
src/backend/parser/parse_clause.c | 18 +++------
src/backend/parser/parse_relation.c | 29 ++++++++++----
src/backend/utils/adt/ruleutils.c | 6 ++-
src/include/parser/parse_node.h | 7 +++-
src/interfaces/ecpg/preproc/ecpg.addons | 6 ---
src/test/regress/expected/subselect.out | 69 +++++++++++++++++++++++++++++++++
src/test/regress/sql/subselect.sql | 31 +++++++++++++++
10 files changed, 164 insertions(+), 81 deletions(-)