pgsql: Make the name optional in CREATE STATISTICS.

Make the name optional in CREATE STATISTICS.

This allows users to omit the statistics name in a CREATE STATISTICS
command, letting the system auto-generate a sensible, unique name,
putting the statistics object in the same schema as the table.

Simon Riggs, reviewed by Matthias van de Meent.

Discussion: https://postgr.es/m/CANbhV-FGD2d_C3zFTfT2aRfX_TaPSgOeKES58RLZx5XzQp5NhA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/624aa2a13bd02dd584bb0995c883b5b93b2152df

Modified Files
--------------
doc/src/sgml/ref/create_statistics.sgml | 12 +++--
src/backend/commands/statscmds.c        |  7 ++-
src/backend/parser/gram.y               | 13 +++--
src/test/regress/expected/stats_ext.out | 87 +++++++++++++++++++--------------
src/test/regress/sql/stats_ext.sql      | 20 +++++---
5 files changed, 86 insertions(+), 53 deletions(-)