pgsql: Increase hash_mem_multiplier default to 2.0.

Increase hash_mem_multiplier default to 2.0.

Double the default setting for hash_mem_multiplier, from 1.0 to 2.0.
This setting makes hash-based executor nodes use twice the usual
work_mem limit.

The PostgreSQL 15 release notes should have a compatibility note about
this change.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-Wzndc_ROk6CY-bC6p9O53q974Y0Ey4WX8jcPbuTZYM4Q3A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8f388f6f554b113f25a53fe3237238d2c58ed1eb

Modified Files
--------------
doc/src/sgml/config.sgml                      |  7 +++----
src/backend/utils/init/globals.c              |  2 +-
src/backend/utils/misc/guc.c                  |  2 +-
src/backend/utils/misc/postgresql.conf.sample |  2 +-
src/test/regress/expected/groupingsets.out    |  2 ++
src/test/regress/expected/join_hash.out       | 18 ++++++++++++++++++
src/test/regress/expected/memoize.out         |  4 +++-
src/test/regress/sql/groupingsets.sql         |  2 ++
src/test/regress/sql/join_hash.sql            | 18 ++++++++++++++++++
src/test/regress/sql/memoize.sql              |  4 +++-
10 files changed, 52 insertions(+), 9 deletions(-)