pgsql: Add SETTINGS option to EXPLAIN, to print modified settings.

Add SETTINGS option to EXPLAIN, to print modified settings.

Query planning is affected by a number of configuration options, and it
may be crucial to know which of those options were set to non-default
values. With this patch you can say EXPLAIN (SETTINGS ON) to include
that information in the query plan. Only options affecting planning,
with values different from the built-in default are printed.

This patch also adds auto_explain.log_settings option, providing the
same capability in auto_explain module.

Author: Tomas Vondra
Reviewed-by: Rafia Sabih, John Naylor
Discussion: https://postgr.es/m/e1791b4c-df9c-be02-edc5-7c8874944be0@2ndquadrant.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ea569d64ac7174d3fe657e3e682d11053ecf1866

Modified Files
--------------
contrib/auto_explain/auto_explain.c | 13 ++
doc/src/sgml/auto-explain.sgml | 18 +++
doc/src/sgml/ref/explain.sgml | 12 ++
src/backend/commands/explain.c | 76 +++++++++++
src/backend/utils/misc/guc.c | 262 +++++++++++++++++++++++++++++-------
src/include/commands/explain.h | 1 +
src/include/utils/guc.h | 2 +
src/include/utils/guc_tables.h | 1 +
8 files changed, 333 insertions(+), 52 deletions(-)