pgsql: Allow extensions to mark an individual index as disabled.

Allow extensions to mark an individual index as disabled.

Up until now, the only way for a loadable module to disable the use of a
particular index was to use build_simple_rel_hook (or, previous to
yesterday's commit, get_relation_info_hook) to remove it from the index
list. While that works, it has some disadvantages. First, the index
becomes invisible for all purposes, and can no longer be used for
optimizations such as self-join elimination or left join removal, which
can severely degrade the resulting plan.

Second, if the module attempts to compel the use of a certain index
by removing all other indexes from the index list and disabling
other scan types, but the planner is unable to use the chosen index
for some reason, it will fall back to a sequential scan, because that
is only disabled, whereas the other indexes are, from the planner's
point of view, completely gone. While this situation ideally shouldn't
occur, it's hard for a loadable module to be completely sure whether
the planner will view a certain index as usable for a certain query.
If it isn't, it may be better to fall back to a scan using a disabled
index rather than falling back to an also-disabled sequential scan.

Reviewed-by: Alexandra Wang <alexandra(dot)wang(dot)oss(at)gmail(dot)com>
Discussion: http://postgr.es/m/CA%2BTgmoYS4ZCVAF2jTce%3DbMP0Oq_db_srocR4cZyO0OBp9oUoGg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0fbfd37cefb7eb30c0fa8a158751c19ddeddf1f0

Modified Files
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src/backend/optimizer/util/pathnode.c | 8 ++++++++
src/include/nodes/pathnodes.h | 2 ++
2 files changed, 10 insertions(+)