pgsql: Again fix initialization of auto-tuned effective_cache_size.

Again fix initialization of auto-tuned effective_cache_size.

The previous method was overly complex and underly correct; in particular,
by assigning the default value with PGC_S_OVERRIDE, it prevented later
attempts to change the setting in postgresql.conf, as noted by Jeff Janes.
We should just assign the default value with source PGC_S_DYNAMIC_DEFAULT,
which will have the desired priority relative to the boot_val as well as
user-set values.

There is still a gap in this method: if there's an explicit assignment of
effective_cache_size = -1 in the postgresql.conf file, and that assignment
appears before shared_buffers is assigned, the code will substitute 4 times
the bootstrap default for shared_buffers, and that value will then persist
(since it will have source PGC_S_FILE). I don't see any very nice way
to avoid that though, and it's not a case to be expected in practice.
The existing comments in guc-file.l look forward to a redesign of the
DYNAMIC_DEFAULT mechanism; if that ever happens, we should consider this
case as one of the things we'd like to improve.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/af930e606a3217db3909029c6c3f8d003ba70920

Modified Files
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src/backend/optimizer/path/costsize.c | 33 ++++++++++++---------------------
src/backend/utils/misc/guc-file.l | 1 +
2 files changed, 13 insertions(+), 21 deletions(-)