pgsql: Reduce the default value of vacuum_cost_page_miss.

Reduce the default value of vacuum_cost_page_miss.

When commit f425b605 introduced cost based vacuum delays back in 2004,
the defaults reflected then-current trends in hardware, as well as
certain historical limitations in PostgreSQL.  There have been enormous
improvements in both areas since that time.  The cost limit GUC defaults
finally became much more representative of current trends following
commit cbccac37, which decreased autovacuum_vacuum_cost_delay's default
by 10x for PostgreSQL 12 (it went from 20ms to only 2ms).

The relative costs have shifted too.  This should also be accounted for
by the defaults.  More specifically, the relative importance of avoiding
dirtying pages within VACUUM has greatly increased, primarily due to
main memory capacity scaling and trends in flash storage.  Within
Postgres itself, improvements like sequential access during index
vacuuming (at least in nbtree and GiST indexes) have also been
contributing factors.

To reflect all this, decrease the default of vacuum_cost_page_miss to 2.
Since the default of vacuum_cost_page_dirty remains 20, dirtying a page
is now considered 10x "costlier" than a page miss by default.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WzmLPFnkWT8xMjmcsm7YS3+_Qi3iRWAb2+_Bc8UhVyHfuA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e19594c5c059d2e071b67d87ae84f569a52d2e32

Modified Files
--------------
doc/src/sgml/config.sgml                      | 2 +-
src/backend/utils/init/globals.c              | 2 +-
src/backend/utils/misc/guc.c                  | 2 +-
src/backend/utils/misc/postgresql.conf.sample | 2 +-
4 files changed, 4 insertions(+), 4 deletions(-)