pgsql: Invent min_parallel_relation_size GUC to replace a hard-wired co

Invent min_parallel_relation_size GUC to replace a hard-wired constant.

The main point of doing this is to allow the cutoff to be set very small,
even zero, to allow parallel-query behavior to be tested on relatively
small tables such as we typically use in the regression tests.  But it
might be of use to users too.  The number-of-workers scaling behavior in
create_plain_partial_paths() is pretty ad-hoc and subject to change, so
we won't expose anything about that, but the notion of not considering
parallel query at all for tables below size X seems reasonably stable.

Amit Kapila, per a suggestion from me

Discussion: <17170.1465830165@sss.pgh.pa.us>

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/75be66464cb1bffa1e5757907b9a04ad5afc7859

Modified Files
--------------
doc/src/sgml/config.sgml                      | 14 ++++++++++++++
src/backend/optimizer/path/allpaths.c         | 15 ++++++++++-----
src/backend/utils/misc/guc.c                  | 11 +++++++++++
src/backend/utils/misc/postgresql.conf.sample |  1 +
src/include/optimizer/paths.h                 |  1 +
5 files changed, 37 insertions(+), 5 deletions(-)