pgsql: Fix tuple_fraction calculation in generate_orderedappend_paths()

Fix tuple_fraction calculation in generate_orderedappend_paths()

6b94e7a6da adjusted generate_orderedappend_paths() to consider fractional
paths. However, it didn't manage to interpret the tuple_fraction value
correctly. According to the header comment of grouping_planner(), the
tuple_fraction >= 1 specifies the absolute number of expected tuples. That
number must be divided by the expected total number of tuples to get the
actual fraction.

Even though this is a bug fix, we don't backpatch it. The risks of the side
effects of plan changes on stable branches are too high.

Reported-by: Andrei Lepikhov <lepihov(at)gmail(dot)com>
Discussion: https://postgr.es/m/3ca271fa-ca5c-458c-8934-eb148622b270%40gmail.com
Author: Andrei Lepikhov <lepihov(at)gmail(dot)com>
Reviewed-by: Junwang Zhao <zhjwpku(at)gmail(dot)com>
Reviewed-by: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3d3a81fc24b07f04df81d898f6b4e85c553ef604

Modified Files
--------------
src/backend/optimizer/path/allpaths.c | 12 +++++++++++-
src/test/regress/expected/partition_join.out | 18 ++++++++++++++++++
src/test/regress/sql/partition_join.sql | 3 +++
3 files changed, 32 insertions(+), 1 deletion(-)