pgsql: Fix cost_incremental_sort for expressions with varno 0

Fix cost_incremental_sort for expressions with varno 0

When estimating the number of pre-sorted groups in cost_incremental_sort
we must not pass Vars with varno 0 to estimate_num_groups, which would
cause failues in find_base_rel. This may happen when sorting output of
set operations, thanks to generate_append_tlist.

Unlike recurse_set_operations we can't easily access the original target
list, so if we find any Vars with varno 0, we fall back to the default
estimate DEFAULT_NUM_DISTINCT.

Reported-by: Justin Pryzby
Discussion: https://postgr.es/m/20200411214639.GK2228%40telsasoft.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/de0dc1a84710f127fdd40f87e783797cc2d69a77

Modified Files
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src/backend/optimizer/path/costsize.c          | 54 ++++++++++++++++++++++++--
src/test/regress/expected/incremental_sort.out | 20 ++++++++++
src/test/regress/sql/incremental_sort.sql      |  4 ++
3 files changed, 75 insertions(+), 3 deletions(-)