pgsql: Consider the "LIMIT 1" optimization with parallel DISTINCT

Consider the "LIMIT 1" optimization with parallel DISTINCT

Similar to what was done in 5543677ec for non-parallel DISTINCT, apply
the same optimization when the distinct_pathkeys are empty for the
partial paths too.

This can be faster than the non-parallel version when the first row
matching the WHERE clause of the query takes a while to find. Parallel
workers could speed that process up considerably.

Author: Richard Guo
Reviewed-by: David Rowley
Discussion: https://postgr.es/m/CAMbWs49JC0qvfUbzs-TVzgMpSSBiMJ_6sN=BaA9iohBgYkr=LA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b588cad688823b1e996ce05af4d88a954c005a3a

Modified Files
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src/backend/optimizer/plan/planner.c | 44 ++++++++++++++++++++++++---
src/test/regress/expected/select_distinct.out | 20 ++++++++++++
src/test/regress/sql/select_distinct.sql | 13 ++++++++
3 files changed, 72 insertions(+), 5 deletions(-)