pgsql: Improve estimation of OR clauses using multiple extended statist

Improve estimation of OR clauses using multiple extended statistics.

When estimating an OR clause using multiple extended statistics
objects, treat the estimates for each set of clauses for each
statistics object as independent of one another. The overlap estimates
produced for each statistics object do not apply to clauses covered by
other statistics objects.

Dean Rasheed, reviewed by Tomas Vondra.

Discussion: https://postgr.es/m/CAEZATCW=J65GUFm50RcPv-iASnS2mTXQbr=CfBvWRVhFLJ_fWA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/88b0898fe35a5a0325fca21bd4f3ed6dffb364c1

Modified Files
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src/backend/statistics/extended_stats.c | 25 +++++++++++++++++--------
src/test/regress/expected/stats_ext.out |  2 +-
2 files changed, 18 insertions(+), 9 deletions(-)