pgsql: Fix IOS planning when only some index columns can return anattr

Fix IOS planning when only some index columns can return an attribute.

Since 9.5, it's possible that some but not all columns of an index
support returning the indexed value for index-only scans.  If the
same indexed column appears in index columns that behave both ways,
check_index_only() supposed that it'd be OK to do an index-only scan
testing that column; but that fails if we have to recheck the indexed
condition on one of the columns that doesn't support this.

In principle we could make this work by remapping the recheck expressions
to pull the value from a column that does support returning the indexed
value.  But such cases are so weird and rare that, at least for now,
it doesn't seem worth the trouble.  Instead, just teach check_index_only
that a value is returnable only if all the index columns containing it
are returnable, rather than any of them.

Per report from David Pereiro Lagares.  Back-patch to 9.5 where the
possibility of this situation appeared.

Kyotaro Horiguchi

Discussion: https://postgr.es/m/1516210494.1798.16.camel@nlpgo.com

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/be55bfc9386da1f7b6b1b64dee0e9e3871383640

Modified Files
--------------
contrib/btree_gist/expected/inet.out  | 39 +++++++++++++++++++++++++++++++++++
contrib/btree_gist/sql/inet.sql       | 18 ++++++++++++++++
src/backend/optimizer/path/indxpath.c | 15 +++++++++++++-
3 files changed, 71 insertions(+), 1 deletion(-)