pgsql: Fix nullability check for a sub-select's upper-level Vars

Fix nullability check for a sub-select's upper-level Vars

When checking whether a sub-select's output columns can produce NULL,
so as to decide whether a NOT IN can be converted to an anti-join,
query_outputs_are_not_nullable() falls back on find_nonnullable_vars()
for targetlist entries that are plain Vars: if the sub-select's own
quals prove the Var non-null, the output is non-nullable. But that
test compared only varno and varattno, without checking varlevelsup.
An outer reference in the targetlist could thus be matched against a
Var of the sub-select's own range table that happens to share the same
varno and varattno, wrongly proving the output non-nullable and
allowing an invalid conversion to an anti-join, which yields wrong
answers when the outer reference is NULL.

To fix, restrict the fallback to Vars of the current query level.

Author: Rui Zhao <zhaorui126(at)gmail(dot)com>
Reviewed-by: Tender Wang <tndrwang(at)gmail(dot)com>
Reviewed-by: Richard Guo <guofenglinux(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAHWVJhGuaFFRpmq4j+mcMcm_HC5QOT7LZsC9bf9b7BCBmvbfMA@mail.gmail.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0d99579320d075eba5733517463d81a2231e373b

Modified Files
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src/backend/optimizer/util/clauses.c | 3 ++-
src/test/regress/expected/subselect.out | 24 ++++++++++++++++++++++++
src/test/regress/sql/subselect.sql | 15 +++++++++++++++
3 files changed, 41 insertions(+), 1 deletion(-)