pgsql: Set hasSubLinks when expanding a whole-row join alias reference

Set hasSubLinks when expanding a whole-row join alias reference

If a subquery has been flattened into its parent, the joinaliasvars
entries of a join above it can be arbitrary expressions rather than
plain Vars, so expanding a reference to such a join alias may insert a
SubLink into a lower-level subquery. flatten_join_alias_vars_mutator
detects that and sets the subquery's hasSubLinks flag, but only in the
single-column code path; the whole-row path just asserted in a comment
that its recursive call would handle this, which is true only when the
alias entry is itself a Var referencing another join.

Hence a whole-row reference to such a join appearing in a sub-select
left that sub-select's hasSubLinks false, so preprocess_expression
skipped SS_process_sublinks for it, and the unprocessed SubLink
reached code that is not prepared for one: this produced "cannot
handle unplanned sub-select" from cost_qual_eval, an assertion failure
in preprocess_aggrefs, or "unrecognized node type" at execution,
depending on where in the sub-select the SubLink ended up.

To fix, make the same check in the whole-row expansion path.

Back-patch to all supported branches.

Author: Richard Guo <guofenglinux(at)gmail(dot)com>
Reviewed-by: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAMbWs49PgeNFhQTZ2gSzwatF0_LXyegMPGt++jSfxUN7NZuQ5Q@mail.gmail.com
Backpatch-through: 14

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/6bd71489d8155def741af656acb0eaee47d25f49

Modified Files
--------------
src/backend/optimizer/util/var.c | 6 +++++-
src/test/regress/expected/subselect.out | 25 +++++++++++++++++++++++++
src/test/regress/sql/subselect.sql | 12 ++++++++++++
3 files changed, 42 insertions(+), 1 deletion(-)