pgsql: Fix join alias expansion in lateral TABLESAMPLE clauses

Fix join alias expansion in lateral TABLESAMPLE clauses

preprocess_expression skips flatten_join_alias_vars for TABLESAMPLE
clauses, on the assumption that they cannot contain Vars of the
current query level. That's not true once a LATERAL subquery
containing a sampled relation has been pulled up: the relation's
TABLESAMPLE clause then becomes a lateral reference of this level, and
if it referred to a join alias Var, that Var is left unexpanded.
extract_lateral_references then pulls the join alias Var out as a
lateral reference and fails with "no relation entry for relid N".

To fix, treat such clauses like lateral function and VALUES RTEs:
introduce EXPRKIND_TABLESAMPLE_LATERAL, used for the TABLESAMPLE
clause of a lateral relation RTE, and keep skipping the alias
expansion only for the non-lateral kind.

Back-patch to all supported branches, as this dates back to the
introduction of TABLESAMPLE.

Author: Richard Guo <guofenglinux(at)gmail(dot)com>
Reviewed-by: Ewan Young <kdbase(dot)hack(at)gmail(dot)com>
Reviewed-by: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAMbWs4834TwOtyyycyvja3MuOBkfpXy3dFAL0zt_vcM1KtqV-Q@mail.gmail.com
Backpatch-through: 14

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/651893c2270fbb963c1a9cbb5fe9880dd84c4aef

Modified Files
--------------
src/backend/optimizer/plan/planner.c | 16 +++++++++++-----
src/test/regress/expected/tablesample.out | 32 +++++++++++++++++++++++++++++++
src/test/regress/sql/tablesample.sql | 12 ++++++++++++
3 files changed, 55 insertions(+), 5 deletions(-)