Re: [sqlsmith] Failed assertion in postgres_fdw/deparse.c:1116

On 2016/06/14 6:51, Robert Haas wrote:
> On Fri, Jun 10, 2016 at 4:14 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> Although I have done a bit of review of this patch, it needs more
>> thought than I have so far had time to give it. I will update again
>> by Tuesday.
>
> I've reviewed this a bit further and have discovered an infelicity.
> The following is all with the patch applied.
>
> By itself, this join can be pushed down:
>
> contrib_regression=# EXPLAIN SELECT 13 FROM ft1 RIGHT JOIN ft2 ON
> ft1.c1 = ft2.c1;
> QUERY PLAN
> ------------------------------------------------------
> Foreign Scan (cost=100.00..137.66 rows=822 width=4)
> Relations: (public.ft2) LEFT JOIN (public.ft1)
> (2 rows)
>
> That's great. However, when that query is used as the outer rel of a
> left join, it can't:
>
> contrib_regression=# explain verbose select * from ft4 LEFT JOIN
> (SELECT 13 FROM ft1 RIGHT JOIN ft2 ON ft1.c1 = ft2.c1) q on true;
> QUERY PLAN
> ---------------------------------------------------------------------------------------------
> Nested Loop Left Join (cost=353.50..920.77 rows=41100 width=19)
> Output: ft4.c1, ft4.c2, ft4.c3, (13)
> -> Foreign Scan on public.ft4 (cost=100.00..102.50 rows=50 width=15)
> Output: ft4.c1, ft4.c2, ft4.c3
> Remote SQL: SELECT c1, c2, c3 FROM "S 1"."T 3"
> -> Materialize (cost=253.50..306.57 rows=822 width=4)
> Output: (13)
> -> Hash Left Join (cost=253.50..302.46 rows=822 width=4)
> Output: 13
> Hash Cond: (ft2.c1 = ft1.c1)
> -> Foreign Scan on public.ft2 (cost=100.00..137.66
> rows=822 width=4)
> Output: ft2.c1
> Remote SQL: SELECT "C 1" FROM "S 1"."T 1"
> -> Hash (cost=141.00..141.00 rows=1000 width=4)
> Output: ft1.c1
> -> Foreign Scan on public.ft1
> (cost=100.00..141.00 rows=1000 width=4)
> Output: ft1.c1
> Remote SQL: SELECT "C 1" FROM "S 1"."T 1"
> (18 rows)
>
> Of course, because of the PlaceHolderVar, there's no way to push down
> the ft1-ft2-ft4 join to the remote side. But we could still push down
> the ft1-ft2 join and then locally perform the join between the result
> and ft4. However, the proposed fix doesn't allow that, because
> ph_eval_at is (b 4 5) and relids for the ft1-ft2 join is also (b 4 5),
> and so the bms_is_subset(phinfo->ph_eval_at, relids) test returns
> true.

You're right. It indeed should be possible to push down ft1-ft2 join.
However it could not be done without also modifying
build_tlist_to_deparse() a little (as Ashutosh proposed [1] to do
upthread). Attached new version of the patch with following changes:

1) Modified the check in foreign_join_ok() so that it is not overly
restrictive. Previously, it used ph_needed as the highest level at which
the PHV is needed (by definition) and checked using it whether it is above
the join under consideration, which ended up being an overkill. ISTM, we
can just decide from joinrel->relids of the join under consideration
whether we are above the lowest level where the PHV could be evaluated
(ph_eval_at) and stop if so. So in the example you provided, it won't now
reject {ft1, ft2}, but will {ft4, ft1, ft2}.

2) Modified build_tlist_to_deparse() to get rid of the PlaceHolderVars in
the targetlist to deparse by using PVC_RECURSE_PLACEHOLDER flag of
pull_var_clause(). That is because we do not yet support anything other
than Vars in deparseExplicitTargetList() (+1 to your patch to change the
Assert to elog).

Thanks,
Amit

[1]
https://www.postgresql.org/message-id/CAFjFpRfQRKNCvfPj8p%3DD9%2BDVZeuTfSN3hnGowKho%3DrKCSeD9dw%40mail.gmail.com