Prevent join removal from removing the query's result relation.
This was not something that required consideration before MERGE
was invented; but MERGE builds a join tree that left-joins to the
result relation, meaning that remove_useless_joins will consider
removing it. That should generally be stopped by the query's use
of output variables from the result relation. However, if the
result relation is inherited (e.g. a partitioned table) then
we don't add any row identity variables to the query until
expand_inherited_rtentry, which happens after join removal.
This was exposed as of commit 3c569049b
, which made it possible
to deduce that a partitioned table could contain at most one row
matching a join key, enabling removal of the not-yet-expanded
result relation. Ooops.
To fix, let's just teach join_is_removable that the query result
rel is never removable. It's a cheap enough test in any case,
and it'll save some cycles that we'd otherwise expend in proving
that it's not removable, even in the cases we got right.
Back-patch to v15 where MERGE was added. Although I think the
case cannot be reached in v15, this seems like cheap insurance.
Per investigation of a report from Alexander Lakhin.
Discussion: https://postgr.es/m/36bee393-b351-16ac-93b2-d46d83637e45@gmail.com
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parent
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@ -183,6 +183,14 @@ join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo)
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if (!bms_get_singleton_member(sjinfo->min_righthand, &innerrelid))
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return false;
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/*
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* Never try to eliminate a left join to the query result rel. Although
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* the case is syntactically impossible in standard SQL, MERGE will build
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* a join tree that looks exactly like that.
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*/
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if (innerrelid == root->parse->resultRelation)
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return false;
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innerrel = find_base_rel(root, innerrelid);
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/*
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@ -1771,6 +1771,41 @@ SELECT * FROM pa_target ORDER BY tid;
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(9 rows)
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ROLLBACK;
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DROP TABLE pa_source;
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DROP TABLE pa_target CASCADE;
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-- Partitioned table with primary key
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CREATE TABLE pa_target (tid integer PRIMARY KEY) PARTITION BY LIST (tid);
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CREATE TABLE pa_targetp PARTITION OF pa_target DEFAULT;
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CREATE TABLE pa_source (sid integer);
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INSERT INTO pa_source VALUES (1), (2);
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EXPLAIN (VERBOSE, COSTS OFF)
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MERGE INTO pa_target t USING pa_source s ON t.tid = s.sid
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WHEN NOT MATCHED THEN INSERT VALUES (s.sid);
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QUERY PLAN
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-------------------------------------------------------------
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Merge on public.pa_target t
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Merge on public.pa_targetp t_1
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-> Hash Left Join
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Output: s.sid, t_1.tableoid, t_1.ctid
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Inner Unique: true
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Hash Cond: (s.sid = t_1.tid)
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-> Seq Scan on public.pa_source s
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Output: s.sid
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-> Hash
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Output: t_1.tid, t_1.tableoid, t_1.ctid
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-> Seq Scan on public.pa_targetp t_1
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Output: t_1.tid, t_1.tableoid, t_1.ctid
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(12 rows)
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MERGE INTO pa_target t USING pa_source s ON t.tid = s.sid
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WHEN NOT MATCHED THEN INSERT VALUES (s.sid);
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TABLE pa_target;
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tid
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-----
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1
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2
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(2 rows)
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DROP TABLE pa_source;
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DROP TABLE pa_target CASCADE;
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-- some complex joins on the source side
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@ -1116,6 +1116,26 @@ ROLLBACK;
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DROP TABLE pa_source;
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DROP TABLE pa_target CASCADE;
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-- Partitioned table with primary key
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CREATE TABLE pa_target (tid integer PRIMARY KEY) PARTITION BY LIST (tid);
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CREATE TABLE pa_targetp PARTITION OF pa_target DEFAULT;
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CREATE TABLE pa_source (sid integer);
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INSERT INTO pa_source VALUES (1), (2);
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EXPLAIN (VERBOSE, COSTS OFF)
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MERGE INTO pa_target t USING pa_source s ON t.tid = s.sid
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WHEN NOT MATCHED THEN INSERT VALUES (s.sid);
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MERGE INTO pa_target t USING pa_source s ON t.tid = s.sid
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WHEN NOT MATCHED THEN INSERT VALUES (s.sid);
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TABLE pa_target;
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DROP TABLE pa_source;
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DROP TABLE pa_target CASCADE;
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-- some complex joins on the source side
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CREATE TABLE cj_target (tid integer, balance float, val text)
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