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Fix an oversight in checking whether a join with LATERAL refs is legal.
In many cases, we can implement a semijoin as a plain innerjoin by first passing the righthand-side relation through a unique-ification step. However, one of the cases where this does NOT work is where the RHS has a LATERAL reference to the LHS; that makes the RHS dependent on the LHS so that unique-ification is meaningless. joinpath.c understood this, and so would not generate any join paths of this kind ... but join_is_legal neglected to check for the case, so it would think that we could do it. The upshot would be a "could not devise a query plan for the given query" failure once we had failed to generate any join paths at all for the bogus join pair. Back-patch to 9.3 where LATERAL was added.
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3 files changed

+54
-2
lines changed

3 files changed

+54
-2
lines changed

src/backend/optimizer/path/joinrels.c

+6-2
Original file line numberDiff line numberDiff line change
@@ -536,7 +536,9 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
536536
if (!bms_is_subset(ljinfo->lateral_lhs, rel1->relids))
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return false; /* rel1 can't compute the required parameter */
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if (match_sjinfo &&
539-
(reversed || match_sjinfo->jointype == JOIN_FULL))
539+
(reversed ||
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unique_ified ||
541+
match_sjinfo->jointype == JOIN_FULL))
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return false; /* not implementable as nestloop */
541543
}
542544
if (bms_is_subset(ljinfo->lateral_rhs, rel1->relids) &&
@@ -549,7 +551,9 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
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if (!bms_is_subset(ljinfo->lateral_lhs, rel2->relids))
550552
return false; /* rel2 can't compute the required parameter */
551553
if (match_sjinfo &&
552-
(!reversed || match_sjinfo->jointype == JOIN_FULL))
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(!reversed ||
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unique_ified ||
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match_sjinfo->jointype == JOIN_FULL))
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return false; /* not implementable as nestloop */
554558
}
555559
}

src/test/regress/expected/join.out

+35
Original file line numberDiff line numberDiff line change
@@ -4527,6 +4527,41 @@ select * from
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Output: 3
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(11 rows)
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4530+
-- check we don't try to do a unique-ified semijoin with LATERAL
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explain (verbose, costs off)
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select * from
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(values (0,9998), (1,1000)) v(id,x),
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lateral (select f1 from int4_tbl
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where f1 = any (select unique1 from tenk1
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where unique2 = v.x offset 0)) ss;
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QUERY PLAN
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----------------------------------------------------------------------
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Nested Loop
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Output: "*VALUES*".column1, "*VALUES*".column2, int4_tbl.f1
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-> Values Scan on "*VALUES*"
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Output: "*VALUES*".column1, "*VALUES*".column2
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-> Nested Loop Semi Join
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Output: int4_tbl.f1
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Join Filter: (int4_tbl.f1 = tenk1.unique1)
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-> Seq Scan on public.int4_tbl
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Output: int4_tbl.f1
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-> Materialize
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Output: tenk1.unique1
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-> Index Scan using tenk1_unique2 on public.tenk1
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Output: tenk1.unique1
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Index Cond: (tenk1.unique2 = "*VALUES*".column2)
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(14 rows)
4554+
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select * from
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(values (0,9998), (1,1000)) v(id,x),
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lateral (select f1 from int4_tbl
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where f1 = any (select unique1 from tenk1
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where unique2 = v.x offset 0)) ss;
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id | x | f1
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----+------+----
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0 | 9998 | 0
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(1 row)
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45304565
-- test some error cases where LATERAL should have been used but wasn't
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select f1,g from int4_tbl a, (select f1 as g) ss;
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ERROR: column "f1" does not exist

src/test/regress/sql/join.sql

+13
Original file line numberDiff line numberDiff line change
@@ -1365,6 +1365,19 @@ select * from
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select * from (select 3 as z offset 0) z where z.z = x.x
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) zz on zz.z = y.y;
13671367

1368+
-- check we don't try to do a unique-ified semijoin with LATERAL
1369+
explain (verbose, costs off)
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select * from
1371+
(values (0,9998), (1,1000)) v(id,x),
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lateral (select f1 from int4_tbl
1373+
where f1 = any (select unique1 from tenk1
1374+
where unique2 = v.x offset 0)) ss;
1375+
select * from
1376+
(values (0,9998), (1,1000)) v(id,x),
1377+
lateral (select f1 from int4_tbl
1378+
where f1 = any (select unique1 from tenk1
1379+
where unique2 = v.x offset 0)) ss;
1380+
13681381
-- test some error cases where LATERAL should have been used but wasn't
13691382
select f1,g from int4_tbl a, (select f1 as g) ss;
13701383
select f1,g from int4_tbl a, (select a.f1 as g) ss;

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