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Fix oversight in outer join removal.
A placeholder that references the outer join's relid in ph_eval_at is logically "above" the join, and therefore we can't remove its PlaceHolderInfo: it might still be used somewhere in the query. This was not an issue pre-v16 because we failed to remove the join at all in such cases. The new outer-join-aware-Var infrastructure permits deducing that it's okay to remove the join, but then we have to clean up correctly afterwards. Report and fix by Richard Guo Discussion: https://postgr.es/m/CAMbWs4_tuVn9EwwMcggGiZJWWstdXX_ci8FeEU17vs+4nLgw3w@mail.gmail.com
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3 files changed

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‎src/backend/optimizer/plan/analyzejoins.c

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -425,7 +425,8 @@ remove_rel_from_query(PlannerInfo *root, int relid, SpecialJoinInfo *sjinfo)
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Assert(!bms_is_member(relid, phinfo->ph_lateral));
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if (bms_is_subset(phinfo->ph_needed, joinrelids) &&
428-
bms_is_member(relid, phinfo->ph_eval_at))
428+
bms_is_member(relid, phinfo->ph_eval_at) &&
429+
!bms_is_member(ojrelid, phinfo->ph_eval_at))
429430
{
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root->placeholder_list = foreach_delete_current(root->placeholder_list,
431432
l);

‎src/test/regress/expected/join.out

Lines changed: 57 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -5842,6 +5842,63 @@ where ss.stringu2 !~* ss.case1;
58425842
doh!
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(1 row)
58445844

5845+
rollback;
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-- test cases where we can remove a join, but not a PHV computed at it
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begin;
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create temp table t (a int unique, b int);
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insert into t values (1,1), (2,2);
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explain (costs off)
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select 1
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from t t1
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left join (select t2.a, 1 as c
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from t t2 left join t t3 on t2.a = t3.a) s
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on true
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left join t t4 on true
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where s.a < s.c;
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QUERY PLAN
5859+
-------------------------------------
5860+
Nested Loop Left Join
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-> Nested Loop
5862+
-> Seq Scan on t t1
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-> Materialize
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-> Seq Scan on t t2
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Filter: (a < 1)
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-> Materialize
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-> Seq Scan on t t4
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(8 rows)
5869+
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explain (costs off)
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select t1.a, s.*
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from t t1
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left join lateral (select t2.a, coalesce(t1.a, 1) as c
5874+
from t t2 left join t t3 on t2.a = t3.a) s
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on true
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left join t t4 on true
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where s.a < s.c;
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QUERY PLAN
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-----------------------------------------------
5880+
Nested Loop Left Join
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-> Nested Loop
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-> Seq Scan on t t1
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-> Seq Scan on t t2
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Filter: (a < COALESCE(t1.a, 1))
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-> Materialize
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-> Seq Scan on t t4
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(7 rows)
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select t1.a, s.*
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from t t1
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left join lateral (select t2.a, coalesce(t1.a, 1) as c
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from t t2 left join t t3 on t2.a = t3.a) s
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on true
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left join t t4 on true
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where s.a < s.c;
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a | a | c
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---+---+---
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2 | 1 | 2
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2 | 1 | 2
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(2 rows)
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58455902
rollback;
58465903
-- test case to expose miscomputation of required relid set for a PHV
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explain (verbose, costs off)

‎src/test/regress/sql/join.sql

Lines changed: 34 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -2153,6 +2153,40 @@ where ss.stringu2 !~* ss.case1;
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rollback;
21552155

2156+
-- test cases where we can remove a join, but not a PHV computed at it
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begin;
2158+
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create temp table t (a int unique, b int);
2160+
insert into t values (1,1), (2,2);
2161+
2162+
explain (costs off)
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select 1
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from t t1
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left join (select t2.a, 1 as c
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from t t2 left join t t3 on t2.a = t3.a) s
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on true
2168+
left join t t4 on true
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where s.a < s.c;
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explain (costs off)
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select t1.a, s.*
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from t t1
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left join lateral (select t2.a, coalesce(t1.a, 1) as c
2175+
from t t2 left join t t3 on t2.a = t3.a) s
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on true
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left join t t4 on true
2178+
where s.a < s.c;
2179+
2180+
select t1.a, s.*
2181+
from t t1
2182+
left join lateral (select t2.a, coalesce(t1.a, 1) as c
2183+
from t t2 left join t t3 on t2.a = t3.a) s
2184+
on true
2185+
left join t t4 on true
2186+
where s.a < s.c;
2187+
2188+
rollback;
2189+
21562190
-- test case to expose miscomputation of required relid set for a PHV
21572191
explain (verbose, costs off)
21582192
select i8.*, ss.v, t.unique2

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