Don't use partial unique indexes for unique proofs in the planner

Here we adjust relation_has_unique_index_for() so that it no longer makes
use of partial unique indexes as uniqueness proofs.  It is incorrect to
use these as the predicates used by check_index_predicates() to set
predOK makes use of not only baserestrictinfo quals as proofs, but also
qual from join conditions.  For relation_has_unique_index_for()'s case, we
need to know the relation is unique for a given set of columns before any
joins are evaluated, so if predOK was only set to true due to some join
qual, then it's unsafe to use such indexes in
relation_has_unique_index_for().  The final plan may not even make use
of that index, which could result in reading tuples that are not as
unique as the planner previously expected them to be.

Bug: #17975
Reported-by: Tor Erik Linnerud
Backpatch-through: 11, all supported versions
Discussion: https://postgr.es/m/17975-98a90c156f25c952%40postgresql.org

2 files changed, 26 insertions, 0 deletions

diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index c50c3826fc7..edc66ea590e 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -5975,6 +5975,23 @@ left join j2 on j1.id1 = j2.id1 where j1.id2 = 1;
          Output: j2.id1, j2.id2
 (8 rows)
 
+create unique index j1_id2_idx on j1(id2) where id2 is not null;
+-- ensure we don't use a partial unique index as unique proofs
+explain (verbose, costs off)
+select * from j1
+inner join j2 on j1.id2 = j2.id2;
+                QUERY PLAN                
+------------------------------------------
+ Nested Loop
+   Output: j1.id1, j1.id2, j2.id1, j2.id2
+   Join Filter: (j1.id2 = j2.id2)
+   ->  Seq Scan on public.j2
+         Output: j2.id1, j2.id2
+   ->  Seq Scan on public.j1
+         Output: j1.id1, j1.id2
+(7 rows)
+
+drop index j1_id2_idx;
 -- validate logic in merge joins which skips mark and restore.
 -- it should only do this if all quals which were used to detect the unique
 -- are present as join quals, and not plain quals.
diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index 737b85925c0..669b2e0bf5a 100644
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -2012,6 +2012,15 @@ explain (verbose, costs off)
 select * from j1
 left join j2 on j1.id1 = j2.id1 where j1.id2 = 1;
 
+create unique index j1_id2_idx on j1(id2) where id2 is not null;
+
+-- ensure we don't use a partial unique index as unique proofs
+explain (verbose, costs off)
+select * from j1
+inner join j2 on j1.id2 = j2.id2;
+
+drop index j1_id2_idx;
+
 -- validate logic in merge joins which skips mark and restore.
 -- it should only do this if all quals which were used to detect the unique
 -- are present as join quals, and not plain quals.