Wrong result when enable_partitionwise_join is on if collation of PartitionKey and Column is different.

Hi all,

I find another issue as $SUBJECT when I work on [1].
I will use [1] SQL to show the problem.
CREATE COLLATION case_insensitive (
provider = icu,
locale = 'und-u-ks-level2',
deterministic = false
);
CREATE TABLE pagg_tab (c text collate case_insensitive) PARTITION BY LIST(c
collate "C");
CREATE TABLE pagg_tab_p1 PARTITION OF pagg_tab FOR VALUES IN ('a', 'b',
'c', 'd');
CREATE TABLE pagg_tab_p2 PARTITION OF pagg_tab FOR VALUES IN ('e', 'f',
'A');
CREATE TABLE pagg_tab_p3 PARTITION OF pagg_tab FOR VALUES IN ('B', 'C',
'D', 'E');
INSERT INTO pagg_tab SELECT substr('abcdeABCDE', (i % 10) +1 , 1) FROM
generate_series(0, 2999) i;
ANALYZE pagg_tab;

set max_parallel_workers_per_gather = 0;
postgres=# show enable_partitionwise_join ;
 enable_partitionwise_join
---------------------------
 off
(1 row)
postgres=# explain select count(*) from pagg_tab t1 join pagg_tab t2 on
t1.c = t2.c;
                                         QUERY PLAN
--------------------------------------------------------------------------------------------
 Aggregate  (cost=24915.00..24915.01 rows=1 width=8)
   ->  Hash Join  (cost=97.50..20415.00 rows=1800000 width=0)
         Hash Cond: (t1.c = t2.c)
         ->  Append  (cost=0.00..60.00 rows=3000 width=2)
               ->  Seq Scan on pagg_tab_p2 t1_1  (cost=0.00..9.00 rows=600
width=2)
               ->  Seq Scan on pagg_tab_p3 t1_2  (cost=0.00..18.00
rows=1200 width=2)
               ->  Seq Scan on pagg_tab_p1 t1_3  (cost=0.00..18.00
rows=1200 width=2)
         ->  Hash  (cost=60.00..60.00 rows=3000 width=2)
               ->  Append  (cost=0.00..60.00 rows=3000 width=2)
                     ->  Seq Scan on pagg_tab_p2 t2_1  (cost=0.00..9.00
rows=600 width=2)
                     ->  Seq Scan on pagg_tab_p3 t2_2  (cost=0.00..18.00
rows=1200 width=2)
                     ->  Seq Scan on pagg_tab_p1 t2_3  (cost=0.00..18.00
rows=1200 width=2)
(12 rows)

postgres=#  select count(*) from pagg_tab t1 join pagg_tab t2 on t1.c =
t2.c;
  count
---------
 1800000
(1 row)

But if we set enable_partitionwise_join is on, the result is different, as
below.
postgres=# set enable_partitionwise_join = on;
SET
postgres=# explain select count(*) from pagg_tab t1 join pagg_tab t2 on
t1.c = t2.c;
                                         QUERY PLAN
--------------------------------------------------------------------------------------------
 Aggregate  (cost=17010.00..17010.01 rows=1 width=8)
   ->  Append  (cost=16.50..14760.00 rows=900000 width=0)
         ->  Hash Join  (cost=16.50..2052.00 rows=180000 width=0)
               Hash Cond: (t1_1.c = t2_1.c)
               ->  Seq Scan on pagg_tab_p2 t1_1  (cost=0.00..9.00 rows=600
width=2)
               ->  Hash  (cost=9.00..9.00 rows=600 width=2)
                     ->  Seq Scan on pagg_tab_p2 t2_1  (cost=0.00..9.00
rows=600 width=2)
         ->  Hash Join  (cost=33.00..4104.00 rows=360000 width=0)
               Hash Cond: (t1_2.c = t2_2.c)
               ->  Seq Scan on pagg_tab_p3 t1_2  (cost=0.00..18.00
rows=1200 width=2)
               ->  Hash  (cost=18.00..18.00 rows=1200 width=2)
                     ->  Seq Scan on pagg_tab_p3 t2_2  (cost=0.00..18.00
rows=1200 width=2)
         ->  Hash Join  (cost=33.00..4104.00 rows=360000 width=0)
               Hash Cond: (t1_3.c = t2_3.c)
               ->  Seq Scan on pagg_tab_p1 t1_3  (cost=0.00..18.00
rows=1200 width=2)
               ->  Hash  (cost=18.00..18.00 rows=1200 width=2)
                     ->  Seq Scan on pagg_tab_p1 t2_3  (cost=0.00..18.00
rows=1200 width=2)
(17 rows)

postgres=#  select count(*) from pagg_tab t1 join pagg_tab t2 on t1.c =
t2.c;
 count
--------
 900000
(1 row)

I think the root cause of this thread and [1] are same.  We don't use the
Partition Key collation but column's
collation to fill the RelOptInfo partexprs field in
set_baserel_partition_key_exprs().
If the Partition Key definition is same as column definition,  which most
times is,
that will be ok. But if it's not, this thread issue will arise.

As far as I know, only partition pruning logic considers not only call
equal(), but also check collation match.
Other codes only call equal() to check if the exprs match the partition key.
For example, in this thread case, match_expr_to_partition_keys() think the
expr match the partition key:
if (equal(lfirst(lc), expr))
    return cnt;

Although We can fix this issue like [1], I think why not directly use the
partkey->partcollation[cnt], which its value is
same with pg_partitioned_table's partcollation. I tried this to fix [1],
but at that time, I was unsure if it was the correct fix.

Until I find this thread issue, I think we should do it this way.
In the attached patch, I include this thread test and [1] test case.

I have two questions in my head:
1.  Does partition pruning logic still check the collation match with this
patch.
2. icu can work on all platform?

Any thoughts?

[1]
https://www.postgresql.org/message-id/18568-2a9afb6b9f7e6ed3%40postgresql.org
-- 
Thanks,
Tender Wang