Re: BUG #18568: BUG: Result wrong when do group by on partition table!

The following bug has been logged on the website:

Bug reference:      18568
Logged by:          Webbo Han
Email address:      1105066510@qq.com
PostgreSQL version: 16.3
Operating system:   centos 7.6
Description:       

First, we create one case-insensitive collation use ICU:
```sql
        CREATE COLLATION case_insensitive (
                provider = icu,
                locale = 'und-u-ks-level2',
                deterministic = false
        );
```

Then, we create the partition table, meanwhile we set the collation of
column c to `case_insensitive`,
and set partkey's collation to 'C'.
```sql
        SET enable_partitionwise_aggregate TO true;
        SET enable_partitionwise_join TO true;
        SET max_parallel_workers_per_gather TO 0;
        SET enable_incremental_sort TO off;
        CREATE TABLE pagg_tab (c text collate case_insensitive) PARTITION BY LIST(c
collate "C");