region is not balance when dispatches to tiflash_storage nodes for static pruning partition table.

[guo-shaoge]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. We use very big internal dataset(ask me for mysql link if you want to reproduce). (topo is: two tiflash_compute nodes and two tiflash_storage nodes
2. run following sqls:

set @@tidb_partition_prune_mode = 'static';
select count(*) from xxx_events;

3. check logs for two tiflash_compute nodes.

2. What did you expect to see? (Required)

region should balance.

3. What did you see instead (Required)

regoin is not balace. We can see there is two aspects of not balancing:

1. same tiflash_compute dispatch to two tiflash_storage nodes is not balance(395 VS 1148, 1768 VS 4900). (Workround is here: *: support different tiflash_compute dispatch policy tidb#41406)
2. regsion need to read between two tiflash_compute nodes.(395+1148=1543 VS 1768+4900=6668)

tiflash_compute-0 -> tiflash_storage-0: 395
tiflash_compute-0 -> tiflash_storage-1: 1148
tiflash_compute-0 -> tiflash_storage-0: 1768
tiflash_compute-1 -> tiflash_storage-1: 4900

4. What is your TiFlash version? (Required)

MySQL [test]> select tidb_version();
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()                                                                                                                                                                                                                                                                                         |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v6.7.0-alpha-111-gb417ad0689
Edition: Community
Git Commit Hash: b417ad0689795a265df9cdba22c40871f50b7e55
Git Branch: master
UTC Build Time: 2023-02-28 08:35:54
GoVersion: go1.19.1
Race Enabled: false
TiKV Min Version: 6.2.0-alpha
Check Table Before Drop: false
Store: unistore |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.001 sec)