ddl, session: using table ID instead of partition ID when calling SplitRegions (#46156)

[ti-chi-bot]

This is an automated cherry-pick of #46156

What problem does this PR solve?

Issue Number: close #46135

Problem Summary:

tidb/ddl/split_region.go

Lines 44 to 46 in 95b4dcc

	for _, def := range parts {
	regionIDs = append(regionIDs, SplitRecordRegion(ctxWithTimeout, store, def.ID, scatter))
	}

tidb/ddl/split_region.go

Lines 121 to 123 in 95b4dcc

	func SplitRecordRegion(ctx context.Context, store kv.SplittableStore, tableID int64, scatter bool) uint64 {
	tableStartKey := tablecodec.GenTablePrefix(tableID)
	regionIDs, err := store.SplitRegions(ctx, [][]byte{tableStartKey}, scatter, &tableID)

https://github.com/tikv/pd/blob/346e7716e2598dfba2db6afa73c3731e15449f49/client/client.go#L1403-L1409

	req := &pdpb.ScatterRegionRequest{
		Header:         c.requestHeader(),
		Group:          options.group,
		RegionsId:      regionsID,
		RetryLimit:     options.retryLimit,
		SkipStoreLimit: options.skipStoreLimit,
	}

As you can see from the above code, the tableID passed to SplitRegions is the ID of the partitioned table. And we use tableID as Group pass to PD. So different partition ID uses different group ID in a table which makes the split region is uneven.

What is changed and how it works?

In the partition table, we use table ID as a group ID to pass PD.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

tidb> select @@global.tidb_scatter_region;
+------------------------------+
| @@global.tidb_scatter_region |
+------------------------------+
|                            1 |
+------------------------------+
1 row in set (0.00 sec)
create table test.t1(id int,b int) shard_row_id_bits=4 partition by hash(id) partitions 2000; 

Before this PR:

After this PR:

• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/check_dev	87b04c7	link	true	/test check-dev
idc-jenkins-ci-tidb/check_dev_2	87b04c7	link	true	/test check-dev2
idc-jenkins-ci-tidb/unit-test	87b04c7	link	true	/test unit-test
idc-jenkins-ci-tidb/build	87b04c7	link	true	/test build
idc-jenkins-ci-tidb/mysql-test	87b04c7	link	true	/test mysql-test

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