release-21.2: sql: pre-split hash sharded indexes ranges before DELETE_AND_WRITE_ONLY

[chengxiong-ruan]

Backport 1/1 commits from #74923.

/cc @cockroachdb/release

---

Fixes #74558

Pre-split ranges on shard boundaries before SchemaChanger move
new hash sharded indexes from DELETE_ONLY to DELETE_AND_WRITE_ONLY
state.

Release note (bug fix): None

---

Release justification: backporting so that existing hash sharded index users would enjoy the performance gain with pre-split ranges.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[chengxiong-ruan]

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @ajwerner)

---

pkg/sql/schema_changer.go, line 1470 at r1 (raw file):

	}

	if sc.execCfg.Codec.ForSystemTenant() {

AdminSplit is not allowed in multitenant in 21.2 :(

---

pkg/sql/logictest/testdata/logic_test/hash_sharded_index_presplit, line 2 at r2 (raw file):

# AdminSplit is not allowed in multi-tenant.
# LogicTest: !3node-tenant

skipif config is not supported in 21.2, so had to create a separate test file.