sql: prohibit DROP COLUMN on REGIONAL BY ROW column reference

[otan]

The foundation of REGIONAL BY ROW is to use a column which dictates the
location to which the row is stored. As such, we should prevent the
column from being used as a DROP COLUMN reference.

Resolves #59117

Release note: None

[cockroach-teamcity]

This change is 

[ajstorm]

Ultra nit: In the commit record, it might be more correct to say "the location in which the rows is stored".

Reviewed 3 of 3 files at r1.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @otan)

---

pkg/sql/alter_table.go, line 409 at r1 (raw file):

						pgerror.Newf(
							pgcode.InvalidColumnReference,
							"cannot drop column %s as it is used as the REGIONAL BY ROW column reference",

Nit: "as it is used to store the region in a REGIONAL BY ROW table"?

---

pkg/sql/alter_table.go, line 412 at r1 (raw file):

							t.Column,
						),
						"You must change the table locality before dropping this table.",

Nit: Or alter the table to choose a different column to use for the region info?

---

pkg/sql/catalog/tabledesc/structured.go, line 4029 at r1 (raw file):

func (desc *wrapper) GetRegionalByRowTableRegionColumnName() (tree.Name, error) {
	if !desc.IsLocalityRegionalByRow() {
		return "", errors.Newf("%s is not REGIONAL BY ROW", desc.Name)

I'd think that this should be an Assertf.

[otan]

bors r=ajstorm

[otan]

bors r-

[craig]

Canceled.

[otan]

bors r=ajstorm

[craig]

Build succeeded:

• GitHub CI (Cockroach)