sql: allow expression indexes to reference computed columns

[mgartner]

Currently we do not allow expression indexes to reference computed columns. This restriction comes from the restriction that computed column cannot reference other computed columns, presumably to prevent cycles. Because expression indexes are backed by inaccessible virtual computed columns, which cannot be referenced by any other schema expressions, we may be able to lift this restriction for expression indexes.

More context from #65703 (review):

rytaft:

statement error index element expression cannot reference computed columns
CREATE TABLE err (a INT, comp INT AS (a + 10) STORED, INDEX ((comp + 10)))

not sure if we care, but this doesn't cause an error in Postgres

mgartner:

Good point. If preventing cycles is the only reason why computed columns can't reference other computed columns, then we may be able to lift this restriction. Because these expression index computed columns will be inaccessible (they can't be referenced in other expressions), then it is guaranteed that there will not be a cycle.

Jira issue: CRDB-8766

[mgartner]

@ianjevans this will likely be a known limitation for expression indexes in the 21.2 release.

[ajwerner]

I think there's two approaches here, but the easiest is to special case the columns which are used for these expression indexes and allow them to have references to regular computed columns because we know there won't be cycles by construction.