sql: avoid materializing all rows from a subquery into a tuple

[yuzefovich]

Depending on the execution mode of the subquery, we might be materializing all rows (which can be arbitrary large number in general case) into a single tuple to be stored as a single tree.Datum. This is suboptimal, and we should find a way to avoid this materialization. See #64464 for an example of OOM caused by this behavior.

Jira issue: CRDB-7208

[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it in
10 days to keep the issue queue tidy. Thank you for your contribution
to CockroachDB!