When evaluating the lookup joins if LookupColumnsAreKey is set, I believe it'd be valid to use Get requests for tables with a single column family in all cases. In particular, currently if we're looking up into the secondary non-UNIQUE index, we issue a Scan request, but given lookup columns form a key, we know that the lookup will result in at most one KV, so I believe it'd be safe to use a Get request.

We should also audit the index join in case we can use Get requests in more cases there too.

We should think through whether presence of NULLs would affect this proposal.

Jira issue: CRDB-30490