[YSQL] Avoid unnecessary secondary index writes for UPDATE on table with check constraints

[pkj415]

Jira Link: DB-1356
About single row update optimization - In case of an UPDATE that touches just a single row (and satisfies a few more requirements such as - no check constraints, no triggers, no secondary indexes etc), we have a bunch of optimizations in place (avoiding multi-row transactions, reducing the number of scans, etc).

Avoid Secondary indexes UPDATE optimization - As part of cf94181, updates to unrelated secondary indexes are avoided in UPDATE execution path - when none of the index/included columns of an index are updated by UPDATE statement.

The second optimization above is orthogonal to the idea of single-row optimization - but it is placed in the code path that checks all requirements for single-row optimization. So, there are bugs where the second optimization doesn't come into affect when some requirements of the single row optimization aren't met. Fix this.

[pkj415]

Right now, we will only fix the case where secondary index UPDATE optimization is not being performed due to CHECK constraints on the table (a requirement for single row update optimization). [D11223]

Untangling these two optimizations for all other cases - such as in presence of triggers, more than one input sources etc, requires more digging into and carful changes. D11192 was an attempt to do that, but is saved for future.