Push down derived tables under route when possible

[systay]

Description

When we are checking if two parts of the query can be merged, either through a join or through a subquery, if we are also dealing with derived tables, things become tricky.

To simplify this logic, we want to separate the decision if we can push a derived table underneath a route or not, from the decision if we can merge two routes into a single one.

This PR pushes down the derived table under the route when possible.
It is possible, when either the derived table is a single-shard query, or all of these conditions must be met:

• The derived table can't do a LIMIT
• If the derived table contains grouping, it has to be grouping on a unique vindex column
• If no grouping is done, there can't be any aggregations.

We also use this logic to figure out when we can have a derived table on the RHS of a join that is performed on the vtgate side. Since these joins are nested loop joins, to guarantee that the RHS of a join returns the correct values, we check the above rules. If the derived table doesn't live up the conditions listed above, we can't have this input on the RHS. The planner will try to switch sides, and if the other side also can't be on the RHS, it will give up.

A lot of this logic has been extracted from #10966 together with @arthurschreiber

More PRs are coming that will build on this logic.

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

Related issues

Fixes #11093
Followed by #10966

[vitess-bot]

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[frouioui]

We could insert only one row in the user table, the row returned by the derived table will always be the same.

[harshit-gangal]

I think we should assert the results as well in all the tests in this file.

[systay]

Sure, can do that. The bugs all showed up when comparing w mysql, but it might be interesting to assert on the values as well