Merge subqueries that "join" on lookup index columns.

[arthurschreiber]

Description

This pull request tries to improve the handling of queries that contain IN or = operations with one operant being a lookup vindex column, and the other operant being a subquery that selects a column from the same or a different table that uses the same lookup vindex.

Here's an example:

SELECT music.id FROM music WHERE music.id IN (SELECT music.id FROM music WHERE music.user_id IN (1, 2, 3))

Without this change, the planner breaks this query into two parts, one part executing the inner query and another for the outer query.

But because we're selecting data from music.id in the inner query, which has a unique lookup index defined, and comparing it to music.id in the outer query, we can merge the inner and outer queries together.

I'm fairly confident the code changes are bad, but I think they show that this change is viable. 😅

Related Issue(s)

Checklist

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[vitess-bot]

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[glortho]

Using this branch, we're seeing this error occasionally when trying to vtexplain some of these types of queries:

[BUG] unknown type encountered: *physical.Table (transformToLogicalPlan)

Eyeballing them, it seems all the problematic ones have a LEFT OUTER JOIN + at least one other join in common.

I'll send examples to you separately @arthurschreiber.

[arthurschreiber]

Using this branch, we're seeing this error occasionally when trying to vtexplain some of these types of queries:

I believe this was fixed with some of the recent changes I pushed. 👍

[arthurschreiber]

This pull request also contains the change described in #11072 (comment)

[harshit-gangal]

This is really great optimization.
See if some helpful comments can be added on top of the functions to help understand the merge rules.