planbuilder bugfix: expose columns through derived tables

[systay]

Description

Somehow we missed adding the relevant tests, and the issue resurfaced.

For this query:

SELECT c.column_name FROM user c JOIN (
    SELECT table_name FROM unsharded LIMIT 1) AS tables 
ON tables.table_name = c.table_name

The planner changes the sides, because we use a nested loop join, and it's much easier to have the limited side on the left - we don't even have the code to support LIMIT on the RHS of our joins.

Somehow, when planning the limited LHS of the join, we did not pull out the table_name column from the derived table, and instead returned a literal 1:

select 1 from (select table_name from unsharded limit 1) as `tables`

This value was then used on the RHS to search for matches, and most often there were none.

Related Issue(s)

Fixes #12649

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.