feat: add a LIMIT 1 on EXISTS subqueries to limit network overhead

[systay]

Description

This pull request introduces an optimization to our query planner by adding a LIMIT 1 clause to EXISTS subqueries. This change aims to minimize the amount of data being transferred over the wire, resulting in improved performance and reduced resource consumption.

Changes

• Optimization Addition: Added a LIMIT 1 clause to all EXISTS subqueries generated by the planner.
• Impact: The optimization reduces the amount of data sent over the network and processed by the database, improving overall query performance.

Example

Before Optimization:

SELECT * FROM orders o WHERE EXISTS (SELECT 1 FROM order_items oi WHERE oi.order_id = o.id);

After Optimization:

SELECT * FROM orders o WHERE EXISTS (SELECT 1 FROM order_items oi WHERE oi.order_id = o.id LIMIT 1);

History

Our planner used to do this, but in #13750 when we refactored the subquery handling in the planner, we lost this optimisation. It was left as a TODO that wasn't picked up until now.

Related Issue(s)

Fixes #16149
Optimisation lost in #13750

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on 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.

[systay]

Paired with @arthurschreiber on this

[codecov]

Codecov Report

Attention: Patch coverage is 70.58824% with 25 lines in your changes missing coverage. Please review.

Project coverage is 68.55%. Comparing base (8a01d03) to head (dd9f965).
Report is 6 commits behind head on main.

Files	Patch %	Lines
.../vt/vtgate/planbuilder/operators/query_planning.go	67.53%	25 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16153      +/-   ##
==========================================
+ Coverage   68.54%   68.55%   +0.01%     
==========================================
  Files        1544     1544              
  Lines      197737   197823      +86     
==========================================
+ Hits       135532   135625      +93     
+ Misses      62205    62198       -7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[GuptaManan100]

Love it 🎉

[harshit-gangal]

should a negative number be not checked here, otherwise the limit engine can panic.

[harshit-gangal]

We should add a test case for it.