Fix dual merging in outer join queries

[GuptaManan100]

Description

This PR resolves the issue described in #15958.

The problem was that we were merging the dual query with the sharded route, and each of the shards ended up returning one row. We should not be merging dual queries into sharded routes. We can only merge them if the other side is a single shard routing.

Related Issue(s)

• Fixes Bug Report: Dual left join query returns more results than it should #15958

Checklist

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Deployment Notes

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[codecov]

Codecov Report

Attention: Patch coverage is 79.16667% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 68.25%. Comparing base (6ff462c) to head (8d67079).
Report is 24 commits behind head on main.

Files	Patch %	Lines
go/vt/vtgate/planbuilder/operators/join_merging.go	79.16%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15959      +/-   ##
==========================================
- Coverage   68.47%   68.25%   -0.22%     
==========================================
  Files        1562     1562              
  Lines      197052   197334     +282     
==========================================
- Hits       134936   134697     -239     
- Misses      62116    62637     +521     

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[harshit-gangal]

Is this not a problem only with left join and that too when the dual table is on the left hand side of the join.

[harshit-gangal]

Is this not a problem only with left join and that too when the dual table is on the left hand side of the join.

I see the code follows this rule, it was missing in the PR description.

[GuptaManan100]

@harshit-gangal @systay Had to change quite a few things to get everything to work. Could I request you two to review it again?

A lot of code has changed since the approval.

[systay]

nitpick: it'd be great to test these in an end to end test as well