Fixing Column aliases in outer join queries

[GuptaManan100]

Description

This PR fixes the issue specified in #15383.

As the issue describes, the problem occurred when we had column aliasing on top of an outer join query. While investigating, it was noticed that we aren't pushing the alias down for outer join queries, and eventually were also getting rid of the Projection operator altogether, causing the aliases to be lost.

The fix in this PR entails 2 things -

1. Projection operator shouldn't be omitted if there are column aliases present that we couldn't push down.
2. Extend SimpleProjection to also change column names rather than just reordering and truncating output.

Related Issue(s)

• Fixes Bug Report: Column Aliases are not respected in some queries that have outer join clauses #15383

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

Deployment Notes

[vitess-bot]

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[harshit-gangal]

We should only keep the SimpleProjection when the query has column with alias otherwise we should remove this if possible.
The new changes seem to [always/most of the time] keep it.

[codecov]

Codecov Report

Attention: Patch coverage is 89.28571% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 65.44%. Comparing base (696fe0e) to head (604503e).
Report is 70 commits behind head on main.

Files	Patch %	Lines
go/vt/vtgate/engine/simple_projection.go	84.61%	2 Missing ⚠️
go/vt/vtgate/planbuilder/operators/projection.go	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15384      +/-   ##
==========================================
- Coverage   67.41%   65.44%   -1.98%     
==========================================
  Files        1560     1562       +2     
  Lines      192752   193937    +1185     
==========================================
- Hits       129952   126923    -3029     
- Misses      62800    67014    +4214     

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[harshit-gangal]

👍

[chhan-coupang]

I would like to ask if this fix can be reverse-merged into v19 version?

[GrahamCampbell]

v19 is not affected by this issue (EDIT: for the same reason).

[chhan-coupang]

@GrahamCampbell Thanks for your reply!
I encountered this problem in v19.0.6-percona80, and it was tested in v20.
v19.0.6-percona80:

v20.0.2-percona80:

CREATE TABLE `item1` (
  `i_id` int NOT NULL,
  `i_im_id` int DEFAULT NULL,
  `i_name` varchar(24) DEFAULT NULL,
  `i_price` decimal(5,2) DEFAULT NULL,
  `i_data` varchar(50) DEFAULT NULL,
  PRIMARY KEY (`i_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

select a.i_name as name1,b.i_name as name2,b.i_id as id2, c.i_name as name3 
from item1 a inner join item2 b on a.i_id = b.i_im_id 
LEFT OUTER JOIN item3 c on a.i_id = c.i_id 
where a.i_id < 10 limit 1;

[chhan-coupang]

I would like to ask if this fix can be reverse-merged into v19 version? Please help confirm.

[nonbb]

v19 is affected by this issue, I also am encountering the same for v19.0.8-mysql80 🙇 @GrahamCampbell

[mattlord]

@GrahamCampbell, @chhan-coupang , and @nonbb this was backported to the v19 release branch here: #17418

Thanks!