Add VEXPLAIN KEYS for improved sharding key selection

[systay]

Description:

This PR introduces a new VEXPLAIN type called KEYS, designed to assist developers and database administrators in making informed decisions about sharding keys. The VEXPLAIN KEYS command provides a concise summary of the query structure, highlighting columns used in joins, filters, and grouping operations.

Key features:

1. Identifies potential sharding key candidates
2. Highlights SARGable columns in the WHERE clause
3. Shows grouping columns for aggregation queries
4. Reveals join conditions between tables

The output focuses on the structural elements of the query that are most relevant to sharding decisions, without executing the query or providing execution statistics.

Example usage:

mysql> vexplain keys select u.foo, ue.bar, count(*) from user u join user_extra ue on u.id = ue.user_id where u.name = 'John Doe' group by 1, 2;
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ColumnUsage                                                                                                                                                                                |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| {
        "groupingColumns": [
                "user.foo",
                "user_extra.bar"
        ],
        "joinColumns": [
                "user.id",
                "user_extra.user_id"
        ],
        "filterColumns": [
                "user.name"
        ],
        "statementType": "SELECT"
} |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

This output provides valuable insights for sharding decisions:

• groupingColumns: Potential candidates for secondary sharding keys
• joinColumns: Important for maintaining data locality across shards
• filterColumns: SARGable columns that could be used as primary sharding keys
• statementType: Helps understand the query type (SELECT, INSERT, UPDATE, DELETE)

By analyzing this information across multiple queries, developers can make more informed decisions about sharding strategies, potentially improving query performance and data distribution.

Related Issue(s)

#16768
Docs: vitessio/website#1853

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• 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

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[codecov]

Codecov Report

Attention: Patch coverage is 94.02985% with 4 lines in your changes missing coverage. Please review.

Project coverage is 69.45%. Comparing base (969e018) to head (ac6138e).

Files with missing lines	Patch %	Lines
go/vt/vtgate/planbuilder/operators/keys.go	93.75%	3 Missing ⚠️
go/vt/vtgate/planbuilder/vexplain.go	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16830      +/-   ##
==========================================
+ Coverage   69.41%   69.45%   +0.03%     
==========================================
  Files        1570     1571       +1     
  Lines      202945   203004      +59     
==========================================
+ Hits       140880   140993     +113     
+ Misses      62065    62011      -54     

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[GuptaManan100]

LGTM!

[deepthi]

TIL SARGable https://dba.stackexchange.com/questions/162263/what-does-the-word-sargable-really-mean