Add vexplain trace

[systay]

Description

This PR introduces a new vexplain trace command that enhances query analysis by tracking data flow between operators during query execution. Instead of returning the normal results from a query, vexplain trace provides an execution plan with detailed runtime statistics, including:

• NoOfCalls: The number of times each operator was invoked during query execution.
• AvgNumberOfRows: The average number of rows returned per call for each operator.
• MedianNumberOfRows: The median number of rows returned per call for each operator.

Example

-- Example query
VEXPLAIN TRACE SELECT
    o.o_orderpriority,
    EXTRACT(YEAR FROM o.o_orderdate) AS order_year,
    COUNT(DISTINCT o.o_orderkey) AS order_count,
    SUM(l.l_extendedprice * (1 - l.l_discount)) AS total_revenue
FROM
    orders o
JOIN
    lineitem l ON o.o_orderkey > l.l_orderkey
WHERE
    o.o_orderdate BETWEEN '1995-01-01' AND '1996-12-31'
GROUP BY
    o.o_orderpriority,
    EXTRACT(YEAR FROM o.o_orderdate)
ORDER BY
    o.o_orderpriority,
    order_year

Here is the output of that query:

{
    "OperatorType": "Aggregate",
    "Variant": "Ordered",
    "NoOfCalls": 1,
    "AvgNumberOfRows": 10,
    "MedianNumberOfRows": 10,
    // ... other fields ...
    "Inputs": [
        {
            "OperatorType": "Sort",
            "Variant": "Memory",
            "NoOfCalls": 1,
            "AvgNumberOfRows": 1995,
            "MedianNumberOfRows": 1995,
            // ... other fields ...
            "Inputs": [
                {
                    "OperatorType": "Route",
                    "Variant": "Scatter",
                    "NoOfCalls": 2,
                    "AvgNumberOfRows": 1000,
                    "MedianNumberOfRows": 1000,
                    // ... other fields ...
                },
                {
                    "OperatorType": "Route",
                    "Variant": "Scatter",
                    "NoOfCalls": 2000,
                    "AvgNumberOfRows": 1.995,
                    "MedianNumberOfRows": 2,
                    // ... other fields ...
                }
            ]
        }
    ]
}

Motivation

Understanding the internal workings of query execution is crucial for performance tuning and optimization. While traditional VEXPLAIN commands show the planned execution path, they lack runtime statistics. The vexplain trace command fills this gap by providing real-time data flow information, allowing for deeper insights into query performance and helping to identify bottlenecks more effectively.

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

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[codecov]

Codecov Report

Attention: Patch coverage is 76.37795% with 30 lines in your changes missing coverage. Please review.

Project coverage is 68.92%. Comparing base (f97ea77) to head (40b1edd).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/engine/vexplain.go	68.33%	19 Missing ⚠️
go/vt/vtgate/vcursor_impl.go	76.00%	6 Missing ⚠️
go/vt/vtgate/engine/plan_description.go	85.18%	4 Missing ⚠️
go/vt/vtgate/planbuilder/vexplain.go	90.90%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16768   +/-   ##
=======================================
  Coverage   68.92%   68.92%           
=======================================
  Files        1566     1566           
  Lines      201886   201983   +97     
=======================================
+ Hits       139149   139222   +73     
- Misses      62737    62761   +24     

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[GuptaManan100]

LGTM!

[harshit-gangal]

There is an extra logging incase of retry. As this is recorded for testing in a controlled way, can be ignored.