How to expose and monitor database query execution times with expvar in Go?

To expose and monitor database query execution times with expvar in Go, you can follow these steps:

Import the necessary packages into your project:

import (
	"expvar"
	"database/sql"
	"time"
)

Create a custom struct to hold the database query execution time metrics:

type DatabaseStats struct {
	TotalQueries   *expvar.Int
	TotalTime      *expvar.Int
	AverageTime    *expvar.Float
	MinTime        *expvar.Duration
	MaxTime        *expvar.Duration
	LastQueryTime  *expvar.Duration
}

Initialize the metric variables in your main function:

func main() {
	dbStats := &DatabaseStats{
		TotalQueries:  expvar.NewInt("total_queries"),
		TotalTime:     expvar.NewInt("total_time"),
		AverageTime:   expvar.NewFloat("average_time"),
		MinTime:       expvar.NewDuration("min_time"),
		MaxTime:       expvar.NewDuration("max_time"),
		LastQueryTime: expvar.NewDuration("last_query_time"),
	}
	// ...
}

Wrap your database query execution code with timing and metric updates:

func executeQuery(db *sql.DB, dbStats *DatabaseStats) {
	startTime := time.Now()

	// Your database query execution code here

	elapsedTime := time.Since(startTime)
	dbStats.LastQueryTime.Set(elapsedTime)
	dbStats.TotalTime.Add(int64(elapsedTime))
	dbStats.TotalQueries.Add(1)

	currentTotalTime := dbStats.TotalTime.Value()
	currentTotalQueries := dbStats.TotalQueries.Value()

	dbStats.AverageTime.Set(float64(currentTotalTime) / float64(currentTotalQueries))
	
	minQueryTime := time.Duration(dbStats.MinTime.Value())
	maxQueryTime := time.Duration(dbStats.MaxTime.Value())
	
	if elapsedTime < minQueryTime || minQueryTime == 0 {
		dbStats.MinTime.Set(int64(elapsedTime))
	}
	
	if elapsedTime > maxQueryTime {
		dbStats.MaxTime.Set(int64(elapsedTime))
	}
}

Finally, periodically expose the database statistics by serving the expvar HTTP endpoint:

func exposeStats() {
	expvar.Publish("db_stats", expvar.Func(func() interface{} {
		return dbStats
	}))
	http.ListenAndServe(":8080", nil)
}

Make sure to call exposeStats() in your main function to start serving the expvar endpoint.

This setup will expose the various database query execution time metrics through the /debug/vars endpoint, which you can view in your browser or fetch programmatically.