What actions should be taken if a database query is very slow?

If a database query is running very slowly, the following steps should be taken:

1. Query profiling: Use database profiling tools (e.g., EXPLAIN in SQL) to analyze the execution plan of the query.
2. Query optimization: Check if the query can be rewritten more efficiently — remove unnecessary JOINs, use subqueries, limit the selection.
3. Indexes: Ensure that indexes are created on the necessary columns, especially for filters and JOINs.
4. Caching: Consider caching query results if the data does not change often.
5. Connection parameters: Check connection settings and connection pool in Go to avoid delays.
6. Parallelism: If possible, split the query into several parallel queries to speed up processing.

Example of using EXPLAIN in Go with PostgreSQL:

rows, err := db.Query("EXPLAIN ANALYZE SELECT * FROM users WHERE age > $1", 30)
if err != nil {
    log.Fatal(err)
}
var plan string
for rows.Next() {
    err := rows.Scan(&plan)
    if err != nil {
        log.Fatal(err)
}
    fmt.Println(plan)
}

This will help understand why the query is slow and where the bottlenecks are.