How do you diagnose and fix the causes of low SQL query performance?

To diagnose low SQL query performance in C# projects, I use the following approach:

1. Query execution plan analysis — using SQL Server Management Studio (SSMS) or similar tools to see how the DBMS executes the query, identifying full table scans and missing indexes.

2. Query profiling — using SQL Profiler or built-in tools to monitor execution time and locks.

3. Query optimization — rewriting queries, removing unnecessary JOINs and subqueries, adding necessary indexes.

4. Parameterization check — avoiding dynamic SQL to enable plan caching.

5. Server load check — ensuring the server is not overloaded, and there are no locks or long transactions.

Example of optimization in C# using parameterized queries:

using (var connection = new SqlConnection(connectionString))
{
    connection.Open();
    var command = new SqlCommand("SELECT * FROM Users WHERE Age > @age", connection);
    command.Parameters.AddWithValue("@age", 18);
    using (var reader = command.ExecuteReader())
    {
        while (reader.Read())
        {
            // data processing
        }
    }
}

This approach helps identify bottlenecks and improve query performance.