How expensive SQLs can impact MySQL Performance?

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Expensive SQLs can have a significant impact on MySQL performance, as they consume a lot of resources and can slow down the entire system. Here are a few ways that expensive SQLs can affect MySQL performance:

1. High CPU usage: Expensive SQLs can consume a lot of CPU resources, which can lead to increased system load and decreased performance for other processes running on the same machine.
2. High memory usage: Expensive SQLs can also consume a lot of memory, which can lead to increased swap usage and decreased performance for other processes running on the same machine.
3. I/O contention: Expensive SQLs can also cause a lot of disk I/O, which can lead to increased disk contention and decreased performance for other processes running on the same machine.
4. Long-running queries: Expensive SQLs can take a long time to complete, which can lead to increased wait times for other queries and decreased performance for other processes running on the same machine.
5. Blocking other queries: Expensive SQLs can also block other queries from being executed, which can lead to increased wait times for other queries and decreased performance for other processes running on the same machine.
6. Deadlocks: Expensive SQLs can also cause deadlocks, which can lead to increased wait times for other queries and decreased performance for other processes running on the same machine.
7. Table Locking: Expensive SQLs can also cause table locking, which can lead to increased wait times for other queries and decreased performance for other processes running on the same machine.

To avoid these problems, it's important to monitor SQL performance and identify and optimize expensive SQLs. This can be done by using tools like the MySQL EXPLAIN command, which allows you to analyze the execution plan of a query and identify potential performance bottlenecks. Additionally, using indexes, partitioning, and denormalizing tables can also help to optimize the performance of SQLs. It's also important to use a good monitoring tool that can provide real-time performance metrics and alerts, enabling you to quickly identify and diagnose performance issues. Additionally, MySQL provides a performance_schema database, where you can find various performance metrics. You can also use the SHOW FULL PROCESSLIST command to see all active processes, including their process IDs and other information such as the user and the current query. Once you have identified the expensive queries, you can use the EXPLAIN command to get more information about the query execution plan, like which indexes are being used, the number of rows being scanned, etc. This can give you more information about what's causing the query to be slow and help you determine the best way to optimize it. I have copied below a Python code for monitoring top processes by latency in MySQL

import mysql.connector

# Connect to the database
cnx = mysql.connector.connect(user='username', password='password', host='hostname', database='dbname')
cursor = cnx.cursor()

# Define the query to retrieve process information
query = "SELECT * FROM information_schema.processlist ORDER BY time DESC LIMIT 10"

while True:
    cursor.execute(query)
    rows = cursor.fetchall()

    # Print the process information
    print("Process ID | User | Host | DB | Command | Time | State | Info")
    for row in rows:
        pid = row[0]
        user = row[1]
        host = row[2]
        db = row[3]
        command = row[4]
        time = row[5]
        state = row[6]
        info = row[7]
        print(f"{pid} | {user} | {host} | {db} | {command} | {time} | {state} | {info}")

    # Wait for a few seconds before running the query again
    time.sleep(5)

cursor.close()
cnx.close()

This script uses the mysql.connector library to connect to the MySQL server and retrieve process information from the information_schema.processlist table. The script use a while loop to continuously execute the query and retrieve the process information, and print the process information to the console. The script will retrieve the top 10 process by latency, it sorts the result by the "time" column in descending order, so it will show the process which are running for the longest time. It's important to note that you need to replace username, password, hostname, dbname with the appropriate values for your MySQL server. You can customize this script as per your requirements, like filtering the process or storing the information in a file or in a database for future reference.