MySQL Performance Monitoring and Query Analysis

In this guide, we will explore various methods and tools to monitor the performance of MySQL databases and analyze query execution plans. By understanding and utilizing these techniques, you can optimize your database performance, identify slow queries, and improve overall efficiency.

Introduction

Efficient database performance is crucial for ensuring the smooth operation of applications. Monitoring system performance parameters, analyzing slow query logs, and understanding query execution plans are essential tasks for database administrators. This guide provides a comprehensive overview of these processes, focusing on MySQL.

Viewing System Performance Parameters

MySQL provides several ways to monitor performance parameters using the SHOW STATUS statements. These parameters help you understand the current state and performance of your MySQL server.

Syntax format:

SHOW [GLOBAL | SESSION] STATUS LIKE 'parameter';

Commonly Used Performance Parameters

Parameter Name	Description
connection	Number of connections to the MySQL server
uptime	MySQL server online time
slow_queries	Number of slow queries
innodb_rows_read	Number of rows returned by select queries
innodb_rows_inserted	Number of rows inserted by insert operations
innodb_rows_updated	Number of rows updated by update operations
innodb_rows_deleted	Number of rows deleted by delete operations
com_select	Number of query operations
com_insert	Number of insert operations (batch inserts count as one)
com_update	Number of update operations
com_delete	Number of delete operations
last_query_cost	SQL query cost

Slow Query Log (Locating Slow Executing SQL)

The slow query log is an essential tool for identifying SQL statements that are performing poorly. This log records statements whose response time exceeds a defined threshold (long_query_time).

Enabling and Using Slow Query Logs

By default, MySQL does not enable the slow query log. You need to manually enable it and set the appropriate parameters.

Check if the slow query log is enabled:

mysql> SHOW VARIABLES LIKE 'slow_query_log';
+----------------+-------+
| Variable_name  | Value |
+----------------+-------+
| slow_query_log | OFF   |
+----------------+-------+
1 row in set (0.01 sec)

Enable the slow query log:

mysql> SET GLOBAL slow_query_log = ON;
Query OK, 0 rows affected (0.12 sec)

mysql> SHOW VARIABLES LIKE 'slow_query_log%';
+---------------------+--------------------------------+
| Variable_name       | Value                          |
+---------------------+--------------------------------+
| slow_query_log      | ON                             |
| slow_query_log_file | /var/lib/mysql/rqtanc-slow.log |
+---------------------+--------------------------------+
2 rows in set (0.00 sec)

Query the long_query_time threshold:

mysql> SHOW VARIABLES LIKE 'long_query_time';
+-----------------+-----------+
| Variable_name   | Value     |
+-----------------+-----------+
| long_query_time | 10.000000 |
+-----------------+-----------+
1 row in set (0.00 sec)

Modify the long_query_time threshold:
Edit /etc/my.cnf:

long_query_time = 5

Restart MySQL:

[root@rqtanc ~]# systemctl restart mysqld.service

Check the number of slow queries:

mysql> SHOW STATUS LIKE 'slow_queries';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Slow_queries  | 0     |
+---------------+-------+
1 row in set (0.00 sec)

Slow Query Log Analysis Tool: mysqldumpslow

The mysqldumpslow tool helps you parse and summarize the MySQL slow query log.

View mysqldumpslow help information:

[root@rqtanc ~]# mysqldumpslow --help
Usage: mysqldumpslow [OPTS...] [LOGS...]

Parse and summarize the MySQL slow query log. Options are

  --verbose    verbose
  --debug      debug
  --help       write this text to standard output

  -v           verbose
  -d           debug
  -s ORDER     what to sort by (al, at, ar, c, l, r, t), 'at' is default
                al: average lock time
                ar: average rows sent
                at: average query time
                 c: count
                 l: lock time
                 r: rows sent
                 t: query time  
  -r           reverse the sort order (largest last instead of first)
  -t NUM       just show the top n queries
  -a           don't abstract all numbers to N and strings to 'S'
  -n NUM       abstract numbers with at least n digits within names
  -g PATTERN   grep: only consider stmts that include this string
  -h HOSTNAME  hostname of db server for *-slow.log filename (can be wildcard),
               default is '*', i.e. match all
  -i NAME      name of server instance (if using mysql.server startup script)
  -l           don't subtract lock time from total time

Source of the analysis file:

mysql> SHOW VARIABLES LIKE 'slow_query_log_file%';
+---------------------+--------------------------------+
| Variable_name       | Value                          |
+---------------------+--------------------------------+
| slow_query_log_file | /var/lib/mysql/rqtanc-slow.log |
+---------------------+--------------------------------+
1 row in set (0.00 sec)

Execute the following statements for analysis:

[root@rqtanc ~]# mysqldumpslow -a -s t -t 5 /var/lib/mysql/rqtanc-slow.log

Reading mysql slow query log from /var/lib/mysql/rqtanc-slow.log

Count: 1  Time=0.00s (0s)  Lock=0.00s (0s)  Rows=0.0 (0), 0 users@0 hosts

Died at /usr/bin/mysqldumpslow line 162, <> chunk 1.

Viewing SQL Execution Costs: SHOW PROFILE

The SHOW PROFILE command provides insights into the execution costs of SQL statements. For more detailed information, refer to MySQL's official documentation on SQL execution processes and principles.

Analyzing the Query Statement: EXPLAIN

The EXPLAIN statement is a powerful tool for analyzing query execution plans. It helps you understand how MySQL executes queries, allowing you to optimize them for better performance.

Basic syntax:

mysql> EXPLAIN SELECT 1;

+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+

| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |

+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+

|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | No tables used |

+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+

1 row in set, 1 warning (0.00 sec)

Relevant Descriptions of EXPLAIN Output Columns

Column	Description
id	Unique id for each select keyword in a query.
select_type	Type of select query (e.g., SIMPLE, PRIMARY, SUBQUERY).
table	Table name involved in the query.
partitions	Matching partition information.
type	Access method for the table.
possible_keys	Possible indexes that might be used.
key	Actual index used.
key_len	Length of the index used.
ref	Columns or constants that are compared to the index.
rows	Estimated number of rows to be read.
filtered	Percentage of rows filtered by the query conditions.
Extra	Additional information about the query execution.

EXPLAIN Output Format Syntax

Traditional format: tabular format

JSON format:
Outputs the query execution plan as data in JSON format, suitable for automated processing and analysis.

Tree format:
Provides a more readable, tree-structured format, with each node representing an operation in the query execution plan.

Extended format (EXTENDED):
Includes additional detailed information, such as operation status, scan methods, and index lengths, useful for in-depth analysis and performance tuning.

Using SHOW WARNINGS

The SHOW WARNINGS command displays warning messages generated by recently executed statements. These warnings can help identify and resolve implementation issues or unexpected situations.

Warning messages may include:

• Warning: The code or number of the warning.
• Level: The severity level (Note, Warning, or Error).
• Message: A description of the warning.

Analyzing the Optimizer Execution Plan: trace

The optimizer_trace functionality tracks various decisions made by the

optimizer (e.g., table access methods, cost calculations, transformations) and records the results in the information_schema.optimizer_trace table. This function is disabled by default and needs to be manually enabled.

Enabling optimizer_trace

Enable trace and set the format to JSON. Also, set the maximum memory size that the trace can use to ensure complete display during the parsing process.

MySQL Monitoring Analysis View: sys.schema

The sys.schema provides various views to monitor and analyze MySQL performance. These views include host-related summaries, InnoDB information, I/O usage, memory usage, connection and session information, table statistics, index usage, and user-related statistics.

Conclusion

By effectively utilizing the tools and techniques described in this guide, you can monitor and optimize the performance of your MySQL databases. Understanding system performance parameters, analyzing slow queries, and interpreting query execution plans are essential skills for any database administrator. Regularly performing these tasks will help ensure your databases run efficiently and meet the performance needs of your applications.

Comments

[BernerT]

This is a fantastic guide on MySQL performance monitoring and query analysis! Could you elaborate more on the difference between the SHOW PROFILE and the EXPLAIN command for understanding query costs?

[Mark Yu]

SHOW PROFILE

Purpose: SHOW PROFILE provides a detailed breakdown of the execution time for a query that has already been executed. It helps diagnose where time is being spent in the various stages of query execution.

Details Provided: SHOW PROFILE returns a step-by-step account of the execution stages of a query, including:

• Starting: Initial setup before the query execution.
• Checking permissions: Verifying user permissions for the query.
• Opening tables: Opening the necessary tables for the query.
• System lock: Acquiring any necessary locks on the tables.
• Optimizing: Query optimization phase.
• Statistics: Gathering statistics about tables and indexes.
• Preparing: Preparing the execution plan.
• Executing: Actual execution of the query.
• Sending data: Retrieving the data and sending it to the client.
• Cleaning up: Cleaning up and releasing resources post-execution.

Use Cases:

• Diagnose Performance Issues: Identify which part of the query execution is taking the most time.
• Fine-tuning: Focus on specific stages that might be optimized further (e.g., optimizing index usage if a lot of time is spent on table scans).

Example Usage:

SET profiling = 1;
SELECT * FROM my_table WHERE id = 123;
SHOW PROFILE FOR QUERY 1;

Output Example:

+------------------------------+----------+
| Status                       | Duration |
+------------------------------+----------+
| starting                     | 0.000027 |
| checking query cache for query| 0.000005|
| Opening tables               | 0.000021 |
| System lock                  | 0.000003 |
| Table lock                   | 0.000009 |
| init                         | 0.000017 |
| optimizing                   | 0.000006 |
| statistics                   | 0.000020 |
| preparing                    | 0.000007 |
| executing                    | 0.000003 |
| Sending data                 | 0.000086 |
| end                          | 0.000002 |
| query end                    | 0.000002 |
| closing tables               | 0.000004 |
| freeing items                | 0.000011 |
| cleaning up                  | 0.000003 |
+------------------------------+----------+

EXPLAIN

Purpose: EXPLAIN analyzes and predicts the execution plan for a query before it is executed. It helps understand how MySQL intends to execute the query, which can guide optimizations before running the query.

Details Provided: EXPLAIN returns a detailed plan of how the query will be executed, including:

• id: The sequence identifier of the query.
• select_type: The type of SELECT query (e.g., SIMPLE, PRIMARY, SUBQUERY).
• table: The table name involved in the query.
• type: The join type, indicating how tables are accessed (e.g., ALL, index, range).
• possible_keys: Possible indexes that might be used.
• key: The actual index used.
• key_len: The length of the index used.
• ref: The columns or constants compared to the index.
• rows: The estimated number of rows to be read.
• filtered: The estimated percentage of rows filtered by the query conditions.
• Extra: Additional information about the query execution (e.g., Using index, Using temporary).

Use Cases:

• Optimize Query Performance: Understand the intended execution path and identify inefficiencies (e.g., full table scans that might benefit from indexing).
• Index Optimization: Ensure the query uses the most efficient indexes available.

Example Usage:

EXPLAIN SELECT * FROM my_table WHERE id = 123;

Output Example:

+----+-------------+----------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table    | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+----------+-------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | my_table | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Using index |
+----+-------------+----------+-------+---------------+---------+---------+-------+------+----------+-------------+

Key Differences

Execution Timing:

• SHOW PROFILE: Provides details after a query has been executed.
• EXPLAIN: Predicts execution details before the query is executed.

Depth of Information:

• SHOW PROFILE: Focuses on the time spent in each stage of query execution, useful for diagnosing performance bottlenecks post-execution.
• EXPLAIN: Provides an overview of how MySQL plans to execute the query, including join types, index usage, and row estimates.

Use Case Scenarios:

• SHOW PROFILE: Ideal for post-execution analysis to identify which parts of the query are slow.
• EXPLAIN: Best for pre-execution analysis to understand and optimize the query execution plan.