Through a case, the author explains in detail the relevant parameters and usage of the MySQL 8.0 field information statistics mechanism.

Author: Yang Qilong

Net name "North in the South", senior DBA, mainly responsible for database architecture design and operation and maintenance platform development, good at database performance tuning and fault diagnosis.

Source of this article: original contribution

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background

A few days ago, a colleague was asking a question: a certain business INFORMATION_SCHEMAinserts data into the table based on the information (such as the maximum value) of the statistical table.

May I ask INFORMATION_SCHEMA.TABLESif AUTO_INCREMENTthe will be updated in time?

Let me talk about the conclusion first: yes!

This involves the information statistics mechanism or the frequency problem, which is mainly information_schema_stats_expirycontrolled by parameters.

Basic logic of table information update

By default, MySQL will efficiently retrieve the cached values of these columns from the system tables mysql.index_statsand mysql.table_statsinstead of obtaining statistics directly from the storage engine. If cached statistics are not available or have expired, MySQL retrieves the latest statistics from the storage engine and updates and caches their statistics in the mysql.index_statsand mysql.table_statsdictionary tables. Subsequent queries will retrieve cached statistics until the cached statistics expire.

It is worth noting that: MySQL restart or opening for the first time mysql.index_statsand mysql.table_statstable will not automatically update the cached statistics.

Core parameters

Core parameters information_schema_stats_expiryThe default is 86400 seconds. That is to say, the relevant statistical information is automatically collected every other day to information_schemathe following table fields in :

STATISTICS.CARDINALITY
TABLES.AUTO_INCREMENT
TABLES.AVG_ROW_LENGTH
TABLES.CHECKSUM
TABLES.CHECK_TIME
TABLES.CREATE_TIME
TABLES.DATA_FREE
TABLES.DATA_LENGTH
TABLES.INDEX_LENGTH
TABLES.MAX_DATA_LENGTH
TABLES.TABLE_ROWS
TABLES.UPDATE_TIME

The value of the parameter information_schema_stats_expirydetermines the time interval for collecting table statistics again, and the default is 86400 seconds. If it is set to 0, it means that the statistics information will be updated in real time , which will definitely affect some performance.

Querying statistics columns does not store or update statistics in mysql.index_statsand dictionary tables in the following cases:mysql.table_stats

When cached statistics have not yet expired.
When information_schema_stas_expiryset to 0.
When the MySQL server is in read-only, super read-only, transactional read-only or innodb_read_onlymode.
When querying also fetches Performance Schemadata from .

information_schema_stas_experityBoth global and session levels are supported, and each session can define its own expiration value. Statistics retrieved from the storage engine and cached by one session are available to other sessions.

test

This article takes MySQL 8.0.30 as an example for analysis.

2.1 Test preparation

  CREATE TABLE `sbtest1` (
  `id` int NOT NULL AUTO_INCREMENT,
  `k` int NOT NULL DEFAULT '0',
  `c` char(120) NOT NULL DEFAULT '',
  `pad` char(60) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB ;

2.2 Testing

View the current information information_schema.tablesin , the maximum value is 1200006, and the maximum value of self-increment in the table structure definition is also 1200006.sbtest1

master [localhost:22031] {msandbox} (test) > show variables like 'information_schema_stats_expiry' ;
+-----------------------------------------+---------+
| Variable_name                   | Value |
+-----------------------------------------+---------+
| information_schema_stats_expiry | 86400 |
+-----------------------------------------+---------+
1 row in set (0.02 sec)

master [localhost:22031] {msandbox} (test) > select  table_name, AUTO_INCREMENT  from information_schema.tables where  table_name='sbtest1';
+---------------+--------------------+
| TABLE_NAME | AUTO_INCREMENT |
+---------------+--------------------+
| sbtest1    |        1200006 |
+---------------+--------------------+
1 row in set (0.01 sec)

master [localhost:22031] {msandbox} (test) > show create table  sbtest1 \G
*************************** 1. row ***************************
       Table: sbtest1
Create Table: CREATE TABLE `sbtest1` (
  `id` int NOT NULL AUTO_INCREMENT,
  `k` int NOT NULL DEFAULT '0',
  `c` char(120) NOT NULL DEFAULT '',
  `pad` char(60) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1200006 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)

Insert new data, increment by 1.

master [localhost:22031] {msandbox} (test) > insert into sbtest1(k,c,pad) values(1,'c','cc');
Query OK, 1 row affected (0.00 sec)

master [localhost:22031] {msandbox} (test) > show create table  sbtest1 \G
*************************** 1. row ***************************
       Table: sbtest1
Create Table: CREATE TABLE `sbtest1` (
  `id` int NOT NULL AUTO_INCREMENT,
  `k` int NOT NULL DEFAULT '0',
  `c` char(120) NOT NULL DEFAULT '',
  `pad` char(60) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1200007 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)

But information_schema.tablesthe value in did not change.

master [localhost:22031] {msandbox} (test) > select  table_name, AUTO_INCREMENT  from information_schema.tables where  table_name='sbtest1';
+------------+----------------+
| TABLE_NAME | AUTO_INCREMENT |
+------------+----------------+
| sbtest1    |        1200006 |
+------------+----------------+
1 row in set (0.00 sec)

Set to update in real time

Modify information_schema_stats_expiryto 0.

master [localhost:22031] {msandbox} (test) > set  information_schema_stats_expiry=0;
Query OK, 0 rows affected (0.00 sec)

master [localhost:22031] {msandbox} (test) > show variables like 'information_schema_stats_expiry' ;
+---------------------------------+-------+
| Variable_name                   | Value |
+---------------------------------+-------+
| information_schema_stats_expiry | 0     |
+---------------------------------+-------+
1 row in set (0.00 sec)

master [localhost:22031] {msandbox} (test) > select  table_name, AUTO_INCREMENT  from information_schema.tables where  table_name='sbtest1';
+------------+----------------+
| TABLE_NAME | AUTO_INCREMENT |
+------------+----------------+
| sbtest1    |        1200007 |
+------------+----------------+
1 row in set (0.00 sec)

master [localhost:22031] {msandbox} (test) > show create table  sbtest1 \G
*************************** 1. row ***************************
       Table: sbtest1
Create Table: CREATE TABLE `sbtest1` (
  `id` int NOT NULL AUTO_INCREMENT,
  `k` int NOT NULL DEFAULT '0',
  `c` char(120) NOT NULL DEFAULT '',
  `pad` char(60) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1200007 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)

The inserted data is automatically incremented by 1, and the statistics of the auto-increment columninformation_schema.tables in the query are also updated in real time.

master [localhost:22031] {msandbox} (test) > insert into sbtest1(k,c,pad) values(1,'c','cc');
Query OK, 1 row affected (0.00 sec)

master [localhost:22031] {msandbox} (test) > select  table_name, AUTO_INCREMENT  from information_schema.tables where  table_name='sbtest1';
+------------+----------------+
| TABLE_NAME | AUTO_INCREMENT |
+------------+----------------+
| sbtest1    |        1200008 |
+------------+----------------+
1 row in set (0.00 sec)

master [localhost:22031] {msandbox} (test) > show create table  sbtest1 \G
*************************** 1. row ***************************
       Table: sbtest1
Create Table: CREATE TABLE `sbtest1` (
  `id` int NOT NULL AUTO_INCREMENT,
  `k` int NOT NULL DEFAULT '0',
  `c` char(120) NOT NULL DEFAULT '',
  `pad` char(60) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1200008 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)

Manually modifying the auto-increment column can also be updated in real time.

master [localhost:22031] {msandbox} (test) > alter table sbtest1  AUTO_INCREMENT=1200010;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

master [localhost:22031] {msandbox} (test) > select  table_name, AUTO_INCREMENT  from information_schema.tables where  table_name='sbtest1';
+------------+----------------+
| TABLE_NAME | AUTO_INCREMENT |
+------------+----------------+
| sbtest1    |        1200010 |
+------------+----------------+
1 row in set (0.00 sec)

summary

MySQL 8.0 provides more technical features to support the statistical information of table fields. The duration of statistical validity and the histogram of the field itself are more and more convenient to use.

Looking back at this requirement, in fact, if it is business monitoring or database monitoring information, such as whether the maximum value of the primary key of the monitoring table overflows, the more practical suggestion is to query the maximum value of the specific table. It is more practical and the query frequency is controllable id.

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Interpretation of new features | MySQL 8.0 field information statistics mechanism