7. The Information Schema – MySQL 8 Query Performance Tuning: A Systematic Method for Improving Execution Speeds

© Jesper Wisborg Krogh 2020
J. W. KroghMySQL 8 Query Performance Tuninghttps://doi.org/10.1007/978-1-4842-5584-1_7

7. The Information Schema

Jesper Wisborg Krogh1 
(1)
Hornsby, NSW, Australia
 

When you need to optimize a query, it is common that you need information about the schema, indexes, and the like. In that case, the Information Schema is a good resource of data. This chapter introduces the Information Schema together with an overview of the views it contains. The Information Schema is used on several occasions in the rest of the book.

What Is the Information Schema?

The Information Schema is a schema common to several of the relational databases including MySQL where it was added in MySQL 5.0. MySQL mostly follows the SQL:2003 standard for F021 Basic information schema with the changes necessary to reflect the unique features of MySQL and with additional views that are not part of the standard.

Note

The Information Schema is virtual in the sense that no data is stored in it. For this reason, this chapter refers to all views and tables as views even if a SHOW CREATE TABLE displays it as if it was a regular table. This is also in line with the information_schema.TABLES view that has the table type set to SYSTEM VIEW for all the objects.

After the introduction of the Performance Schema in MySQL 5.5, the aim is to make relatively static data such as schema information available through the Information Schema and more volatile data belonging to the Performance Schema. That said, it is not always clear-cut what belongs where, for example, index statistics are relatively volatile, but are also part of the schema information. There is also some information such as the InnoDB metrics that for historical reasons still reside in the Information Schema.

As such, you can consider the Information Schema a collection of data describing the MySQL instance. In MySQL 8 with the relational data dictionary, several of the views are simple SQL views on the underlying data dictionary tables. This means that the performance of many Information Schema queries in MySQL 8 will be vastly superior to what you may have experienced in older versions. This is particularly the case when querying schema data that does not require retrieving information from the storage engine.

Caution

If you are still using MySQL 5.7 or earlier, be careful with queries against views such as the TABLES and COLUMNS views in the Information Schema. They can take a long time if the tables they contain data for are not yet in the table definition cache or if the cache is not large enough to hold all tables. An example of the performance difference of the Information Schema between MySQL 5.7 and 8 is discussed in a blog by the MySQL Server team: https://mysqlserverteam.com/mysql-8-0-scaling-and-performance-of-information_schema/.

Privileges

The Information Schema is a virtual database, and the access to the views works a little different from other tables. All users will see that the information_schema schema exists, and they will see all views. However, the result of querying the views depends on the privileges assigned to the account. For example, an account that has no other privileges than the global USAGE privilege will only see the Information Schema views when querying the information_schema.TABLES view.

Some views require additional privileges in which case an ER_SPECIFIC_ACCESS_DENIED_ERROR (error number 1227) error is returned with a description of which privilege is missing. For example, the INNODB_METRICS view requires the PROCESS privilege, so if a user without the PROCESS privilege queries that view, the following error occurs:
mysql> SELECT *
         FROM information_schema.INNODB_METRICS;
ERROR: 1227: Access denied; you need (at least one of) the PROCESS privilege(s) for this operation

Now, it is time to look at what kind of information you can find in the Information Schema views.

Views

The data that is available in the Information Schema ranges from high-level information about the system to low-level InnoDB metrics. This section provides an overview of the views but will not go into detail as the most important of the views from a performance tuning perspective are discussed in the relevant parts of later chapters.

Note

Some plugins add their own views to the Information Schema. The extra plugin views are not considered here.

System Information

The highest level of information that is available in the Information Schema concerns the whole MySQL instance. This includes such information as which character sets are available and which plugins are installed.

The views with system information are summarized in Table 7-1.
Table 7-1

Information Schema views with system information

View Name

Description

CHARACTER_SETS

The character sets available.

COLLATIONS

The collations available for each character set. This includes the id of the collation which in some cases (e.g., in the binary log) is used to uniquely specify both the collation and character set.

COLLATION_CHARACTER_SET_APPLICABILITY

The mapping of collations to character sets (the same as the first two columns of COLLATIONS).

ENGINES

The storage engines that are known and whether they are loaded.

INNODB_FT_DEFAULT_STOPWORD

A list of the default stopwords that are used when creating a full text index on an InnoDB table.

KEYWORDS

A list of the keywords in MySQL and whether the keyword is reserved.

PLUGINS

The plugins known to MySQL including the status.

RESOURCE_GROUPS

The resource groups that are used by threads to do their work. A resource group specifies the priority of a thread and which CPUs it can use.

ST_SPATIAL_REFERENCE_SYSTEMS

A list of the spatial reference systems including the SRS_ID column which contains the id used to specify a reference system for spatial columns.

The system-related views largely work as reference views with the RESOURCE_GROUPS table being somewhat a difference as it is possible to add resource groups as it will be discussed in Chapter 17.

The KEYWORDS view is, for example, useful when testing an upgrade as you can use it to verify whether any of your schema, table, column, routine, or parameter names matches a keyword in the new version. If that is the case, you will need to update the application to quote the identifier, if that is not already the case. To find all column names matching a keyword:
SELECT TABLE_SCHEMA, TABLE_NAME,
       COLUMN_NAME, RESERVED
  FROM information_schema.COLUMNS
       INNER JOIN information_schema.KEYWORDS
          ON KEYWORDS.WORD = COLUMNS.COLUMN_NAME
 WHERE TABLE_SCHEMA NOT IN ('mysql',
                            'information_schema',
                            'performance_schema',
                            'sys'
                           )
 ORDER BY TABLE_SCHEMA, TABLE_NAME, COLUMN_NAME;

The query uses the COLUMNS view to find all column names except for the system schemas (you can choose to include those, if you use those in your application or in scripts). The COLUMNS view is one of several views describing the schema objects.

Schema Information

The views with information about the schema objects are among the most useful views in the Information Schema. These are also the source for several of the SHOW statements . You can use the views to find information from everything from parameters for a stored routine to database names. The views with schema information are summarized in Table 7-2.
Table 7-2

Information Schema views with schema information

View Name

Description

CHECK_CONSTRAINTS

This view contains information about the CHECK constraints and is available in MySQL 8.0.16 and later.

COLUMN_STATISTICS

The definition of histograms including the statistics. This is a very useful view for query performance tuning.

COLUMNS

The column definitions.

EVENTS

The definitions of the stored events.

FILES

Information about InnoDB tablespace files.

INNODB_COLUMNS

Metadata information for columns in InnoDB tables.

INNODB_DATAFILES

This view links the InnoDB tablespace ids to the file system paths.

INNODB_FIELDS

Metadata for columns included in InnoDB indexes.

INNODB_FOREIGN

Metadata for the InnoDB foreign keys.

INNODB_FOREIGN_COLS

Lists the child and parent columns of InnoDB foreign keys.

INNODB_FT_BEING_DELETED

A snapshot of the INNODB_FT_DELETED view during an OPTIMIZE TABLE statement for the InnoDB table specified in the innodb_ft_aux_table option .

INNODB_FT_CONFIG

Configuration information for full text indexes on the InnoDB table specified in the innodb_ft_aux_table option.

INNODB_FT_DELETED

Rows that have been deleted from full text indexes for the InnoDB table specified in the innodb_ft_aux_table option. InnoDB uses this extra list for performance reasons to avoid having to update the index itself for each DML statement.

INNODB_FT_INDEX_CACHE

Newly inserted rows into the full text indexes for the InnoDB table specified in the innodb_ft_aux_table option. InnoDB uses this extra list for performance reasons to avoid having to update the index itself for each DML statement.

INNODB_FT_INDEX_TABLE

The inverted full text index for the InnoDB table specified in the innodb_ft_aux_table option.

INNODB_INDEXES

Information about indexes on InnoDB tables. This includes internal information such as the page number of the root page and the merge threshold.

INNODB_TABLES

Metadata for the InnoDB tables.

INNODB_TABLESPACES

Metadata for the InnoDB tablespaces.

INNODB_TABLESPACES_BRIEF

This view combines the SPACE, NAME, FLAG, and SPACE_TYPE columns from INNODB_TABLESPACES with the PATH column from INNODB_DATAFILES to provide a summary of the InnoDB tablespace.

INNODB_TABLESTATS

Table statistics for InnoDB tables. Some of these statistics are updated at the same time as index statistics; others are maintained at an ongoing basis.

INNODB_TEMP_TABLE_INFO

Metadata for InnoDB temporary tables (both internal and explicit).

INNODB_VIRTUAL

Internal metadata information about virtual generated columns on InnoDB tables.

KEY_COLUMN_USAGE

Information about the primary keys, unique keys, and foreign keys.

PARAMETERS

Information about the parameters for stored functions and stored procedures.

PARTITIONS

Information about table partitions.

REFERENTIAL_CONSTRAINTS

Information about foreign keys.

ROUTINES

The definition of stored functions and stored procedures.

SCHEMATA

Information about the schemas (databases). (Schemata is technically the correct word for the plural form of schema, but most use schemas nowadays.)

ST_GEOMETRY_COLUMNS

Information about columns with a spatial data type.

STATISTICS

The index definitions and statistics. This is one of the most useful views when it comes to query performance turning.

TABLE_CONSTRAINTS

Summary of the primary, unique, and foreign keys and CHECK constraints.

TABLES

Information about tables and views and their properties.

TABLESPACES

This view is only used for NDB Cluster tablespaces.

TRIGGERS

The trigger definitions.

VIEW_ROUTINE_USAGE

Lists the stored functions used in views. This table was added in 8.0.13.

VIEW_TABLE_USAGE

Lists the tables referenced by views. This view was added in 8.0.13.

VIEWS

The view definitions.

Several of the views are closely related, for example, the columns are in tables which are in schemas and constraints refer to tables and columns. This means that some of the column names are present in several of the views. The most commonly used column names that relate to these views are
  • TABLE_NAME: Used in the views not specific to InnoDB for the table name.

  • TABLE_SCHEMA: Used in the views not specific to InnoDB for the schema name.

  • COLUMN_NAME: Used in the views not specific to InnoDB for the column name.

  • SPACE: Used in the InnoDB-specific views for the tablespace id.

  • TABLE_ID: Used in the InnoDB-specific views to uniquely identify the table. This is also used internally in InnoDB.

  • NAME: The InnoDB-specific views use a column called NAME to give the name of the object irrespective of the object type.

In addition to the use of the names as in this list, there are also examples where these column names are slightly modified like in the view KEY_COLUMN_USAGE where you find the columns REFERENCED_TABLE_SCHEMA, REFERENCED_TABLE_NAME, and REFERENCED_COLUMN_NAME that are used in the description of foreign keys. As an example, if you want to use the KEY_COLUMN_USAGE view to find the tables with foreign keys referencing the sakila.film table, you can use a query like this:
mysql> SELECT TABLE_SCHEMA, TABLE_NAME
    FROM information_schema.KEY_COLUMN_USAGE
    WHERE REFERENCED_TABLE_SCHEMA = 'sakila'
        AND REFERENCED_TABLE_NAME = 'film';
+--------------+---------------+
| TABLE_SCHEMA | TABLE_NAME    |
+--------------+---------------+
| sakila       | film_actor    |
| sakila       | film_category |
| sakila       | inventory     |
+--------------+---------------+
3 rows in set (0.0078 sec)
This shows that the film_actor, film_category, and inventory tables all have foreign keys where the film table is the parent table. For example, if you look at the table definition for film_actor:
mysql> SHOW CREATE TABLE sakila.film_actor\G
*************************** 1. row ***************************
       Table: film_actor
Create Table: CREATE TABLE `film_actor` (
  `actor_id` smallint(5) unsigned NOT NULL,
  `film_id` smallint(5) unsigned NOT NULL,
  `last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`actor_id`,`film_id`),
  KEY `idx_fk_film_id` (`film_id`),
  CONSTRAINT `fk_film_actor_actor` FOREIGN KEY (`actor_id`) REFERENCES `actor` (`actor_id`) ON DELETE RESTRICT ON UPDATE CASCADE,
  CONSTRAINT `fk_film_actor_film` FOREIGN KEY (`film_id`) REFERENCES `film` (`film_id`) ON DELETE RESTRICT ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.0097 sec)

The fk_film_actor_film constraint references the film_id column in the film table. You can use this as the starting point for finding the full chain of foreign keys either by manually executing the query for each table returned in the query against the KEY_COLUMN_USAGE view or by creating a recursive common table expression (CTE). This is left as an exercise for the reader.

Tip

For an example where the KEY_COLUMN_USAGE view is used in a recursive common table expression to find the chain of foreign key dependencies, see https://mysql.wisborg.dk/tracking-foreign-keys.

For completeness, a visual representation of the tables depending on the film table through foreign keys can be found in Figure 7-1.
Figure 7-1

A visual representation of the foreign key chain from sakila.film

The diagram is created using the reverse engineering feature of MySQL Workbench.

The views with information specific to InnoDB use the SPACE and TABLE_ID to identify the tablespace and table. Each tablespace has a unique id with ranges reserved for different tablespace types. For example, the data dictionary tablespace file (<datadir>/mysql.ibd) has space id 4294967294, the temporary tablespace has id 4294967293, undo log tablespaces start with 4294967279 and decrement, and user tablespaces start at 1.

The views with information about InnoDB full text indexes are special as they require you to set the innodb_ft_aux_table global variable with the name of the table you want to get information for. For example, to get the full text index configuration of the sakila.film_text table :
mysql> SET GLOBAL innodb_ft_aux_table = 'sakila/film_text';
Query OK, 0 rows affected (0.0685 sec)
mysql> SELECT *
         FROM information_schema.INNODB_FT_CONFIG;
+---------------------------+-------+
| KEY                       | VALUE |
+---------------------------+-------+
| optimize_checkpoint_limit | 180   |
| synced_doc_id             | 1002  |
| stopword_table_name       |       |
| use_stopword              | 1     |
+---------------------------+-------+
4 rows in set (0.0009 sec)

The values in the INNODB_FT_CONFIG view may differ for you.

InnoDB also includes views with information that relates to performance. These will be discussed together with a few other performance-related tables.

Performance Information

The group of views that relate to performance are those that you will likely use the most in your performance tuning work together with the COLUMN_STATISTICS and STATISTICS views from the previous group of views. The views with performance-related information are listed in Table 7-3.
Table 7-3

Information Schema views with performance-related information

View Name

Description

INNODB_BUFFER_PAGE

A list of the pages in the InnoDB buffer pool which can be used to determine which tables and indexes are currently cached.

Warning: There is a high overhead of querying this table particularly for large buffer pools and many tables and indexes. It is best used on test systems.

INNODB_BUFFER_PAGE_LRU

Information about the pages in the InnoDB buffer pool and how they are ordered in the least recently used (LRU) list.

Warning: There is a high overhead of querying this table particularly for large buffer pools and many tables and indexes. It is best used on test systems.

INNODB_BUFFER_POOL_STATS

Statistics about the usage of the InnoDB buffer pool. The information is similar to what can be found in the SHOW ENGINE INNODB STATUS output in the BUFFER POOL AND MEMORY section. This is one of the most useful views.

INNODB_CACHED_INDEXES

A summary of the number of index pages cached in the InnoDB buffer pool for each index.

INNODB_CMP

INNODB_CMP_RESET

Statistics about operations related to compressed InnoDB tables.

INNODB_CMP_PER_INDEX

INNODB_CMP_PER_INDEX_RESET

The same as INNODB_CMP but grouped by the index.

INNODB_CMPMEM

INNODB_CMPMEM_RESET

Statistics about compressed pages in the InnoDB buffer pool.

INNODB_METRICS

Similar to the global status variables but specific to InnoDB.

INNODB_SESSION_TEMP_TABLESPACES

Metadata including the connection id, file path, and size for InnoDB temporary tablespace files (each session gets its own file in MySQL 8.0.13 and later). It can be used to link a session to a tablespace file which is very useful if you notice one file becoming large. The view was added in 8.0.13.

INNODB_TRX

Information about InnoDB transactions.

OPTIMIZER_TRACE

When the optimizer trace is enabled, the trace can be queried from this view.

PROCESSLIST

The same as SHOW PROCESSLIST.

PROFILING

When profiling is enabled, the profiling statistics can be queried from this view. This is deprecated, and it is recommended to use the Performance Schema instead.

For the views with information about InnoDB compressed tables, the table with _RESET as the suffix returns the operation and timing statistics as deltas since the last time the view was queried.

The INNODB_METRICS view includes metrics similar to the global status variables but specific to InnoDB. The metrics are grouped into subsystems (the SUBSYSTEM column), and for each metric there is a description of what the metric measures in the COMMENT column. You can enable, disable, and reset the metrics using global system variables:
  • innodb_monitor_disable: Disable one or more metrics.

  • innodb_monitor_enable: Enable one or more metrics.

  • innodb_monitor_reset: Reset the counter for one or more metrics.

  • innodb_monitor_reset_all: Reset all statistics including the counter, minimum, and maximum values for one or more metrics.

The metrics can be turned on and off as needed with the current status found in the STATUS column. You specify the name of the metric as the value to the innodb_monitor_enable or innodb_monitor_disable variable, and you can use % as a wildcard. The value all works as a special value to affect all metrics. Listing 7-1 shows an example of enabling and using all the metrics matching %cpu% (which happens to be the metrics in the cpu subsystem). The counter values depend on the workload you have at the time of the query.
mysql> SET GLOBAL innodb_monitor_enable = '%cpu%';
Query OK, 0 rows affected (0.0005 sec)
mysql> SELECT NAME, COUNT, MIN_COUNT,
              MAX_COUNT, AVG_COUNT,
              STATUS, COMMENT
         FROM information_schema.INNODB_METRICS
        WHERE NAME LIKE '%cpu%'\G
*************************** 1. row ***************************
     NAME: module_cpu
    COUNT: 0
MIN_COUNT: NULL
MAX_COUNT: NULL
AVG_COUNT: 0
   STATUS: enabled
  COMMENT: CPU counters reflecting current usage of CPU
*************************** 2. row ***************************
     NAME: cpu_utime_abs
    COUNT: 51
MIN_COUNT: 0
MAX_COUNT: 51
AVG_COUNT: 0.4358974358974359
   STATUS: enabled
  COMMENT: Total CPU user time spent
*************************** 3. row ***************************
     NAME: cpu_stime_abs
    COUNT: 7
MIN_COUNT: 0
MAX_COUNT: 7
AVG_COUNT: 0.05982905982905983
   STATUS: enabled
  COMMENT: Total CPU system time spent
*************************** 4. row ***************************
     NAME: cpu_utime_pct
    COUNT: 6
MIN_COUNT: 0
MAX_COUNT: 6
AVG_COUNT: 0.05128205128205128
   STATUS: enabled
  COMMENT: Relative CPU user time spent
*************************** 5. row ***************************
     NAME: cpu_stime_pct
    COUNT: 0
MIN_COUNT: 0
MAX_COUNT: 0
AVG_COUNT: 0
   STATUS: enabled
  COMMENT: Relative CPU system time spent
*************************** 6. row ***************************
     NAME: cpu_n
    COUNT: 8
MIN_COUNT: 8
MAX_COUNT: 8
AVG_COUNT: 0.06837606837606838
   STATUS: enabled
  COMMENT: Number of cpus
6 rows in set (0.0011 sec)
mysql> SET GLOBAL innodb_monitor_disable = '%cpu%';
Query OK, 0 rows affected (0.0004 sec)
Listing 7-1

Using the INNODB_METRICS view

First, the metrics are enabled using the innodb_monitor_enable variable; then the values are retrieved. In addition to the values shown, there is also a set of columns with the _RESET suffix which are reset when you set the innodb_monitor_reset (only the counter) or innodb_monitor_reset_all system variable. Finally, the metrics are disabled again.

Caution

The metrics have varying overheads, so you are recommended to test with your workload before enabling metrics in production.

The InnoDB metrics are also included in the sys.metrics view together with the global status variables and a few other metrics and when the metrics are retrieved.

The remaining Information Schema views contain information about privileges.

Privilege Information

MySQL uses privileges assigned to the accounts to determine which accounts can access which schemas, tables, and columns. The common way to determine the privileges for a given account is to use the SHOW GRANTS statement, but the Information Schema also includes views that allow you to query the privileges.

The Information Schema privilege views are summarized in Table 7-4. The views are ordered from global privileges to column privileges.
Table 7-4

Information Schema tables with privilege information

Table Name

Description

USER_PRIVILEGES

The global privileges.

SCHEMA_PRIVILEGES

Privileges to access schemas.

TABLE_PRIVILEGES

Privileges to access tables.

COLUMN_PRIVILEGES

Privileges to access columns.

In all views, the account is called GRANTEE and is in the form 'username'@'hostname' with the quotes always present. Listing 7-2 shows an example of retrieving the privileges for the mysql.sys@localhost account and comparing it to the output of the SHOW GRANTS statement.
mysql> SHOW GRANTS FOR 'mysql.sys'@'localhost'\G
*************************** 1. row ***************************
Grants for mysql.sys@localhost: GRANT USAGE ON *.* TO `mysql.sys`@`localhost`
*************************** 2. row ***************************
Grants for mysql.sys@localhost: GRANT TRIGGER ON `sys`.* TO `mysql.sys`@`localhost`
*************************** 3. row ***************************
Grants for mysql.sys@localhost: GRANT SELECT ON `sys`.`sys_config` TO `mysql.sys`@`localhost`
3 rows in set (0.2837 sec)
mysql> SELECT *
         FROM information_schema.USER_PRIVILEGES
        WHERE GRANTEE = '''mysql.sys''@''localhost'''\G
*************************** 1. row ***************************
       GRANTEE: 'mysql.sys'@'localhost'
 TABLE_CATALOG: def
PRIVILEGE_TYPE: USAGE
  IS_GRANTABLE: NO
1 row in set (0.0006 sec)
mysql> SELECT *
         FROM information_schema.SCHEMA_PRIVILEGES
        WHERE GRANTEE = '''mysql.sys''@''localhost'''\G
*************************** 1. row ***************************
       GRANTEE: 'mysql.sys'@'localhost'
 TABLE_CATALOG: def
  TABLE_SCHEMA: sys
PRIVILEGE_TYPE: TRIGGER
  IS_GRANTABLE: NO
1 row in set (0.0005 sec)
mysql> SELECT *
         FROM information_schema.TABLE_PRIVILEGES
        WHERE GRANTEE = '''mysql.sys''@''localhost'''\G
*************************** 1. row ***************************
       GRANTEE: 'mysql.sys'@'localhost'
 TABLE_CATALOG: def
  TABLE_SCHEMA: sys
    TABLE_NAME: sys_config
PRIVILEGE_TYPE: SELECT
  IS_GRANTABLE: NO
1 row in set (0.0005 sec)
mysql> SELECT *
         FROM information_schema.COLUMN_PRIVILEGES
        WHERE GRANTEE = '''mysql.sys''@''localhost'''\G
Empty set (0.0005 sec)
Listing 7-2

Using the Information Schema privilege views

Notice how the single quotes around the username and hostname are escaped by doubling the quotes.

While the views with the privilege information are not directly usable for performance tuning, they are very useful for maintaining a stable system as you can use them to easily identify whether any accounts have privileges that they do not need.

Tip

It is best practice to limit accounts to have just the privileges they need and no more. That is one of the steps to keep the system secure.

The last topic to consider about the Information Schema is how data related to index statistics are cached.

Caching of Index Statistics Data

One thing that is important to understand is where the information in the index statistics–related views (and the equivalent SHOW statements) comes from. Most of the data comes from the MySQL data dictionary. In MySQL 8, the data dictionary is stored in an InnoDB table, so the views are just normal SQL views on top of the data dictionary. (You can, for example, try to execute SHOW CREATE VIEW information_schema.STATISTICS to get the definition of the STATISTICS view.)

The index statistics themselves are however still originating from the storage engine layer, so it is relatively expensive to query those. To improve the performance, the statistics are cached in the data dictionary. You can control how old the statistics are allowed to be before MySQL refreshes the cache. This is done with the information_schema_stats_expiry variable which defaults to 86400 seconds (one day). If you set the value to 0, you will always get the latest values available from the storage engine; this is the equivalent of the MySQL 5.7 behavior. The variable can be set both at the global and session scopes, so you can set it to 0 for the session, if you are investigating an issue where it is important to see the current statistics, for example, if the optimizer is not using the index you expect.

Tip

Use the information_schema_stats_expiry variable to control how long index statistics can be cached in the data dictionary. This is only for displaying purposes – the optimizer always uses the latest statistics. Setting information_schema_stats_expiry to 0 to disable caching can, for example, be useful when investigating an issue with the wrong index being used by the optimizer. You can change the value both at the global and session scopes as needed.

The caching affects the columns listed in Table 7-5. The SHOW statements displaying the same data are also affected.
Table 7-5

Columns affected by information_schema_stats_expiry

View Name

Column Name

Description

STATISTICS

CARDINALITY

The estimate for the number of unique values for the part of the index up and including to the column in the same row.

TABLES

AUTO_INCREMENT

The next value for the auto-increment counter for the table.

AVG_ROW_LENGTH

The estimated data length divided with the estimated number of rows.

CHECKSUM

The table checksum. It is not used by InnoDB, so the value is NULL.

CHECK_TIME

When the table was last checked (CHECK TABLE). For partitioned tables, InnoDB always returns NULL.

CREATE_TIME

When the table was created.

DATA_FREE

An estimate of the amount of free space in the tablespace the table belongs to. For InnoDB, this is the size of completely free extents minus a safety margin.

DATA_LENGTH

The estimated size of the row data. For InnoDB, it is the size of the clustered index, which is found as the number of pages in the clustered index multiplied with the page size.

INDEX_LENGTH

The estimated size of secondary indexes. For InnoDB, this is the sum of pages in non-clustered indexes times the page size.

MAX_DATA_LENGTH

The maximum allowed size of the data length. It is not used by InnoDB, so the value is NULL.

TABLE_ROWS

The estimated number of rows. For InnoDB tables, this comes from the cardinality of the primary key or clustered index.

UPDATE_TIME

When the tablespace file was last updated. For tables in the InnoDB system tablespace, the value is NULL. As data is written to the tablespace asynchronously, the time will not in general reflect the time of the last statement changing the data.

You can force an update of this data for a given table by executing ANALYZE TABLE for the table.

There are times when querying the data does not update the cached data:
  • When the cached data has not yet expired, that is, it was refreshed less than information_schema_stats_expiry seconds ago

  • When information_schema_stats_expiry is set to 0

  • When MySQL or InnoDB is running in a read-only mode, that is, when one of the modes, read_only, super_read_only, transaction_read_only, or innodb_read_only, is enabled.

  • When the query also includes data from the Performance Schema

Summary

This chapter introduced the Information Schema by first discussing what the Information Schema is and how the user privileges work. The remainder of the chapter walked through the standard views and how caching works. The Information Schema views can be grouped by the type of information they contain: system, schema, performance, and privilege information.

The system information includes the character sets and collations, resource groups, keywords, and information related to spatial data. This is useful as an alternative to using the reference manual.

The schema information is the largest group of views and includes all the information available from schema data down to columns, indexes, and constraints. These views together with the performance views that have information such as metrics and InnoDB buffer pool statistics are the most commonly used views in performance tuning. The privilege-related views are not so often used for performance tuning, but they are very useful to help maintain a stable system.

A common shortcut to obtain information from the Information Schema views is to use a SHOW statement . These will be discussed in the next chapter.