Introduction to Statistics

Based on the statistics, the TiDB optimizer chooses the most efficient query execution plan. The statistics collect table-level and column-level information.

• The statistics of a table include the total number of rows and the number of updated rows.
• The statistics of a column include the number of different values, the number of NULL, the histogram, the TOPN value that occurs most frequently in the column, and the Count-Min Sketch of the column.

Collect statistics

Manual collection

You can run the ANALYZE statement to collect statistics.

Full collection

Note:

The execution time of ANALYZE TABLE in TiDB is longer than that in MySQL or InnoDB. In InnoDB, only a small number of pages are sampled, while in TiDB a comprehensive set of statistics is completely rebuilt. Scripts that were written for MySQL may naively expect ANALYZE TABLE will be a short-lived operation.

For quicker analysis, you can set tidb_enable_fast_analyze to 1 to enable the Quick Analysis feature. The default value for this parameter is 0.

After Quick Analysis is enabled, TiDB randomly samples approximately 10,000 rows of data to build statistics. Therefore, in the case of uneven data distribution or a relatively small amount of data, the accuracy of statistical information is relatively poor. It might lead to poor execution plans, such as choosing the wrong index. If the execution time of the normal ANALYZE statement is acceptable, it is recommended to disable the Quick Analysis feature.

You can perform full collection using the following syntax.

• To collect statistics of all the tables in TableNameList:

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  ANALYZE TABLE TableNameList [WITH NUM BUCKETS];

  WITH NUM BUCKETS specifies the maximum number of buckets in the generated histogram.

• To collect statistics of the index columns on all IndexNameLists in TableName:

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  ANALYZE TABLE TableName INDEX [IndexNameList] [WITH NUM BUCKETS];

  The statement collects statistics of all index columns when IndexNameList is empty.

• To collect statistics of partition in all PartitionNameLists in TableName:

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  ANALYZE TABLE TableName PARTITION PartitionNameList [WITH NUM BUCKETS];

• To collect statistics of index columns for the partitions in all PartitionNameLists in TableName:

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  ANALYZE TABLE TableName PARTITION PartitionNameList [IndexNameList] [WITH NUM BUCKETS];

Incremental collection

To improve the speed of analysis after full collection, incremental collection could be used to analyze the newly added sections in monotonically non-decreasing columns such as time columns.

Note:

• Currently, the incremental collection is only provided for index.
• When using the incremental collection, you must ensure that only INSERT operations exist on the table, and that the newly inserted value on the index column is monotonically non-decreasing. Otherwise, the statistical information might be inaccurate, affecting the TiDB optimizer to select an appropriate execution plan.

You can perform incremental collection using the following syntax.

• To incrementally collect statistics for index columns in all IndexNameLists in TableName:

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  ANALYZE INCREMENTAL TABLE TableName INDEX [IndexNameList] [WITH NUM BUCKETS];

• To incrementally collect statistics of index columns for partitions in all PartitionNameLists in TableName:

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  ANALYZE INCREMENTAL TABLE TableName PARTITION PartitionNameList INDEX [IndexNameList] [WITH NUM BUCKETS];

Automatic update

For the INSERT, DELETE, or UPDATE statements, TiDB automatically updates the number of rows and updated rows. TiDB persists this information regularly and the update cycle is 20 * stats-lease. The default value of stats-lease is 3s. If you specify the value as 0, it does not update automatically.

Three system variables related to automatic update of statistics are as follows:

When the ratio of the number of modified rows to the total number of rows of tbl in a table is greater than tidb_auto_analyze_ratio, and the current time is between tidb_auto_analyze_start_time and tidb_auto_analyze_end_time, TiDB executes the ANALYZE TABLE tbl statement in the background to automatically update the statistics of this table.

When the query is executed, TiDB collects feedback with the probability of feedback-probability and uses it to update the histogram and Count-Min Sketch. You can modify the value of feedback-probability in the configuration file. The default value is 0.05. You can set the value to 0.0 to disable this feature.

Note:

If you set the value of feedback-probability to 0 in the configuration file, a failure will occur and an error will be reported. To disable feedback-probability, you need to set the value to 0.0.

Control ANALYZE concurrency

When you run the ANALYZE statement, you can adjust the concurrency using the following parameters, to control its effect on the system.

tidb_build_stats_concurrency

Currently, when you run the ANALYZE statement, the task is divided into multiple small tasks. Each task only works on one column or index. You can use the tidb_build_stats_concurrency parameter to control the number of simultaneous tasks. The default value is 4.

tidb_distsql_scan_concurrency

When you analyze regular columns, you can use the tidb_distsql_scan_concurrency parameter to control the number of Region to be read at one time. The default value is 15.

tidb_index_serial_scan_concurrency

When you analyze index columns, you can use the tidb_index_serial_scan_concurrency parameter to control the number of Region to be read at one time. The default value is 1.

View ANALYZE state

When executing the ANALYZE statement, you can view the current state of ANALYZE using the following SQL statement:

SHOW ANALYZE STATUS [ShowLikeOrWhere]

This statement returns the state of ANALYZE. You can use ShowLikeOrWhere to filter the information you need.

Currently, the SHOW ANALYZE STATUS statement returns the following 7 columns:

View statistics

You can view the statistics status using the following statements.

Metadata of tables

You can use the SHOW STATS_META statement to view the total number of rows and the number of updated rows.

Syntax as follows:

SHOW STATS_META [ShowLikeOrWhere]

This statement returns the total number of all the rows in all the tables and the number of updated rows. You can use ShowLikeOrWhere to filter the information you need.

Currently, the SHOW STATS_META statement returns the following 6 columns:

Note:

When TiDB automatically updates the total number of rows and the number of modified rows according to DML statements, update_time is also updated. Therefore, update_time does not necessarily indicate the last time when the ANALYZE statement is executed.

Health state of tables

You can use the SHOW STATS_HEALTHY statement to check the health state of tables and roughly estimate the accuracy of the statistics. When modify_count >= row_count, the health state is 0; when modify_count < row_count, the health state is (1 - modify_count/row_count) * 100.

The syntax is as follows. You can use ShowLikeOrWhere to filter the information you need:

SHOW STATS_HEALTHY [ShowLikeOrWhere];

Currently, the SHOW STATS_HEALTHY statement returns the following 4 columns:

Metadata of columns

You can use the SHOW STATS_HISTOGRAMS statement to view the number of different values and the number of NULL in all the columns.

Syntax as follows:

SHOW STATS_HISTOGRAMS [ShowLikeOrWhere]

This statement returns the number of different values and the number of NULL in all the columns. You can use ShowLikeOrWhere to filter the information you need.

Currently, the SHOW STATS_HISTOGRAMS statement returns the following 8 columns:

Buckets of histogram

You can use the SHOW STATS_BUCKETS statement to view each bucket of the histogram.

Syntax as follows:

SHOW STATS_BUCKETS [ShowLikeOrWhere]

This statement returns information about all the buckets. You can use ShowLikeOrWhere to filter the information you need.

Currently, the SHOW STATS_BUCKETS statement returns the following 10 columns:

Delete statistics

You can run the DROP STATS statement to delete statistics.

Syntax as follows:

DROP STATS TableName

The statement deletes statistics of all the tables in TableName.

Import and export statistics

Export statistics

The interface to export statistics is as follows:

• To obtain the JSON format statistics of the ${table_name} table in the ${db_name} database:

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  http://${tidb-server-ip}:${tidb-server-status-port}/stats/dump/${db_name}/${table_name}

• To obtain the JSON format statistics of the ${table_name} table in the ${db_name} database at specific time:

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  http://${tidb-server-ip}:${tidb-server-status-port}/stats/dump/${db_name}/${table_name}/${yyyyMMddHHmmss}

Import statistics

Generally, the imported statistics refer to the JSON file obtained using the export interface.

Syntax:

LOAD STATS 'file_name'

file_name is the file name of the statistics to be imported.