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, and the Count-Min Sketch of the column.

Collect statistics

Manual collection

You can run the ANALYZE statement to collect statistics. Note that ANALYZE TABLE in TiDB takes considerably longer than in MySQL/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.

Syntax:

ANALYZE TABLE TableNameList [WITH NUM BUCKETS]
> The statement collects statistics of all the tables in `TableNameList`.
> `WITH NUM BUCKETS` specifies the maximum number of buckets in the generated histogram.

ANALYZE TABLE TableName INDEX [IndexNameList] [WITH NUM BUCKETS]
> The statement collects statistics of the index columns on all `IndexNameList`s in `TableName`.
> The statement collects statistics of all index columns when `IndexNameList` is empty.

ANALYZE TABLE TableName PARTITION PartitionNameList [WITH NUM BUCKETS]
> The statement collects partition statistics of all `PartitionNameList`s in `TableName`.

ANALYZE TABLE TableName PARTITION PartitionNameList [IndexNameList] [WITH NUM BUCKETS]
> The statement collects index column statistics of the partitions in all `PartitionNameList`s in `TableName`.

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 5 * 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:

System Variable Default Value Description
tidb_auto_analyze_ratio| 0.5 the threshold value of automatic update
tidb_auto_analyze_start_time 00:00 +0000 the start time in a day when TiDB can perform automatic update
tidb_auto_analyze_end_time 23:59 +0000 the end time in a day when TiDB can perform automatic update

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 0.0.

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 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:

SHOW STATS_META [ShowLikeOrWhere]
> The statement returns the total number of rows 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:

Syntax Element Description
db_name database name
table_name table name
partition_name| partition name
update_time the time of the update
modify_count the number of modified rows
row_count the total number of rows

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.

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:

SHOW STATS_HISTOGRAMS [ShowLikeOrWhere]
> The 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:

Syntax Element Description
db_name database name
table_name table name
partition_name partition name
column_name the column name (when is_index is 0) or the index name (when is_index is 1)
is_index whether it is an index column or not
update_time the time of the update
distinct_count the number of different values
null_count the number of NULL
avg_col_size the average length of columns

Buckets of histogram

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

Syntax:

SHOW STATS_BUCKETS [ShowLikeOrWhere]
> The 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:

Syntax Element Description
db_name database name
table_name table name
partition_name partition name
column_name the column name (when is_index is 0) or the index name (when is_index is 1)
is_index whether it is an index column or not
bucket_id the ID of a bucket
count the number of all the values that falls on the bucket and the previous buckets
repeats the occurrence number of the maximum value
lower_bound the minimum value
upper_bound the maximum value

Delete statistics

You can run the DROP STATS statement to delete statistics.

Syntax:

DROP STATS TableName
> The statement deletes statistics of all the tables in `TableName`.

Import and export statistics

Export statistics

The interface to export statistics:

http://${tidb-server-ip}:${tidb-server-status-port}/stats/dump/${db_name}/${table_name}
> Use this interface to obtain the JSON format statistics of the `${table_name}` table in the `${db_name}` database.

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.