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, and the histogram of the column.
Collect statistics
Manual collection
You can run the ANALYZE statement to collect statistics.
Syntax:
ANALYZE TABLE TableNameList
> The statement collects statistics of all the tables in `TableNameList`.
ANALYZE TABLE TableName INDEX IndexNameList
> The statement collects statistics of the index columns on all `IndexNameList` 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.
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 10.
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 5 columns:
| Syntax Element | Description |
|---|---|
db_name |
database name |
table_name |
table name |
update_time |
the time of the update |
modify_count |
the number of modified rows |
row_count |
the total number of rows |
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 7 columns:
| Syntax Element | Description |
|---|---|
db_name |
database name |
table_name |
table name |
column_name |
column name |
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 |
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 9 columns:
| Syntax Element | Description |
|---|---|
db_name |
database name |
table_name |
table name |
column_name |
column name |
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`。