statistics: add info about tidb_auto_analyze_partition_batch_size (#1…

…5032) (#15123)

statistics.md

@@ -377,9 +377,10 @@ 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 |
+| [`tidb_auto_analyze_ratio`](/system-variables.md#tidb_auto_analyze_ratio) | 0.5 | The threshold value of automatic update |
+| [`tidb_auto_analyze_start_time`](/system-variables.md#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`](/system-variables.md#tidb_auto_analyze_end_time)   | `23:59 +0000` | The end time in a day when TiDB can perform automatic update |
+| [`tidb_auto_analyze_partition_batch_size`](/system-variables.md#tidb_auto_analyze_partition_batch_size-new-in-v640) | `1` | The number of partitions that TiDB automatically analyzes when analyzing a partitioned table (that is, when automatically updating statistics on a partitioned table) |
 
 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 on this table.