statistics: add description of update_time (#1622)

* toc: refine the sequence of sql statements

* *: remove transaction kv count limit

* Revert "Merge remote-tracking branch 'upstream/master'"

This reverts commit f1844b1, reversing
changes made to 13f7cc8.

* Revert "Revert "Merge remote-tracking branch 'upstream/master'""

This reverts commit 097b78a.

* statistics: add description for `update_time`

* address comment

dev/reference/performance/statistics.md

@@ -177,6 +177,10 @@ Currently, the `SHOW STATS_META` statement returns the following 6 columns:
 | `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.

v2.1/reference/performance/statistics.md

@@ -93,6 +93,10 @@ Currently, the `SHOW STATS_META` statement returns the following 6 columns:
 | `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.

v3.0/reference/performance/statistics.md

@@ -178,6 +178,10 @@ Currently, the `SHOW STATS_META` statement returns the following 6 columns:
 | `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.