tidb: update tidb_index_serial_scan_concurrency docs (#21938)

statistics.md

@@ -763,23 +763,29 @@ The relationships of the relevant system variables are shown below:
 
 #### `tidb_build_stats_concurrency`
 
-When you run the `ANALYZE` statement, the task is divided into multiple small tasks. Each task only works on statistics of one column or index. You can use the [`tidb_build_stats_concurrency`](/system-variables.md#tidb_build_stats_concurrency) variable to control the number of simultaneous small tasks. The default value is `2`. The default value is `4` for v7.4.0 and earlier versions.
+This variable controls the concurrency for building statistics during manual `ANALYZE`, such as the number of table or partition analysis tasks that can be processed simultaneously. The default value is `2`. The default value is `4` for v7.4.0 and earlier versions.
 
 #### `tidb_build_sampling_stats_concurrency`
 
-When analyzing ordinary columns, you can use [`tidb_build_sampling_stats_concurrency`](/system-variables.md#tidb_build_sampling_stats_concurrency-new-in-v750) to control the concurrency of executing sampling tasks. The default value is `2`.
+This variable controls the following aspects of `ANALYZE` concurrency:
+
+- The concurrency for merging samples collected from different Regions.
+- The concurrency for collecting statistics on special indexes (such as indexes on generated virtual columns), for example, the number of indexes that TiDB can concurrently collect statistics for.
+
+The default value is `2`.
 
 #### `tidb_analyze_partition_concurrency`
 
-When running the `ANALYZE` statement, you can use [`tidb_analyze_partition_concurrency`](/system-variables.md#tidb_analyze_partition_concurrency) to control the concurrency of reading and writing statistics for a partitioned table. The default value is `2`. The default value is `1` for v7.4.0 and earlier versions.
+This variable controls the concurrency for saving `ANALYZE` results (writing TopN and histograms to system tables). The default value is `2`. The default value is `1` for v7.4.0 and earlier versions.
 
-#### `tidb_distsql_scan_concurrency`
+#### `tidb_analyze_distsql_scan_concurrency`
 
-When you analyze regular columns, you can use the [`tidb_distsql_scan_concurrency`](/system-variables.md#tidb_distsql_scan_concurrency) variable to control the number of Regions to be read at one time. The default value is `15`. Note that changing the value will affect query performance. Adjust the value carefully.
+This variable controls the following aspects of `ANALYZE` concurrency:
 
-#### `tidb_index_serial_scan_concurrency`
+- The concurrency of scanning TiKV Regions.
+- The concurrency of scanning Regions for special indexes (indexes generated from virtual columns).
 
-When you analyze index columns, you can use the [`tidb_index_serial_scan_concurrency`](/system-variables.md#tidb_index_serial_scan_concurrency) variable to control the number of Regions to be read at one time. The default value is `1`. Note that changing the value will affect query performance. Adjust the value carefully.
+The default value is `4`.
 
 ## See also
 

system-variables.md

@@ -1114,21 +1114,28 @@ MPP is a distributed computing framework provided by the TiFlash engine, which a
 
 ### tidb_analyze_distsql_scan_concurrency <span class="version-mark">New in v7.6.0</span>
 
+> **Note:**
+>
+> In versions earlier than v7.6.0, regular `ANALYZE` region scans are controlled by `tidb_distsql_scan_concurrency`, and index statistics scans are controlled by `tidb_index_serial_scan_concurrency`. Therefore, to adjust the concurrency of scanning TiKV Regions for these versions, consider changing the value of `tidb_distsql_scan_concurrency`.
+
 - Scope: SESSION | GLOBAL
 - Persists to cluster: Yes
 - Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
 - Type: Integer
 - Default value: `4`
 - Range: `[1, 4294967295]`
-- This variable is used to set the concurrency of the `scan` operation when executing the `ANALYZE` operation.
+- This variable controls the following aspects of `ANALYZE` concurrency:
+    - The concurrency of scanning TiKV Regions.
+    - The concurrency of scanning Regions for special indexes, such as indexes on generated virtual columns.
 
 ### tidb_analyze_partition_concurrency
 
 - Scope: SESSION | GLOBAL
 - Persists to cluster: Yes
 - Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
 - Default value: `2`. The default value is `1` for v7.4.0 and earlier versions.
-- This variable specifies the concurrency of reading and writing statistics for a partitioned table when TiDB analyzes the partitioned table.
+- Range: `[1, 18446744073709551615]`
+- For manual and auto `ANALYZE`, this variable controls the concurrency for saving `ANALYZE` results, including writing TopN and histograms to system tables.
 
 ### tidb_analyze_version <span class="version-mark">New in v5.1.0</span>
 
@@ -1250,7 +1257,7 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
 - Type: Integer
 - Default value: `1`
 - Range: `[1, 256]`
-- This variable is used to set the concurrency of executing the automatic update of statistics.
+- This variable controls the concurrency for building statistics during auto `ANALYZE`, such as the number of table or partition analysis tasks that can be processed simultaneously.
 
 ### tidb_backoff_lock_fast
 
@@ -1355,8 +1362,7 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
 - Default value: `2`. The default value is `4` for v7.4.0 and earlier versions.
 - Range: `[1, 256]`
 - Unit: Threads
-- This variable is used to set the concurrency of executing the `ANALYZE` statement.
-- When the variable is set to a larger value, the execution performance of other queries is affected.
+- This variable controls the concurrency for building statistics during manual `ANALYZE`, such as the number of table or partition analysis tasks that can be processed simultaneously.
 
 ### tidb_build_sampling_stats_concurrency <span class="version-mark">New in v7.5.0</span>
 
@@ -1367,8 +1373,9 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
 - Unit: Threads
 - Default value: `2`
 - Range: `[1, 256]`
-- This variable is used to set the sampling concurrency in the `ANALYZE` process.
-- When the variable is set to a larger value, the execution performance of other queries is affected.
+- This variable controls the following aspects of `ANALYZE` concurrency:
+    - The concurrency for merging samples collected from different Regions.
+    - The concurrency for collecting statistics on special indexes (such as indexes on generated virtual columns), for example, the number of indexes that TiDB can concurrently collect statistics for.
 
 ### tidb_capture_plan_baselines <span class="version-mark">New in v4.0</span>
 
@@ -3339,15 +3346,18 @@ For a system upgraded to v5.0 from an earlier version, if you have not modified
 
 ### tidb_index_serial_scan_concurrency
 
+> **Warning:**
+>
+> This variable is deprecated and no longer controls execution behavior. The concurrency of sequential index scans is now controlled by [`tidb_executor_concurrency`](#tidb_executor_concurrency-new-in-v50), and [`ANALYZE TABLE`](/sql-statements/sql-statement-analyze-table.md) uses [`tidb_analyze_distsql_scan_concurrency`](#tidb_analyze_distsql_scan_concurrency-new-in-v760) to control the concurrency of index statistics scans.
+
 - Scope: SESSION | GLOBAL
 - Persists to cluster: Yes
 - Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): Yes
 - Type: Integer
 - Default value: `1`
 - Range: `[1, 256]`
 - Unit: Threads
-- This variable is used to set the concurrency of the `serial scan` operation.
-- Use a bigger value in OLAP scenarios, and a smaller value in OLTP scenarios.
+- This variable remains only for backward compatibility. Use [`tidb_executor_concurrency`](#tidb_executor_concurrency-new-in-v50) to control the concurrency of sequential index scans, or [`tidb_analyze_distsql_scan_concurrency`](#tidb_analyze_distsql_scan_concurrency-new-in-v760) to control the concurrency of index statistics scans.
 
 ### tidb_init_chunk_size
 
@@ -3824,7 +3834,7 @@ For a system upgraded to v5.0 from an earlier version, if you have not modified
 - Persists to cluster: Yes
 - Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
 - Default value: `1`
-- This variable specifies the concurrency of merging statistics for a partitioned table when TiDB analyzes the partitioned table.
+- This variable controls the concurrency for merging TopN results of partitioned tables.
 
 ### tidb_enable_async_merge_global_stats <span class="version-mark">New in v7.5.0</span>
 

troubleshoot-tidb-oom.md

@@ -99,7 +99,6 @@ You can take the following measures to reduce the memory usage of SQL statements
 
 - Reduce the number of Regions to be read simultaneously or reduce the concurrency of operators to avoid memory problems caused by high concurrency. The corresponding system variables include:
     - [`tidb_distsql_scan_concurrency`](/system-variables.md#tidb_distsql_scan_concurrency)
-    - [`tidb_index_serial_scan_concurrency`](/system-variables.md#tidb_index_serial_scan_concurrency)
     - [`tidb_executor_concurrency`](/system-variables.md#tidb_executor_concurrency-new-in-v50)
 
 - The concurrency of sessions is too high near the time point when the problem occurs. In this case, consider scaling out the TiDB cluster by adding more TiDB nodes.
@@ -195,4 +194,4 @@ To locate the root cause of an OOM issue, you need to collect the following info
 ## See also
 
 - [TiDB Memory Control](/configure-memory-usage.md)
-- [Tune TiKV Memory Parameter Performance](/tune-tikv-memory-performance.md)
+- [Tune TiKV Memory Parameter Performance](/tune-tikv-memory-performance.md)