*: add more doc for slow log (pingcap#3886)

* *: add more doc for slow log

* Apply suggestions from code review

Co-authored-by: Keke Yi <[email protected]>

Co-authored-by: Keke Yi <[email protected]>

Author: TomShawn

Diff for: identify-slow-queries.md

@@ -109,6 +109,38 @@ TiKV Coprocessor Task fields:
 * `Cop_backoff_{backoff-type}_avg_time`: The average time of backoff caused by an error.
 * `Cop_backoff_{backoff-type}_p90_time`: The P90 percentile backoff time caused by an error.
 
+## Related system variables
+
+* [`tidb_slow_log_threshold`](/system-variables.md#tidb_slow_log_threshold): Sets the threshold for the slow log. The SQL statement whose execution time exceeds this threshold is recorded in the slow log. The default value is 300 (ms).
+* [`tidb_query_log_max_len`](/system-variables.md#tidb_query_log_max_len): Sets the maximum length of the SQL statement recorded in the slow log. The default value is 4096 (byte).
+* [`tidb_log_desensitization`](/system-variables.md#tidb_log_desensitization): Determines whether to desensitize user data using `?` in the SQL statement recorded in the slow log. The default value is `0`, which means to disable the feature.
+* [`tidb_enable_collect_execution_info`](/system-variables.md#tidb_enable_collect_execution_info): Determines whether to record the execution information of each operator in the execution plan. The default value is `1`. This feature impacts the performance by approximately 3%. After enabling this feature, you can view the `Plan` information as follows:
+
+    ```sql
+    > select tidb_decode_plan('jAOIMAk1XzE3CTAJMQlmdW5jczpjb3VudChDb2x1bW4jNyktPkMJC/BMNQkxCXRpbWU6MTAuOTMxNTA1bXMsIGxvb3BzOjIJMzcyIEJ5dGVzCU4vQQoxCTMyXzE4CTAJMQlpbmRleDpTdHJlYW1BZ2dfOQkxCXQRSAwyNzY4LkgALCwgcnBjIG51bTogMQkMEXMQODg0MzUFK0hwcm9jIGtleXM6MjUwMDcJMjA2HXsIMgk1BWM2zwAAMRnIADcVyAAxHcEQNQlOL0EBBPBbCjMJMTNfMTYJMQkzMTI4MS44NTc4MTk5MDUyMTcJdGFibGU6dCwgaW5kZXg6aWR4KGEpLCByYW5nZTpbLWluZiw1MDAwMCksIGtlZXAgb3JkZXI6ZmFsc2UJMjUBrgnQVnsA');
+    +------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+    | tidb_decode_plan('jAOIMAk1XzE3CTAJMQlmdW5jczpjb3VudChDb2x1bW4jNyktPkMJC/BMNQkxCXRpbWU6MTAuOTMxNTA1bXMsIGxvb3BzOjIJMzcyIEJ5dGVzCU4vQQoxCTMyXzE4CTAJMQlpbmRleDpTdHJlYW1BZ2dfOQkxCXQRSAwyNzY4LkgALCwgcnBjIG51bTogMQkMEXMQODg0MzUFK0hwcm9jIGtleXM6MjUwMDcJMjA2HXsIMg |
+    +------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+    |     id                    task    estRows               operator info                                                  actRows    execution info                                                                  memory       disk                              |
+    |     StreamAgg_17          root    1                     funcs:count(Column#7)->Column#5                                1          time:10.931505ms, loops:2                                                       372 Bytes    N/A                               |
+    |     └─IndexReader_18      root    1                     index:StreamAgg_9                                              1          time:10.927685ms, loops:2, rpc num: 1, rpc time:10.884355ms, proc keys:25007    206 Bytes    N/A                               |
+    |       └─StreamAgg_9       cop     1                     funcs:count(1)->Column#7                                       1          time:11ms, loops:25                                                             N/A          N/A                               |
+    |         └─IndexScan_16    cop     31281.857819905217    table:t, index:idx(a), range:[-inf,50000), keep order:false    25007      time:11ms, loops:25                                                             N/A          N/A                               |
+    +------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+    ```
+
+If you are conducting a performance test, you can disable the feature of automatically collecting the execution information of operators:
+
+{{< copyable "sql" >}}
+
+```sql
+set @@tidb_enable_collect_execution_info=0;
+```
+
+The returned result of the `Plan` field has roughly the same format with that of `EXPLAIN` or `EXPLAIN ANALYZE`. For more details of the execution plan, see [`EXPLAIN`](/sql-statements/sql-statement-explain.md) or [`EXPLAIN ANALYZE`](/sql-statements/sql-statement-explain-analyze.md).
+
+For more information, see [TiDB specific variables and syntax](/system-variables.md).
+
 ## Memory mapping in slow log
 
 You can query the content of the slow query log by querying the `INFORMATION_SCHEMA.SLOW_QUERY` table. Each column name in the table corresponds to one field name in the slow log. For table structure, see the introduction to the `SLOW_QUERY` table in [Information Schema](/information-schema/information-schema-slow-query.md).

Diff for: system-variables.md

@@ -6,7 +6,12 @@ aliases: ['/tidb/dev/tidb-specific-system-variables','/docs/dev/system-variables
 
 # System Variables
 
-TiDB system variables behave similar to MySQL, in that settings may apply on a `SESSION`, `GLOBAL` or both `SESSION` and `GLOBAL` scope. Changes to `GLOBAL` scoped variables **only apply** to new connections to TiDB. Variables can be set with the [`SET` statement](/sql-statements/sql-statement-set-variable.md) on a per-session or global basis:
+TiDB system variables behave similar to MySQL with some differences, in that settings might apply on a `SESSION`, `INSTANCE`, or `GLOBAL` scope, or on a scope that combines `SESSION`, `INSTANCE`, or `GLOBAL`.
+
+- Changes to `GLOBAL` scoped variables **only apply to new connection sessions with TiDB**. Currently active connection sessions are not affected. These changes are persisted and valid after restarts.
+- Changes to `INSTANCE` scoped variables apply to all active or new connection sessions with the current TiDB instance immediately after the changes are made. Other TiDB instances are not affected. These changes are not persisted and become invalid after TiDB restarts.
+
+Variables can be set with the [`SET` statement](/sql-statements/sql-statement-set-variable.md) on a per-session, instance or global basis:
 
 ```sql
 # These two identical statements change a session variable
@@ -39,7 +44,7 @@ SET  GLOBAL tidb_distsql_scan_concurrency = 10;
 
 ### ddl_slow_threshold
 
-- Scope: SESSION
+- Scope: INSTANCE
 - Default value: 300
 - DDL operations whose execution time exceeds the threshold value are output to the log. The unit is millisecond.
 
@@ -171,7 +176,7 @@ SET  GLOBAL tidb_distsql_scan_concurrency = 10;
 
 ### tidb_check_mb4_value_in_utf8
 
-- Scope: SERVER
+- Scope: INSTANCE
 - Default value: 1, indicating check the validity of UTF-8 data. This default behavior is compatible with MySQL.
 - This variable is used to set whether to check the validity of UTF-8 data.
 - To upgrade an earlier version (TiDB v2.1.1 or earlier), you may need to disable this option. Otherwise, you can successfully write invalid strings in an earlier version but fail to do this in a later version, because there is no data validity check in the earlier version. For details, see [FAQs After Upgrade](/faq/upgrade-faq.md).
@@ -336,7 +341,7 @@ Constraint checking is always performed in place for pessimistic transactions (d
 
 ### tidb_enable_slow_log
 
-- Scope: SESSION
+- Scope: INSTANCE
 - Default value: `1`
 - This variable is used to control whether to enable the slow log feature. It is enabled by default.
 
@@ -410,22 +415,22 @@ Constraint checking is always performed in place for pessimistic transactions (d
 
 ### tidb_expensive_query_time_threshold
 
-- Scope: SERVER
+- Scope: INSTANCE
 - Default value: 60
 - This variable is used to set the threshold value that determines whether to print expensive query logs. The unit is second. The difference between expensive query logs and slow query logs is:
     - Slow logs are printed after the statement is executed.
     - Expensive query logs print the statements that are being executed, with execution time exceeding the threshold value, and their related information.
 
 ### tidb_force_priority
 
-- Scope: SESSION
+- Scope: INSTANCE
 - Default value: `NO_PRIORITY`
 - This variable is used to change the default priority for statements executed on a TiDB server. A use case is to ensure that a particular user that is performing OLAP queries receives lower priority than users performing OLTP queries.
 - You can set the value of this variable to `NO_PRIORITY`, `LOW_PRIORITY`, `DELAYED` or `HIGH_PRIORITY`.
 
 ### tidb_general_log
 
-- Scope: SERVER
+- Scope: INSTANCE
 - Default value: 0
 - This variable is used to set whether to record all the SQL statements in the log.
 
@@ -679,7 +684,7 @@ mysql> desc select count(distinct a) from test.t;
 
 ### tidb_query_log_max_len
 
-- Scope: SESSION
+- Scope: INSTANCE
 - Default value: 4096 (bytes)
 - The maximum length of the SQL statement output. When the output length of a statement is larger than the `tidb_query-log-max-len` value, the statement is truncated to output.
 
@@ -691,13 +696,13 @@ set tidb_query_log_max_len = 20
 
 ### tidb_pprof_sql_cpu <span class="version-mark">New in v4.0</span>
 
-- Scope: SESSION
+- Scope: INSTANCE
 - Default value: 0
 - This variable is used to control whether to mark the corresponding SQL statement in the profile output to identify and troubleshoot performance issues.
 
 ### tidb_record_plan_in_slow_log
 
-- Scope: SESSION
+- Scope: INSTANCE
 - Default value: `1`
 - This variable is used to control whether to include the execution plan of slow queries in the slow log.
 
@@ -756,7 +761,7 @@ Query OK, 0 rows affected, 1 warning (0.00 sec)
 
 ### tidb_slow_log_threshold
 
-- Scope: SESSION
+- Scope: INSTANCE
 - Default value: 300ms
 - This variable is used to output the threshold value of the time consumed by the slow log. When the time consumed by a query is larger than this value, this query is considered as a slow log and its log is output to the slow query log.
 
@@ -766,6 +771,19 @@ Usage example:
 SET tidb_slow_log_threshold = 200;
 ```
 
+### tidb_enable_collect_execution_info
+
+- Scope: INSTANCE
+- Default value: 1
+- This variable controls whether to record the execution information of each operator in the slow query log.
+
+### tidb_log_desensitization
+
+- Scope: GLOBAL
+- Default value: 0
+- This variable controls whether to hide user information in the SQL statement being recorded into the TiDB log and slow log.
+- When you set the variable to `1`, user information is hidden. For example, if the executed SQL statement is `insert into t values (1,2)`, the statement is recorded as `insert into t values (?,?)` in the log.
+
 ### tidb_slow_query_file
 
 - Scope: SESSION
@@ -810,7 +828,7 @@ SET tidb_slow_log_threshold = 200;
 
 ### tidb_store_limit <span class="version-mark">New in v3.0.4 and v4.0</span>
 
-- Scope: SESSION | GLOBAL
+- Scope: INSTANCE | GLOBAL
 - Default value: 0
 - This variable is used to limit the maximum number of requests TiDB can send to TiKV at the same time. 0 means no limit.
 
@@ -845,7 +863,7 @@ SET tidb_slow_log_threshold = 200;
 
 - Scope: SESSION | GLOBAL
 - Default value: 4
-- This variable is used to set the concurrency degree of the window operator. 
+- This variable is used to set the concurrency degree of the window operator.
 
 ### time_zone