cherry pick pingcap#4162 to release-4.0

Signed-off-by: ti-srebot <ti-srebot@pingcap.com>

identify-slow-queries.md

@@ -15,11 +15,13 @@ TiDB enables the slow query log by default. You can enable or disable the featur
 ```sql
 # Time: 2019-08-14T09:26:59.487776265+08:00
 # Txn_start_ts: 410450924122144769
-# User: root@127.0.0.1
+# User@Host: root[root] @ localhost [127.0.0.1]
 # Conn_ID: 3086
+# Exec_retry_time: 5.1 Exec_retry_count: 3
 # Query_time: 1.527627037
 # Parse_time: 0.000054933
 # Compile_time: 0.000129729
+# Rewrite_time: 0.000000003 Preproc_subqueries: 2 Preproc_subqueries_time: 0.000000002
 # Process_time: 0.07 Request_count: 1 Total_keys: 131073 Process_keys: 131072 Prewrite_time: 0.335415029 Commit_time: 0.032175429 Get_commit_ts_time: 0.000177098 Local_latch_wait_time: 0.106869448 Write_keys: 131072 Write_size: 3538944 Prewrite_region: 1
 # DB: test
 # Is_internal: false
@@ -37,6 +39,7 @@ TiDB enables the slow query log by default. You can enable or disable the featur
 # Plan_from_cache: false
 # Succ: true
 # Plan: tidb_decode_plan('ZJAwCTMyXzcJMAkyMAlkYXRhOlRhYmxlU2Nhbl82CjEJMTBfNgkxAR0AdAEY1Dp0LCByYW5nZTpbLWluZiwraW5mXSwga2VlcCBvcmRlcjpmYWxzZSwgc3RhdHM6cHNldWRvCg==')
+use test;
 insert into t select * from t;
 ```
 
@@ -62,6 +65,11 @@ Slow query basics:
 * `Plan`: The execution plan of the statement. Use the `select tidb_decode_plan('xxx...')` statement to parse the specific execution plan.
 * `Prepared`: Whether this statement is a `Prepare` or `Execute` request or not.
 * `Plan_from_cache`: Whether this statement hits the execution plan cache.
+* `Rewrite_time`: The time consumed for rewriting the query of this statement.
+* `Preproc_subqueries`: The number of subqueries (in the statement) that are executed in advance. For example, the `where id in (select if from t)` subquery might be executed in advance.
+* `Preproc_subqueries_time`: The time consumed for executing the subquery of this statement in advance.
+* `Exec_retry_count`: The retry times of this statement. This field is usually for pessimistic transactions in which the statement is retried when the lock is failed.
+* `Exec_retry_time`: The execution retry duration of this statement. For example, if a statement has been executed three times in total (failed for the first two times), `Exec_retry_time` means the total duration of the first two executions. The duration of the last execution is `Query_time` minus `Exec_retry_time`.
 
 The following fields are related to transaction execution:
 
@@ -114,7 +122,7 @@ TiKV Coprocessor Task fields:
 * [`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:
+* [`tidb_enable_collect_execution_info`](/system-variables.md#tidb_enable_collect_execution_info): Determines whether to record the physical 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');