include storage engines in slow query logs and statements summary (#21155)

Author: henrybw

identify-slow-queries.md

@@ -168,6 +168,11 @@ Fields related to Resource Control:
 * `Request_unit_write`: the total write RUs consumed by the statement.
 * `Time_queued_by_rc`: the total time that the statement waits for available resources.
 
+Fields related to storage engines:
+
+- `Storage_from_kv`: introduced in v9.0.0, indicates whether this statement read data from TiKV.
+- `Storage_from_mpp`: introduced in v9.0.0, indicates whether this statement read data from TiFlash.
+
 ## 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).

statement-summary-tables.md

@@ -467,6 +467,11 @@ Fields related to network traffic:
 - `SUM_UNPACKED_BYTES_SENT_TIFLASH_CROSS_ZONE`: bytes sent from SQL statements to TiFlash across availability zones, including bytes sent between TiFlash nodes across availability zones.
 - `SUM_UNPACKED_BYTES_RECEIVED_TIFLASH_CROSS_ZONE`: bytes received by SQL statements from TiFlash across availability zones, including bytes received between TiFlash nodes across availability zones.
 
+Fields related to storage engines:
+
+- `STORAGE_KV`: introduced in v9.0.0, indicates whether the previous execution of SQL statements of this category read data from TiKV.
+- `STORAGE_MPP`: introduced in v9.0.0, indicates whether the previous execution of SQL statements of this category read data from TiFlash.
+
 ### `statements_summary_evicted` fields description
 
 - `BEGIN_TIME`: Records the starting time.