Change the description for max_execution_time. (#13939) (#13944)

develop/dev-guide-timeouts-in-tidb.md

@@ -25,7 +25,7 @@ SQL statements such as `INSERT INTO t10 SELECT * FROM t1` are not affected by GC
 
 ## SQL execution timeout
 
-TiDB also provides a system variable (`max_execution_time`, `0` by default, indicating no limit) to limit the execution time of a single SQL statement. `max_execution_time` currently takes effect for all types of statements, not just the `SELECT` statements. The unit is `ms`, but the actual precision is at the `100ms` level instead of the millisecond level.
+TiDB also provides a system variable (`max_execution_time`, `0` by default, indicating no limit) to limit the execution time of a single SQL statement. Currently, the system variable only takes effect for read-only SQL statements. The unit of `max_execution_time` is `ms`, but the actual precision is at the `100ms` level instead of the millisecond level.
 
 ## JDBC query timeout
 
@@ -35,7 +35,7 @@ TiDB provides the following MySQL-compatible timeout control parameters.
 
 - **wait_timeout**, controls the non-interactive idle timeout for the connection to Java applications. Since TiDB v5.4, the default value of `wait_timeout` is `28800` seconds, which is 8 hours. For TiDB versions earlier than v5.4, the default value is `0`, which means the timeout is unlimited.
 - **interactive_timeout**, controls the interactive idle timeout for the connection to Java applications. The value is `8 hours` by default.
-- **max_execution_time**, controls the timeout for SQL execution in the connection. The value is `0` by default, which allows the connection to be infinitely busy, that is, an SQL statement is executed for an infinitely long time.
+- **max_execution_time**, controls the timeout for SQL execution in the connection, only effective for read-only SQL statements. The value is `0` by default, which allows the connection to be infinitely busy, that is, an SQL statement is executed for an infinitely long time.
 
 However, in a real production environment, idle connections and indefinitely executing SQL statements have a negative effect on both the database and the application. You can avoid idle connections and indefinitely executing SQL statements by configuring these two session-level variables in your application's connection string. For example, set the following:
 

system-variables.md

@@ -439,7 +439,7 @@ This variable is an alias for [`last_insert_id`](#last_insert_id).
 
 > **Note:**
 >
-> Unlike in MySQL, the `max_execution_time` system variable currently works on all kinds of statements in TiDB, not only restricted to the `SELECT` statement. The precision of the timeout value is roughly 100ms. This means the statement might not be terminated in accurate milliseconds as you specify.
+> The `max_execution_time` system variable currently only controls the maximum execution time for read-only SQL statements. The precision of the timeout value is roughly 100ms. This means the statement might not be terminated in accurate milliseconds as you specify.
 
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