sql-statements: update claims on optimizer/explain compatibility (#6695…

…) (#7296)
pingcap · Dec 20, 2021 · 7dcaf94 · 7dcaf94
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diff --git a/mysql-compatibility.md b/mysql-compatibility.md
@@ -81,7 +81,11 @@ TiDB uses a combination of [Prometheus and Grafana](/tidb-monitoring-api.md) to
 
 ### Query Execution Plan
 
-The output format, output content, and the privilege setting of Query Execution Plan (`EXPLAIN`/`EXPLAIN FOR`) in TiDB is greatly different from those in MySQL. See [Understand the Query Execution Plan](/explain-overview.md) for more details.
+The output format, output content, and the privilege setting of Query Execution Plan (`EXPLAIN`/`EXPLAIN FOR`) in TiDB is greatly different from those in MySQL.
+
+The MySQL system variable `optimizer_switch` is read-only in TiDB and has no effect on query plans. You can also use [optimizer hints](/optimizer-hints.md) in similar syntax to MySQL, but the available hints and implementation might differ.
+
+See [Understand the Query Execution Plan](/explain-overview.md) for more details.
 
 ### Built-in functions
 

diff --git a/sql-statements/sql-statement-explain.md b/sql-statements/sql-statement-explain.md
@@ -243,10 +243,10 @@ If the `dot` program is not installed on your computer, copy the result to [this
 ## MySQL compatibility
 
 * Both the format of `EXPLAIN` and the potential execution plans in TiDB differ substaintially from MySQL.
-* TiDB does not support the `EXPLAIN FORMAT=JSON` as in MySQL.
+* TiDB does not support the `FORMAT=JSON`  or `FORMAT=TREE` options.
 * TiDB does not currently support `EXPLAIN` for insert statements.
 
-## `EXPLAIN FOR CONNECTION`
+### `EXPLAIN FOR CONNECTION`
 
 `EXPLAIN FOR CONNECTION` is used to get the execution plan of the currently executed SQL query or the last executed SQL query in a connection. The output format is the same as that of `EXPLAIN`. However, the implementation of `EXPLAIN FOR CONNECTION` in TiDB is different from that in MySQL. Their differences (apart from the output format) are listed as follows: