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sql-statements: update claims on optimizer/explain compatibility (#6695) #7297

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Dec 20, 2021
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sql-statements: update claims on optimizer/explain compatibility (#6695) #7297

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sql-statements: update claims on explain compatibility
morgo Oct 29, 2021
34d0e8a
Add additional notes on optimizer
morgo Oct 29, 2021
16be97b
Update mysql-compatibility.md
morgo Dec 17, 2021
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6 changes: 5 additions & 1 deletion mysql-compatibility.md
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Expand Up @@ -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

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4 changes: 2 additions & 2 deletions sql-statements/sql-statement-explain.md
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Expand Up @@ -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:

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