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SQL statement, system variables: update expression indexes #6185

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SQL statement, system variables: update expression indexes #6185

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SQL statement, TiDB configuration: update expression indexes
en-jin19 Aug 17, 2021
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Update sql-statements/sql-statement-create-index.md
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en-jin19 committed Aug 18, 2021
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2 changes: 1 addition & 1 deletion sql-statements/sql-statement-create-index.md
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Expand Up @@ -227,7 +227,7 @@ The global variables associated with the `CREATE INDEX` statement are `tidb_ddl_
* Descending indexes are not supported (similar to MySQL 5.7).
* Adding the primary key of the `CLUSTERED` type to a table is not supported. For more details about the primary key of the `CLUSTERED` type, refer to [clustered index](/clustered-indexes.md).
* Expression indexes are incompatible with the views. When you query contents via a view, the expression index can not be used at the same time.
* There are compatibility issues between expression indexes and bindings. When an expression in an expression index has constants, the binding created for the corresponding query expands its scope. For example, suppose the expression in the expression index is `a+1`, and the corresponding query condition is `a+1 > 2`. In this case, the binding is `a+? > ?`, which means that the queries such as `a+2 > 2` are also forced to select expression indexes. It results in a poor execution plan. Besides, this also affects the baseline capturing and baseline evolution belonging to SQL Plan Management.
* There are compatibility issues between expression indexes and bindings. When an expression in an expression index has constants, the binding created for the corresponding query expands its scope. For example, suppose the expression in the expression index is `a+1`, and the corresponding query condition is `a+1 > 2`. In this case, the binding is `a+? > ?`, which means that the queries such as `a+2 > 2` are also forced to use the expression index. It results in a poor execution plan. Besides, this also affects the baseline capturing and baseline evolution belonging to SQL Plan Management.

## See also

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2 changes: 1 addition & 1 deletion system-variables.md
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Expand Up @@ -505,7 +505,7 @@ Constraint checking is always performed in place for pessimistic transactions (d

- Scope: SESSION | GLOBAL
- Default value: `OFF`
- This variable is used to determine whether to involve the `AUTO_INCREMENT` columns when creating a generated column or an expression index.
- This variable is used to determine whether can include the `AUTO_INCREMENT` columns when creating a generated column or an expression index.

### tidb_enable_cascades_planner

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