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Description

This error indicates that MySQL does not directly support creating an index on an expression or function applied to a column. It typically occurs when attempting to define an index using syntax common in other database systems for functional indexes.

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Error Message

Functional index on a column is not supported. Consider using a regular index instead.

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Known Causes

3 known causes

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Attempting Functional Index Creation

You tried to define an index on an expression, function, or calculation involving one or more columns, which is a feature not natively supported by MySQL's indexing engine.

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Misunderstanding MySQL Indexing

The error arises from using index creation syntax that works in other SQL databases (e.g., PostgreSQL, Oracle) but is not valid for MySQL's current implementation of indexes.

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Direct Expression Indexing

You attempted to index a computed value directly within the CREATE INDEX statement, instead of indexing a stored column or using a generated column for the expression.

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Solutions

3 solutions available

1. Replace Functional Index with Regular Index easy

Create a standard index on the column(s) used in the functional index.

1

Identify the table and column(s) where the functional index was attempted.

2

Drop the attempted functional index (if it was partially created or if you are re-attempting).

DROP INDEX index_name ON table_name;

3

Create a regular index on the relevant column(s). For example, if you tried to index `UPPER(column_name)`, create an index on `column_name`.

CREATE INDEX index_name ON table_name (column_name);

4

If the functional index was on multiple columns, create a composite index.

CREATE INDEX index_name ON table_name (column1, column2);

2. Materialize Derived Values in a Separate Column medium

Store the computed value of the functional expression in a new column and index that column.

1

Identify the functional expression and the table involved. For example, `UPPER(email)`.

2

Add a new column to the table to store the materialized value. Ensure the data type is appropriate.

ALTER TABLE table_name ADD COLUMN materialized_column VARCHAR(255);

3

Populate the new column with the computed values from the existing column(s). This can be done with an `UPDATE` statement.

UPDATE table_name SET materialized_column = UPPER(email);

4

Create a regular index on the newly created materialized column.

CREATE INDEX index_name ON table_name (materialized_column);

5

Implement a trigger or application-level logic to keep the `materialized_column` synchronized with the source column(s) on INSERT and UPDATE operations.

CREATE TRIGGER update_materialized_column BEFORE INSERT ON table_name FOR EACH ROW SET NEW.materialized_column = UPPER(NEW.email);

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Modify your queries to use the `materialized_column` for filtering and joining.

SELECT * FROM table_name WHERE materialized_column = 'EXAMPLE@DOMAIN.COM';

3. Review and Simplify Query Logic medium

Re-evaluate queries to avoid the need for functional indexes altogether.

1

Analyze the queries that were intended to use the functional index. Understand why the functional expression was necessary.

2

Consider if the data can be stored in a way that avoids the functional transformation. For example, if you're always storing email addresses in lowercase, you don't need `LOWER()` in your queries or indexes.

3

If the functional transformation is for case-insensitivity, consider setting the column's collation to a case-insensitive one (e.g., `utf8mb4_general_ci`). This allows standard indexes to work for case-insensitive comparisons.

ALTER TABLE table_name MODIFY COLUMN column_name VARCHAR(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;

4

After simplifying the query or adjusting data storage, create regular indexes on the relevant columns as needed.

CREATE INDEX index_name ON table_name (column_name);

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Related Errors

5 related errors

MySQL Error 3762: Functional Index Not Supported