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Description

This error occurs when you attempt to drop a column from a MySQL table, but that specific column is currently being utilized by a functional index. MySQL prevents the column drop to maintain the integrity and functionality of the dependent index. To resolve this, the functional index must be removed before the column can be dropped.

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Error Message

Cannot drop column '%s' because it is used by a functional index. In order to drop the column, you must remove the functional index.

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Known Causes

3 known causes

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Direct Column Drop Attempt

You tried to remove a table column using `ALTER TABLE ... DROP COLUMN` without first checking for its dependencies.

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Unrecognized Index Dependency

The existence or specific reliance of a functional index on the target column was not identified prior to the drop operation.

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Automated Schema Modification

An automated script or migration attempted to drop the column without including preceding steps to manage dependent functional indexes.

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Solutions

3 solutions available

1. Identify and Drop the Functional Index easy

Find the functional index using the problematic column and then drop it.

1

Identify the functional index. You can query the `information_schema.STATISTICS` table to find indexes that reference your column. Look for columns of type 'FUNCTION' in the `INDEX_COL_NAME` or `SUB_PART` fields.

SELECT INDEX_NAME, TABLE_NAME, COLUMN_NAME, SUB_PART FROM information_schema.STATISTICS WHERE TABLE_SCHEMA = 'your_database_name' AND COLUMN_NAME = 'your_column_name';

2

Once you've identified the `INDEX_NAME`, drop the functional index using the `DROP INDEX` statement.

DROP INDEX functional_index_name ON your_table_name;

3

After successfully dropping the functional index, you can now drop the column.

ALTER TABLE your_table_name DROP COLUMN your_column_name;

2. Temporarily Rename Column, Drop Index, Then Rename Back medium

A workaround to avoid immediate data loss or application downtime if the index is critical.

1

Identify the functional index as described in the previous solution.

SELECT INDEX_NAME, TABLE_NAME, COLUMN_NAME, SUB_PART FROM information_schema.STATISTICS WHERE TABLE_SCHEMA = 'your_database_name' AND COLUMN_NAME = 'your_column_name';

2

Temporarily rename the column to something that won't be used by the functional index. This might involve renaming it to a similar name or a temporary placeholder.

ALTER TABLE your_table_name RENAME COLUMN your_column_name TO your_column_name_temp;

3

Drop the functional index. Since the column has been renamed, the index will no longer be actively referencing it.

DROP INDEX functional_index_name ON your_table_name;

4

Now, rename the column back to its original name.

ALTER TABLE your_table_name RENAME COLUMN your_column_name_temp TO your_column_name;

5

You can now proceed with dropping the column if that was your original intent, or re-evaluate its necessity.

ALTER TABLE your_table_name DROP COLUMN your_column_name;

3. Recreate Table Without the Column and Index advanced

A more disruptive but thorough approach for a complete cleanup.

1

Identify the functional index and the column to be dropped.

SELECT INDEX_NAME, TABLE_NAME, COLUMN_NAME, SUB_PART FROM information_schema.STATISTICS WHERE TABLE_SCHEMA = 'your_database_name' AND COLUMN_NAME = 'your_column_name';

2

Create a new table with the desired schema, excluding the column you want to drop and the functional index.

CREATE TABLE your_table_name_new LIKE your_table_name;

3

Copy the data from the original table to the new table, omitting the column to be dropped. Be mindful of data types and any transformations needed.

INSERT INTO your_table_name_new (column1, column2, ...) SELECT column1, column2, ... FROM your_table_name;

4

Add any necessary indexes to the new table, excluding the functional index that used the dropped column.

ALTER TABLE your_table_name_new ADD INDEX index_name (column_name);

5

Verify the data and schema in `your_table_name_new` thoroughly.

SELECT COUNT(*) FROM your_table_name;
SELECT COUNT(*) FROM your_table_name_new;

6

Once confident, drop the original table and rename the new table to take its place.

DROP TABLE your_table_name;
RENAME TABLE your_table_name_new TO your_table_name;

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Related Errors

5 related errors

MySQL Error 3755: Column Used by Functional Index