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Description

MySQL Error 1491, symbolized as `ER_PARTITION_FUNC_NOT_ALLOWED_ERROR`, occurs when a function specified in a `PARTITION BY` clause for table partitioning returns a data type that is not supported or expected by the database. This typically happens when MySQL requires an integer return type for partitioning (e.g., for `RANGE` or `LIST` partitioning), but the function provides a different type.

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Error Message

The %s function returns the wrong type

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Known Causes

3 known causes

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Non-Integer Return Type

The function used within the `PARTITION BY` expression evaluates to a data type (e.g., string, date, or float) that is incompatible with MySQL's partitioning requirements, which primarily expect an integer.

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Unsupported Function Usage

Certain MySQL functions, even if they return an integer, might not be permitted within a `PARTITION BY` clause due to their non-deterministic nature or other internal restrictions.

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Partitioning Type Mismatch

The chosen partitioning type (e.g., `RANGE` or `LIST`) explicitly requires the partitioning function to yield an integer value, but the function provided produces a different data type.

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Solutions

3 solutions available

1. Review Partitioning Function and Column Type Compatibility medium

Ensure the data type of the column used in the partitioning function matches the expected return type of that function.

1

Identify the table and the partitioning function causing the error. This is usually indicated in the error message or by examining the `CREATE TABLE` or `ALTER TABLE` statement that failed.

2

Examine the data type of the column specified in the `PARTITION BY` clause.

DESCRIBE your_table_name;

3

Examine the expected return type of the partitioning function. For example, `YEAR()` returns an integer, `TO_DAYS()` returns an integer, `MD5()` returns a string, etc. Consult the MySQL documentation for the specific function being used.

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If there's a mismatch, you have two primary options:

1. **Modify the column's data type** to be compatible with the function's return type. This might involve casting or converting the column data.
2. **Change the partitioning function** to one that is compatible with the current column's data type.

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If you need to change the column's data type, use `ALTER TABLE MODIFY COLUMN`. For example, if `PARTITION BY YEAR(date_column)` failed because `date_column` was a `VARCHAR` and you want to fix it, you might do:

sql
ALTER TABLE your_table_name MODIFY COLUMN date_column DATE;

If you need to change the partitioning function, you'll likely need to drop and recreate the partitions or the table.

2. Recreate Table with Correct Partitioning advanced

For complex partitioning schemes or when data type changes are difficult, recreating the table with the correct partitioning is often the cleanest solution.

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Create a new table with the same structure as the problematic table, but with the correct partitioning definition. Pay close attention to the `PARTITION BY` clause and the data types of the partitioning columns.

CREATE TABLE your_new_table LIKE your_old_table;

ALTER TABLE your_new_table PARTITION BY HASH(column_name) PARTITIONS 4; -- Example: Replace with your correct partitioning strategy

2

Insert all data from the old table into the new table.

INSERT INTO your_new_table SELECT * FROM your_old_table;

3

Verify that the data has been transferred correctly and that queries on the new table are performing as expected.

4

Once confirmed, drop the old table and rename the new table to take its place.

DROP TABLE your_old_table;
RENAME TABLE your_new_table TO your_old_table;

3. Use Compatible Built-in Functions easy

Choose partitioning functions that are explicitly designed for the data types you are using.

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Review the MySQL documentation for partitioning functions and their supported data types. For example, for date/time columns, `YEAR()`, `MONTH()`, `DAYOFMONTH()`, `TO_DAYS()`, and `UNIX_TIMESTAMP()` are common and generally compatible with `DATE`, `DATETIME`, and `TIMESTAMP` types.

2

If your partitioning column is a string and you're trying to use a date function, you'll encounter this error. Instead, consider partitioning by a derived numeric value if possible, or rethink your partitioning strategy.

3

Example: If you have a `VARCHAR` column storing dates like 'YYYY-MM-DD' and want to partition by year, you might need to extract the year first or ensure the column is a proper `DATE` type. If you cannot change the column type, consider using string manipulation functions if they return a type compatible with the partitioning strategy (e.g., `SUBSTRING` if used with a numeric partitioning type).

4

If you were trying to partition by a custom function or a non-standard expression, try to replace it with a standard, well-supported MySQL partitioning function.

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Related Errors

5 related errors

MySQL Error 1491: Incorrect Partitioning Function Type