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Description

MySQL Error 3756 occurs when you attempt to define a PRIMARY KEY constraint on an expression or a functional index. Primary keys must be based directly on one or more columns, not on the result of a function or computation. This error prevents the creation or alteration of tables with such an invalid primary key definition.

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Error Message

The primary key cannot be a functional index

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Known Causes

3 known causes

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Defining PK on an Expression

You attempted to create a PRIMARY KEY directly using an expression or a function's output in your CREATE TABLE or ALTER TABLE statement.

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Misunderstanding Index Types

There's a misunderstanding of how primary keys differ from functional indexes, leading to an attempt to combine their definitions incorrectly.

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Incorrect Uniqueness Enforcement

You tried to enforce uniqueness on a derived or computed column by defining it as a primary key, which is not supported for functional indexes.

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Solutions

3 solutions available

1. Remove Functional Index from Primary Key medium

Recreate the table without the functional index on the primary key.

1

Identify the table and the primary key column(s) that are defined as a functional index.

SHOW CREATE TABLE your_table_name;

2

Create a new table with the same schema but without the functional index definition on the primary key.

CREATE TABLE your_table_name_new (
    column1 datatype,
    column2 datatype,
    -- ... other columns ...
    PRIMARY KEY (column1, column2) -- Assuming column1, column2 are your primary key
);

3

Copy the data from the old table to the new table.

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

4

Rename the tables to replace the old one with the new one.

RENAME TABLE your_table_name TO your_table_name_old, your_table_name_new TO your_table_name;

5

Optionally, drop the old table after verifying the data.

DROP TABLE your_table_name_old;

2. Modify Primary Key to be Non-Functional medium

Recreate the primary key using standard column definitions.

1

Identify the table and the primary key column(s).

SHOW CREATE TABLE your_table_name;

2

Drop the existing primary key constraint.

ALTER TABLE your_table_name DROP PRIMARY KEY;

3

Add a new primary key constraint using the actual column(s) without any functional index expressions.

ALTER TABLE your_table_name ADD PRIMARY KEY (column1, column2); -- Replace column1, column2 with your actual primary key columns

3. Re-evaluate Indexing Strategy medium

Separate functional indexing needs from primary key requirements.

1

Analyze the queries that benefit from the functional index. Understand what data is being derived or manipulated.

EXPLAIN your_query_using_functional_index;

2

If the functional index is essential for performance, create it as a separate index on the table, not as part of the primary key.

CREATE INDEX idx_functional_expression ON your_table_name ((expression_using_columns)); -- Example for generated columns or functional indexes

3

Ensure the primary key is defined on the actual base columns that uniquely identify a row.

ALTER TABLE your_table_name DROP PRIMARY KEY;
ALTER TABLE your_table_name ADD PRIMARY KEY (base_column1, base_column2);

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Related Errors

5 related errors

MySQL Error 3756: Primary Key Functional Index