Addressing MySQL Error 1073: BLOB Columns and Key Specifications

MySQL Error 1073, marked by SQLSTATE code 42000 (ER_BLOB_USED_AS_KEY), occurs when a BLOB or TEXT column is specified as a key in a table that does not support such columns as keys. The error message “BLOB column ‘%s’ can’t be used in key specification with the used table type” indicates that the operation is not allowed due to the nature of BLOB or TEXT data types, which are typically large and not suitable for indexing in their entirety. This guide will help you understand the problem and provide solutions to fix it.

Understanding the Error

BLOB and TEXT data types are used to store large amounts of data, such as images or large text. Because of their size, MySQL does not allow these data types to be used as primary keys or in a full index.

Diagnosing the Issue

The error message will specify the column name that is causing the issue. Review your table schema or the query that triggered the error to identify the BLOB or TEXT column being used incorrectly.

Fixing the Error

Scenario 1: Changing the Column Data Type

If you have control over the table’s design and the BLOB or TEXT column does not need to store large data, consider changing the column data type to VARCHAR or CHAR, which are indexable:

-- Attempting to create a table with a BLOB as a primary key
CREATE TABLE my_table (
  content BLOB,
  PRIMARY KEY (content)

-- Corrected table creation with a VARCHAR as a primary key
CREATE TABLE my_table (
  content VARCHAR(255),
  PRIMARY KEY (content)

Scenario 2: Using a Prefix Index

If you must use a BLOB or TEXT column in an index, you can specify a prefix length for the index. The prefix length determines how many characters of the column MySQL should use for the index:

-- Attempting to create an index on a BLOB column without a prefix length
CREATE INDEX content_index ON my_table (content);

-- Corrected index creation with a specified prefix length
CREATE INDEX content_index ON my_table (content(100));

Choose an appropriate prefix length that balances performance with the need for uniqueness.

Scenario 3: Adding a Separate Key Column

Instead of using a BLOB or TEXT column as a key, add a separate column to act as the primary key or index. This can be an auto-increment integer or a unique identifier:

-- Original table with a BLOB column
CREATE TABLE my_table (
  content BLOB,

Use the id column as the primary key, and reference it in your queries.

Scenario 4: Using FULLTEXT Indexes

For full-text searches in TEXT columns, use a FULLTEXT index, which is designed for this purpose:

-- Creating a FULLTEXT index on a TEXT column
CREATE TABLE my_table (
  content TEXT,
  FULLTEXT (content)

FULLTEXT indexes are only available for MyISAM and InnoDB table types as of MySQL 5.6.

Scenario 5: Evaluating Table Type

Ensure that the table type you are using supports the operations you are attempting. For example, older versions of MySQL with the MyISAM table type do not support indexing BLOB or TEXT columns without a prefix. Consider altering the table to use a different storage engine if necessary:

-- Altering the table to use the InnoDB storage engine

InnoDB allows prefix indexing for BLOB and TEXT columns.


Error 1073 in MySQL signals an attempt to perform an unsupported operation with BLOB or TEXT columns in key specifications. By adjusting the column data type, using prefix indexing, adding separate key columns, employing FULLTEXT indexes, or evaluating the table type, you can resolve this error and ensure your database schema is optimized for performance and functionality. Always remember to create backups before making structural changes to your database.

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