How to fix ERROR 1073: BLOB column can't be used in key in MySQL

MySQLINTERMEDIATEHIGHUpdated Dec 22, 2025· 148 views

MySQL Error 1073 occurs when trying to use a BLOB column as a key or index without specifying a prefix length. This happens because BLOB columns store variable-length data that MySQL cannot fully index, requiring either a table engine change, prefix length specification, or column type modification.

Quick fix

Fixes ERROR 1073: BLOB column can't be used in key

DESC your_table_name;
  1. Verify the table structure and identify the BLOB column
  2. Specify a prefix length for the BLOB column in the index
  3. Consider using VARCHAR instead of BLOB/TEXT for keys
  4. Use a separate surrogate key and remove BLOB from key constraints
mysql — error
DESC your_table_name;
ERROR 1073: BLOB column can't be used in key

What this error means

MySQL Error 1073 (ER_BLOB_USED_AS_KEY) occurs when you attempt to create an index, primary key, or unique constraint on a BLOB column without specifying a prefix length. BLOB (Binary Large Object) columns are designed to store large binary data like images, video files, or other binary content. Because BLOB data is variable-length and can be very large, MySQL cannot use the entire column as a key—it needs to know exactly how many bytes to index. The error indicates that your table engine (like MyISAM) doesn't support BLOB indexing, or you've tried to use a BLOB column as a primary key without providing a length constraint.

Typical symptoms

  • Error message "BLOB column can't be used in key specification" appears when creating tables or altering table structure
  • CREATE TABLE or ALTER TABLE commands fail when BLOB columns are referenced in PRIMARY KEY, UNIQUE, or INDEX definitions
  • The error occurs specifically with BLOB, TEXT, TINYBLOB, MEDIUMBLOB, LONGBLOB, TINYTEXT, MEDIUMTEXT, or LONGTEXT column types
  • Migration tools or ORM frameworks fail when generating schemas with BLOB-based keys

Common causes

  1. Attempting to use a BLOB or TEXT column as a PRIMARY KEY without specifying a prefix length (e.g., `data BLOB` in PRIMARY KEY instead of `data BLOB(100)` in an index)
  2. Creating a UNIQUE constraint on a full BLOB column without a length prefix
  3. Using MyISAM or other older table engines that don't support BLOB/TEXT indexing
  4. Incorrectly converting VARCHAR(size) to TEXT implicitly—when size exceeds 255, MySQL auto-converts to TEXT, which causes key specification errors
  5. Including BLOB columns in FOREIGN KEY constraints, which require indexed columns

How to fix "ERROR 1073: BLOB column can't be used in key"

1Verify the table structure and identify the BLOB column

First, examine your CREATE TABLE statement to locate which column is causing the error:

sql
DESC your_table_name;

Look for any BLOB, TEXT, or similar variable-length binary types that are part of a PRIMARY KEY or UNIQUE constraint.

2Specify a prefix length for the BLOB column in the index

The most common solution is to add a length prefix when using BLOB/TEXT in indexes. Instead of:

sql
CREATE TABLE my_table (
  data BLOB,
  PRIMARY KEY (data)
);

Use a prefix length:

sql
CREATE TABLE my_table (
  id INT PRIMARY KEY AUTO_INCREMENT,
  data BLOB,
  UNIQUE KEY (data(100))
);

The number (100) specifies that only the first 100 bytes of the BLOB column should be indexed.

3Consider using VARCHAR instead of BLOB/TEXT for keys

If the column doesn't need to store truly large binary data, consider using VARCHAR instead:

sql
CREATE TABLE my_table (
  id INT PRIMARY KEY AUTO_INCREMENT,
  hash_value VARCHAR(255) UNIQUE
);

VARCHAR(255) can be fully indexed without a prefix length. VARCHAR(256) or larger will be auto-converted to TEXT, causing the error. Keep VARCHAR columns under 255 characters for full indexing.

4Use a separate surrogate key and remove BLOB from key constraints

The safest approach is to use an auto-increment integer as the primary key and remove BLOB columns from key definitions:

sql
CREATE TABLE my_table (
  id INT PRIMARY KEY AUTO_INCREMENT,
  data BLOB,
  metadata VARCHAR(255)
);

CREATE INDEX idx_metadata ON my_table (metadata(100));

This allows you to store BLOB data without key constraint issues.

5Verify the solution with a test table

After making changes, test the corrected statement:

sql
DROP TABLE IF EXISTS test_blob_table;

CREATE TABLE test_blob_table (
  id INT PRIMARY KEY AUTO_INCREMENT,
  file_data BLOB,
  file_hash VARCHAR(255) UNIQUE,
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

INSERT INTO test_blob_table (file_data, file_hash) VALUES (
x'48656C6C6F', 'abc123');

SELECT * FROM test_blob_table;

If no errors appear, the fix is successful.

Advanced notes

PREFIX LENGTH CONSIDERATION: When specifying a prefix length for BLOB/TEXT columns in indexes, choose a balance between specificity and index size. Shorter prefixes (like 10-50) create smaller indexes but may have more hash collisions. Longer prefixes (like 255-500) are more specific but use more disk space. For most use cases, 100-255 bytes is appropriate.

FOREIGN KEY CONSTRAINT: BLOB/TEXT columns cannot be referenced in FOREIGN KEY constraints at all, even with prefix length, because indexes on those columns must include a prefix. If you need a foreign key on a string column, use VARCHAR with a length under 255 bytes instead.

INN0DB STORAGE: While InnoDB technically supports prefix indexes on BLOB columns better than MyISAM, the same prefix length rule applies. InnoDB is the default in modern MySQL (5.7+) and handles BLOB columns more gracefully, but the indexing limitation remains.

ORMFRAMEWORK CONSIDERATIONS: ORMs like Sequelize, TypeORM, or Doctrine may auto-generate problematic schema statements. If using an ORM, explicitly define column indexes in your model configuration rather than relying on automatic @PrimaryKey or @Unique decorators on BLOB columns.

VARCHAR AUTO-CONVERSION: Be aware that VARCHAR(256) and larger are automatically converted to TEXT in MySQL, triggering the same error. If you need a TEXT field as a key, always use a prefix length in your index definition.

Frequently asked questions

What does "ERROR 1073: BLOB column can't be used in key" mean?
MySQL Error 1073 (ER_BLOB_USED_AS_KEY) occurs when you attempt to create an index, primary key, or unique constraint on a BLOB column without specifying a prefix length. BLOB (Binary Large Object) columns are designed to store large binary data like images, video files, or other binary content. Because BLOB data is variable-length and can be very large, MySQL cannot use the entire column as a key—it needs to know exactly how many bytes to index.
How do I fix "ERROR 1073: BLOB column can't be used in key"?
MySQL Error 1073 occurs when trying to use a BLOB column as a key or index without specifying a prefix length. This happens because BLOB columns store variable-length data that MySQL cannot fully index, requiring either a table engine change, prefix length specification, or column type modification.
What causes "ERROR 1073: BLOB column can't be used in key"?
Attempting to use a BLOB or TEXT column as a PRIMARY KEY without specifying a prefix length (e.g., data BLOB in PRIMARY KEY instead of data BLOB(100) in an index) Creating a UNIQUE constraint on a full BLOB column without a length prefix Using MyISAM or other older table engines that don't support BLOB/TEXT indexing
Is the "ERROR 1073: BLOB column can't be used in key" error serious?
Yes. This is a high-severity error that usually blocks work until it is fixed, but the steps above resolve it.

Official resources & further reading