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Description

This error indicates an attempt to create a functional index on an expression that resolves to a BLOB or TEXT data type. MySQL explicitly prohibits functional indexes on these large object types due to their inherent storage characteristics and variable length, preventing the index creation.

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Error Message

Cannot create a functional index on an expression that returns a BLOB or TEXT. Please consider using CAST.

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Known Causes

3 known causes

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Indexing BLOB/TEXT Expression

Directly attempting to create a functional index on an expression derived from a BLOB or TEXT column without proper type conversion.

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Implicit LOB Return Type

Using a function within a functional index definition that implicitly returns a BLOB or TEXT data type, even if the source column is not.

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Misunderstanding Index Limitations

Lack of awareness regarding MySQL's specific restrictions against creating functional indexes on BLOB or TEXT data types.

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Solutions

3 solutions available

1. Cast LOB to a String Type for Indexing easy

Convert the LOB data to a smaller, indexable string type using CAST.

1

Identify the column with the BLOB or TEXT type that you intend to index. Let's assume it's a column named `large_text_column` in a table named `my_table`.

DESCRIBE my_table;

2

Instead of directly indexing the LOB column, create a functional index on a CAST expression that converts a portion of the LOB to a CHAR or VARCHAR. You'll need to decide on a reasonable length for the cast. For example, casting the first 255 characters.

CREATE INDEX idx_large_text_prefix ON my_table ((CAST(large_text_column AS CHAR(255))));

3

If you need to index the entire LOB (which is generally not recommended for performance reasons and might still hit limitations depending on the exact LOB type and MySQL version), you might need to explore full-text indexing or other specialized solutions. However, for typical use cases where you're searching for prefixes or specific keywords, casting a prefix is the most practical approach.

2. Create a Separate Indexed Column for LOB Content medium

Maintain a separate, smaller column that stores a relevant excerpt or hash of the LOB for indexing.

1

Add a new column to your table that will store a manageable representation of the LOB data. This could be a prefix, a hash, or a summary.

ALTER TABLE my_table ADD COLUMN indexed_text_excerpt VARCHAR(255) NULL AFTER large_text_column;

2

Create an index on this new column.

CREATE INDEX idx_indexed_text ON my_table (indexed_text_excerpt);

3

Implement application logic or a trigger to populate and maintain the `indexed_text_excerpt` column whenever `large_text_column` is inserted or updated. This could involve extracting a prefix, generating a hash (e.g., MD5 or SHA1), or creating a summary. Example using application logic (conceptual):

UPDATE my_table SET indexed_text_excerpt = LEFT(large_text_column, 255) WHERE ...;

4

Alternatively, use a trigger for automatic updates. Example trigger (conceptual):

DELIMITER $$
CREATE TRIGGER trg_update_indexed_text BEFORE INSERT ON my_table
FOR EACH ROW BEGIN
  SET NEW.indexed_text_excerpt = LEFT(NEW.large_text_column, 255);
END;$$
DELIMITER ;

DELIMITER $$
CREATE TRIGGER trg_update_indexed_text_update BEFORE UPDATE ON my_table
FOR EACH ROW BEGIN
  IF NEW.large_text_column <> OLD.large_text_column THEN
    SET NEW.indexed_text_excerpt = LEFT(NEW.large_text_column, 255);
  END IF;
END;$$
DELIMITER ;

3. Utilize Full-Text Search for LOB Content medium

Leverage MySQL's built-in full-text indexing for efficient searching within LOBs.

1

Ensure your table uses a storage engine that supports full-text indexing, such as InnoDB (default in modern MySQL versions).

2

Add a `FULLTEXT` index to the BLOB or TEXT column. Note that full-text indexing is typically only supported for `TEXT`, `MEDIUMTEXT`, and `LONGTEXT` column types, not `BLOB` types directly. If you have a `BLOB` that needs full-text indexing, you might need to convert it to `LONGTEXT` or store it as a `LONGTEXT` in the first place.

ALTER TABLE my_table ADD FULLTEXT(large_text_column);

3

Use `MATCH() AGAINST()` syntax for searching. For example, to find rows containing the word 'example':

SELECT * FROM my_table WHERE MATCH(large_text_column) AGAINST('example' IN NATURAL LANGUAGE MODE);

4

Consider the `ft_min_word_len` and `ft_max_word_len` server variables if you need to index very short or very long words.

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Related Errors

5 related errors

MySQL Error 3757: Functional Index on LOB Type