Let's assume that I have table data:

CREATE  TABLE `test`.`data` (
`row_id` INT NOT NULL ,
`data_id` INT NOT NULL ,
PRIMARY KEY (`row_id`) );

Now, let's assume that data_id has unique values. Is there any difference(space, performance, memory) between having B-Tree index on data_id and UNIQUE index on it? (besides the case when InnoDB is used as storage engine and UNIQUE index can be used as clustering key so row_id is uneccesary)

1 Answer 1


According to the Book

Understanding MySQL Internals

Page 198 says the following:

  • Paragraph 6 : A MyISAM B-tree consists of leaf and nonleaf nodes, or pages.
  • Paragraph 7 : Both leaf and nonleaf nodes key values and pointers to the record positions in the datafile. Nonleaf nodes additionally contains pointers to child nodes.

Given this description

  • a unique index would have more nonleaf nodes. This would lend itself mode towards ordering and searching.
  • nonunique index would requires less nonleaf nodes. This would lend itself mode towards doing range scans (tables and index)
  • A covering index (which would contain all needed columns for specific SELECT queries) would combine the best and worst of both. This would allow for a range scans that would need more nonleaf nodes and provide for ordering/searching. The additional benefit would be bypassing the need for reading table data if all needed columns are present and accounted for in the index.
  • I mentioned other aspects in my past post Benefits of BTREE in MySQL


  • Unique Indexes would take up more space for nonleaf nodes.
  • Covering indexes would need more space than a nonunqiue index but would have a higher multi-columns index cardinality that can approach the actual row count of the table.

PERFORMANCE (Heavy-Write Environment)

  • SELECTs would be very fast for exact keys
  • SELECTs that only touch a Covering Index would be extremely fast
  • Random INSERTs can be a nightmare due to rotations/rebalancing of nonleaf nodes
  • UPDATEs that change key values can be a nightmare due to rotations/rebalancing of nonleaf nodes
  • The higher the cardinality, the more nonleaf nodes, the better the performance
  • The lower the cardinality, the fewer the nonleaf nodes, the worse the performance
  • The more columns in the index increases the effects of the cardinality


Depends on Storage Engine


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.