title: Max size of UNIQUE index in MySQL
body: <p>Is there a max size of a <code>UNIQUE</code> index in MySQL?
  I have six dimensions in my index, which leads to a massive index.
  Is there a cardinality limit to <code>UNIQUE</code> index? </p>
  
  <p>I am getting weird unexplained errors and wonder if there is a limit from the <code>UNIQUE</code>.
  Could it come from a uniqueness limit?
  Here is what it looks like (I also don't understand why the cardinality of the last three columns does not increase).</p>
  
  <pre><code>+-------+------------+----------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
  | Table | Non_unique | Key_name | Seq_in_index | Column_name   | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
  +-------+------------+----------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
  | table |          0 | PRIMARY  |            1 | column_1      | A         |          15 |     NULL | NULL   |      | BTREE      |         |               |
  | table |          0 | PRIMARY  |            2 | column_2      | A         |       91948 |     NULL | NULL   |      | BTREE      |         |               |
  | table |          0 | PRIMARY  |            3 | column_3      | A         |      924889 |     NULL | NULL   |      | BTREE      |         |               |
  | table |          0 | PRIMARY  |            4 | column_4      | A         |    15723114 |     NULL | NULL   |      | BTREE      |         |               |
  | table |          0 | PRIMARY  |            5 | column_5      | A         |    15723114 |     NULL | NULL   |      | BTREE      |         |               |
  | table |          0 | PRIMARY  |            6 | column_6      | A         |    15723114 |     NULL | NULL   |      | BTREE      |         |               |
  +-------+------------+----------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
  </code></pre>