The storage for rows and associated columns affects performance for queries and DML operations. As more rows fit into a single disk page, queries and index lookups can work faster, less cache memory is required in the InnoDB buffer pool, and less I/O is required to write out updated values for the numeric and short string columns.
The data in each InnoDB table is divided into pages. The pages that make up each table are arranged in a tree data structure called a B-tree index. Table data and secondary indexes both use this type of structure. The B-tree index that represents an entire table is known as the clustered index, which is organized according to the primary key columns. The nodes of the index data structure contain the values of all the columns in that row (for the clustered index) or the index columns and the primary key columns (for secondary indexes).
Variable-length columns are an exception to this rule. Columns
that are too long to fit on a B-tree page are stored on separately
allocated disk pages called
overflow pages. We call
such columns off-page
columns. The values of these columns are stored in
singly-linked lists of overflow pages, and each such column has
its own list of one or more overflow pages. In some cases, all or
a prefix of the long column value is stored in the B-tree, to
avoid wasting storage and eliminating the need to read a separate
The Barracuda file format
provides a new option (
control how much column data is stored in the clustered index, and
how much is placed on overflow pages.
The next section describes the clauses you can use with the
CREATE TABLE and
ALTER TABLE statements to control
how these variable-length columns are represented:
KEY_BLOCK_SIZE. To use these clauses, you might
also need to change the settings for the