With the exception of spatial indexes,
indexes are B-tree data
structures. Spatial indexes use
R-trees, which are
specialized data structures for indexing multi-dimensional data.
Index records are stored in the leaf pages of their B-tree or
R-tree data structure. The default size of an index page is
When new records are inserted into an
InnoDB tries to leave 1/16 of the page free
for future insertions and updates of the index records. If index
records are inserted in a sequential order (ascending or
descending), the resulting index pages are about 15/16 full. If
records are inserted in a random order, the pages are from 1/2
to 15/16 full.
InnoDB performs a bulk load when creating or
rebuilding B-tree indexes. This method of index creation is
known as a sorted index build. The
configuration option defines the percentage of space on each
B-tree page that is filled during a sorted index build, with the
remaining space reserved for future index growth. Sorted index
builds are not supported for spatial indexes. For more
information, see Section 184.108.40.206, “Sorted Index Builds”. An
innodb_fill_factor setting of
100 leaves 1/16 of the space in clustered index pages free for
future index growth.
If the fill factor of an
InnoDB index page
drops below the
MERGE_THRESHOLD, which is 50%
by default if not specified,
InnoDB tries to
contract the index tree to free the page. The
MERGE_THRESHOLD setting applies to both
B-tree and R-tree indexes. For more information, see
Section 15.6.12, “Configuring the Merge Threshold for Index Pages”.
You can define the page
size for all
InnoDB tablespaces in a
MySQL instance by setting the
option prior to initializing the MySQL instance. Once the page
size for an instance is defined, you cannot change it without
reinitializing the instance. Supported sizes are 64KB, 32KB,
16KB (default), 8KB, and 4KB.
A MySQL instance using a particular
page size cannot use data files or log files from an instance
that uses a different page size.