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15.6.2.2 The Physical Structure of an InnoDB Index
With the exception of spatial indexes, InnoDB
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 16KB.
When new records are inserted into an InnoDB
clustered index,
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
innodb_fill_factor 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 15.6.2.3, “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 15.8.11, “Configuring the Merge Threshold for Index Pages”.
You can define the page size
for all InnoDB tablespaces in a MySQL instance
by setting the innodb_page_size
configuration 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 InnoDB page
size cannot use data files or log files from an instance that uses
a different page size.
Document created the 26/06/2006, last modified the 26/10/2018
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