14.6.2.2 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 14.6.2.3, “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 14.8.12, “Configuring the Merge Threshold for Index Pages”.
You can define the page size for all InnoDB
tablespaces in a MySQL instance by setting the 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.
Support for 32KB and 64KB pages sizes was added in MySQL 5.7. For more information, refer to the innodb_page_size
documentation.
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.