B-tree indexes

High Performance MySQL, Third Edition

by Baron Schwartz, Peter Zaitsev, and Vadim Tkachenko

http://dev.mysql.com/doc/refman/5.7/en/

https://zh.wikipedia.org/wiki/ISAM

https://en.wikipedia.org/wiki/ISAM

ISAM (an acronym for Indexed Sequential Access Method) is a method for indexing data for fast retrieval.

索引顺序存取方法（ISAM, Indexed Sequential Access Method）最初是IBM公司发展起来的一个文件系统，可以连续地（按照他们进入的顺序）或者任意地（根据索引）记录任何访问。每个索引定义了一次不同排列的记录。现在这个概念用在许多场合：

• 特指IBM公司的ISAM产品
• 数据库系统中提供用户接口从数据文件中检索数据。
• 通常指，数据库的索引，这种索引被大多数数据库所采用，包括关系数据库或其它。

在ISAM系统，数据组织成有固定长度的记录，按顺序存储的。

• MySQL implements and extends ISAM as MyISAM
• //MySQL实现ISAM为MyISAM

In an ISAM system, data is organized into records which are composed of fixed length fields. Records are stored sequentially, originally to speed access on a tape system. A secondary set of hash tables known as indexes contain "pointers" into the tables, allowing individual records to be retrieved without having to search the entire data set. This is a departure from the contemporaneous navigational databases, in which the pointers to other data were stored inside the records themselves. The key improvement in ISAM is that the indexes are small and can be searched quickly, thereby allowing the database to access only the records it needs. Additionally modifications to the data do not require changes to other data, only the table and indexes in question.

When an ISAM file is created, index nodes are fixed, and their pointers do not change during inserts and deletes that occur later (only content of leaf nodes change afterwards). As a consequence of this, if inserts to some leaf node exceed the node's capacity, new records are stored in overflow chains. If there are many more inserts than deletions from a table, these overflow chains can gradually become very large, and this affects the time required for retrieval of a record.^[4]

Table 16.2 MyISAM Storage Engine Features

Storage limits	256TB	Transactions	No	Locking granularity	Table
MVCC	No	Geospatial data type support	Yes	Geospatial indexing support	Yes
B-tree indexes	Yes	T-tree indexes	No	Hash indexes	No
Full-text search indexes	Yes	Clustered indexes	No	Data caches	No
Index caches	Yes	Compressed data	Yes[a]	Encrypted data[b]	Yes
Cluster database support	No	Replication support[c]	Yes	Foreign key support	No
Backup / point-in-time recovery[d]	Yes	Query cache support	Yes	Update statistics for data dictionary	Yes
[a] Compressed MyISAM tables are supported only when using the compressed row format. Tables using the compressed row format with MyISAM are read only. [b] Implemented in the server (via encryption functions). Data-at-rest tablespace encryption is available in MySQL 5.7 and higher. [c] Implemented in the server, rather than in the storage engine. [d] Implemented in the server, rather than in the storage engine.

Each MyISAM table is stored on disk in three files. The files have names that begin with the table name and have an extension to indicate the file type. An .frm file stores the table format. The data file has an .MYD (MYData) extension. The index file has an .MYI(MYIndex) extension.

Table 15.1 InnoDB Storage Engine Features

Storage limits	64TB	Transactions	Yes	Locking granularity	Row
MVCC	Yes	Geospatial data type support	Yes	Geospatial indexing support	Yes[a]
B-tree indexes	Yes	T-tree indexes	No	Hash indexes	No[b]
Full-text search indexes	Yes[c]	Clustered indexes	Yes	Data caches	Yes
Index caches	Yes	Compressed data	Yes[d]	Encrypted data[e]	Yes
Cluster database support	No	Replication support[f]	Yes	Foreign key support	Yes
Backup / point-in-time recovery[g]	Yes	Query cache support	Yes	Update statistics for data dictionary	Yes
[a] InnoDB support for geospatial indexing is available in MySQL 5.7.5 and higher. [b] InnoDB utilizes hash indexes internally for its Adaptive Hash Index feature. [c] InnoDB support for FULLTEXT indexes is available in MySQL 5.6.4 and higher. [d] Compressed InnoDB tables require the InnoDB Barracuda file format. [e] Implemented in the server (via encryption functions). Data-at-rest tablespace encryption is available in MySQL 5.7 and higher. [f] Implemented in the server, rather than in the storage engine. [g] Implemented in the server, rather than in the storage engine.

Storage engines use B-Tree indexes in various ways, which can affect performance. For instance, MyISAM uses a prefix compression technique that makes indexes smaller,but InnoDB leaves values uncompressed in its indexes. Also, MyISAM indexes refer to the indexed rows by their physical storage locations, but InnoDB refers to them by their primary key values. Each variation has benefits and drawbacks.

The general idea of a B-Tree is that all the values are stored in order, and each leaf page is the same distance from the root.