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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：
在ISAM系统，数据组织成有固定长度的记录，按顺序存储的。
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 |
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[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. |
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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 |
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[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. |
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原文地址：http://www.cnblogs.com/yuanjiangw/p/5797772.html
