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CHAPTER 2 THE ANATOMY OF A DATABASE SYSTEM
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Note Practice has shown that the LRU algorithm performs within 80 percent of the best algorithms. In a
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The privilege cache is used to store grant data on a user account. This data is stored in the same manner as an access control list (ACL), which lists all of the privileges a user has for an object in the system. The privilege cache is implemented as a structure stored in a first in, last out (FILO) hash table. Data for the cache is gathered when the grant tables are read during user authentication and initialization. It is important to store this data in memory as it saves a lot of time reading the grant tables.
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The hostname cache is another of the helper caches, like the privilege cache. It too is implemented as a stack of a structure. It contains the hostnames of all the connections to the server. It may seem surprising, but this data is frequently requested and therefore in high demand and a candidate for a dedicated cache.
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A number of other small cache mechanisms are implemented throughout the MySQL source code. One example is the join buffer cache used during complex join operations. For example, some join operations require comparing one tuple to all the tuples in the second table. A cache in this case can store the tuples read so that the join can be implemented without having to reread the second table into memory multiple times.
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File Access via Pluggable Storage Engines
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One of the best features of MySQL is the ability to support different storage engines, or file types. This allows database professionals to tune their database performance by selecting the storage engine that best meets their application needs. Examples include using storage engines that provide transaction control for highly active databases where transaction processing is required or using the memory storage engine whenever a table is read many times but seldom updated (e.g., a lookup table). MySQL AB added a new architectural design in version 5 that makes it easier to add new storage types. The new mechanism is called the MySQL pluggable storage engine. MySQL AB has worked hard to make the server extensible via the pluggable storage engine. The pluggable storage engine was created as an abstraction of the file access layer and built as an API that MySQL AB (or anyone) can use to build specialized file-access mechanisms called storage engines. The API provides a set of methods and access utilities for reading and writing data. These methods combine to form a standardized modular architecture that permits storage engines to use the same methods for every storage engine (this is the essence of why it is called pluggable the storage engines all plug into the server using the same API). What is perhaps most interesting of all is the fact that it is possible to assign a different storage engine to each table in a given database. It is even possible to change storage engines
CHAPTER 2 THE ANATOMY OF A DATABASE SYSTEM
after a table is created. This flexibility and modularity permits database implementers (you!) to create new storage engines as the need arises. To change storage engines for a table, you can issue a command like the following: ALTER TABLE MyTable ENGINE = InnoDB; The pluggable storage engine is perhaps the most unique feature of MySQL. No other database system comes close to having this level of flexibility and extensibility for the file access layer of the architecture. The following sections describe all of the storage engines available in the server and present a brief overview of how you can create your own storage engine. I ll show you how to create your own storage engine in 7. The strengths and weaknesses of the storage engines are many and varied. For example, some of the storage engines offered in MySQL support concurrency. The default storage engine for MySQL is MyISAM. It supports table-level locking for concurrency control. That is, when an update is in progress no other processes can access any data from the same table until the operation is completed. The MyISAM storage engine is also the fastest of the available types due to optimizations made using indexed sequential access method (ISAM) principles. The Berkeley Database (BDB) tables support page-level locking for concurrency control; when an update is in progress, no other processes can access any data from the same page as that of the data being modified until the operation is complete. The InnoDB tables support record locking (sometimes called row-level locking) for concurrency control; when an update is in progress, no other processes can access that row in the table until the operation is complete. Thus, the InnoDB table type provides an advantage for use in situations where many concurrent updates are expected. However, any of these storage engines will perform well in read-only environments such as web servers or kiosk applications. Concurrency operations like those we ve discussed are implemented in database systems using specialized commands that form a transaction subsystem. Currently, only three of the storage engines listed support transactions: BDB, InnoDB, and NDB. Transactions provide a mechanism that permits a set of operations to execute as a single atomic operation. For example, if a database was built for a banking institution the macro operations of transferring money from one account to another would preferably be executed completely (money removed from one account and placed in another) without interruption. Transactions permit these operations to be encased in an atomic operation that will back out any changes should an error occur before all operations are complete, thus avoiding data being removed from one table and never making it to the next table. A sample set of operations in the form of SQL statements encased in transactional commands is shown here: START TRANSACTION; UPDATE SavingsAccount SET Balance = Balance 100 WHERE AccountNum = 123; UPDATE CheckingAccount SET Balance = Balance + 100 WHERE AccountNum = 345; COMMIT; In practice, most database professionals specify the MyISAM table type if they require faster access and InnoDB if they need transaction support. Fortunately, MySQL provides facilities to specify a table type for each table in a database. In fact, tables within a database do not
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