Session 5 Storage EnginesRAM N SANGWAN

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MySQL Storage Engines

• A storage engine can be though of as a File System in an Operating System.

• Storage engines differ in the way

◦ that they use locking to manage query contention, or

◦ whether the tables that they provide are transactional or non-transactional.

• These engine properties have implications for query processing performance,concurrency, and deadlock prevention.

• When you create a table, you can choose what storage engine to use.

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Specify a storage engine..

• Explicitly in a CREATE TABLE statement, use an ENGINE option:

CREATE TABLE t (i INT) ENGINE = InnoDB;

• Without using an ENGINE option, the default engine given by storage_engine system variable willbe used.

• To determine which storage engine is used for a given table, you can use the SHOW CREATETABLE or the SHOW TABLE STATUS statement:

mysql> SHOW CREATE TABLE City\G

mysql> SHOW TABLE STATUS LIKE ‘CountryLanguage‘ \G

• The INFORMATION_SCHEMA TABLES table contains storage engine information as well:

mysql> SELECT TABLE_NAME, ENGINE FROM INFORMATION_SCHEMA.TABLES

-> WHERE TABLE_SCHEMA ='world';

• In most respects the way that you use the table after creating it, is engine independent.

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Storage Engine Prerequisites

• It must be compiled into the server and enabled. Each storage engine is asoftware module that is compiled into the server.

• The use of this modular approach allows storage engines to be easily selectedfor inclusion in the server at configuration time.

• Some storage engines are always available, such as InnoDB, MyISAM, MERGE,and MEMORY.

• Support for optional engines typically can be selected when MySQL isconfigured and built.

• To reduce memory use, don't configure unneeded storage engines into theserver. To see what storage engines:

mysql> SHOW ENGINES\G

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MyISAM Storage Engine

• MyISAM is the improved version of the original storage engine of MySQL, ISAM

• The MyISAM engine is fast and thus, preferred for web and other applicationenvironments.

• It is also used for data warehousing

• MyISAM is not transaction-safe and supports 64 keys per table with maximumkey length of 1024 bytes

• The size of MyISAM table depends on the host operating system

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MyISAM Storage Engine Contd..

• MyISAM table allows table level locking only.

• There are no limitations on data file transfer and the data files can be portedfrom system to system

• The foreign key constraint cannot be defined

• MyISAM is the only storage engine that supports Full-text search

• It also supports one auto increment column per table

• A high-byte-first pattern for saving numeric key values ensures faster indexing

• It can be used where fulltext indexing is needed

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MyISAM Storage Engine Contd..

• On disk, MySQL represents each MyISAM table using three files:

◦ a format file that stores the definition of the table structure,

◦ a data file that stores the contents of table rows, and

◦ an index file that stores any indexes on the table.

Files for a table named mytable are called mytable.frm, mytable.MYD, and mytable.MYI.

• MyISAM tables can be used to set up MERGE tables.

• MyISAM tables can be converted into fast, compressed, read-only tables to save space.

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MyISAM Storage Engine Contd..

• Query performance is very fast for retrievals.

• Multiple queries can read the same table simultaneously.

• You can influence the scheduling mechanism for queries thatuse MyISAM tables by using LOW_PRIORITY/HIGH_PRIORITY Query Modifier.

• You can specify that a MyISAM table must be able to hold at least a certainnumber of rows, which allows MyISAM to adjust the table's internal rowpointer size accordingly.

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MyISAM Storage Engine Contd..

• When loading data into an empty MyISAM table, you can disable updating of non-unique indexes and enable the indexes after loading.

• If you run out of disk space while adding rows to a MyISAM table, no error occurs. The server suspends the operation until space becomes available, and then completes the operation.

• MyISAM tables use the indexed sequential access method for indexing.

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MyISAM Storage Engine Contd..

• MyISAM locking occurs at the table level. Deadlock cannot occur with tablelocking as it can row locking.

• When processing queries on MyISAM tables, the server manages contention forthe tables by simultaneous clients by implicitly acquiring any locks it needs.

• You can also lock tables explicitly with the LOCK TABLES and UNLOCKTABLES statements.

• MyISAM tables support concurrent inserts.

• Concurrent inserts can take place even for a table that has been read-lockedexplicitly if the locking client acquired a READ LOCAL lock rather than a regularREAD lock.

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MyISAM Storage Engine Contd..

• If a table does have holes, concurrent inserts cannot be performed. However,you can remove the holes by using OPTIMIZE TABLE to defragment the table.

• For applications that use MyISAM tables, you can change the priority ofstatements that retrieve or modify data.

• By default, the server schedules queries for execution as follows :

◦ Write requests take priority over read requests.

◦ The server tries to perform write requests in the order that it receives them.

• However, if a table is being read from when a write request arrives, the writerequest cannot be processed until all current readers have finished.

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When working with MyISAM tables..

Certain scheduling modifiers are available to change the priority of requests:

◦ The LOW_PRIORITY modifier may be applied to statements that update tables(INSERT, DELETE, REPLACE, or UPDATE).

A low-priority write request waits not only until all current readers have finished, but for any pendingread requests that arrive while the write request itself is waiting.

◦ HIGH_PRIORITY may be used with a SELECT statement to move it ahead of updates and ahead ofother SELECT statements that do not use the HIGH_PRIORITY modifier.

◦ DELAYED may be used with INSERT (and REPLACE). The server buffers the rows in memory and insertsthem when the table is not being used.

◦ Delayed inserts increase efficiency because they're done in batches rather than individually.

◦ Using DELAYED allows the client to proceed immediately after issuing the INSERT statement ratherthan waiting until it completes.

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MyISAM Row-Storage Formats

Three storage formats: fixed-row, dynamic-row, and compressed.

Characteristics:

• Fixed-row format:◦ All rows have the same size.

◦ Rows are stored within the table at positions that are multiples of the row size.

◦ Fixed-size rows take more space.

• Dynamic-row format:◦ Rows take varying amounts of space.

◦ Rows cannot be looked up as efficiently.

◦ Dynamic-rows tables usually take less space because rows are not padded to a fixed size.

◦ Fragmentation can occur more easily than for fixed-row tables.

• Compressed format:◦ Tables are packed to save space.

◦ Storage is optimized for quick retrieval.

◦ Tables are read-only.

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The MERGE Engine

The MERGE storage engine manages tables that have the following characteristics:

• A MERGE table is a collection of identically structured MyISAM tables. EachMERGE table is represented on disk by an .frm format file and an .MRG file thatlists the names of the constituent MyISAM files.

• Logically, a query on a MERGE table acts as a query on all the MyISAM tables ofwhich it consists.

• A MERGE table creates a logical entity that can exceed the maximumMyISAM table size.

• MySQL manages contention between queries for MERGE table access usingtable-level locking.

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The MERGE Engine

• A MERGE table is portable because the .MRG file is a text file andthe MyISAM tables that it names are portable.

• The MERGE engine supports SELECT, DELETE, UPDATE, and INSERT statements.For INSERT, the CREATE TABLE statement can specify whether records should beinserted into the first or last table, or disallowed.

• Disadvantage:

◦ They increase the number of file descriptors required because each of theunderlying tables must be opened along with the MERGE table.

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Example of Merge Engine

mysql> CREATE TABLE NACountry SELECT Code, Name

-> FROM Country WHERE Continent ='North America';

mysql> CREATE TABLE SACountry SELECT Code, Name

-> FROM Country WHERE Continent ='South America';

mysql> DESCRIBE NACountry;

mysql> CREATE TABLE NorthAndSouth

-> (Code CHAR(3) NOT NULL, Name CHAR(52) NOT NULL)

-> ENGINE = MERGE UNION = (NACountry, SACountry);

mysql> SELECT COUNT(*) FROM NACountry;

mysql> SELECT COUNT(*) FROM SACountry;

mysql> SELECT COUNT(*) FROM NorthAndSouth;

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Other Storage Engines - BDB

• The BDB storage engine provides transactional tables.

◦ Each BDB table is represented on disk by an .frm format file and a .db file that stores data and index information.

◦ BDB supports transactions with full ACID compliance.

◦ The BDB engine provides auto-recovery after a crash of the MySQL server or the host where the server runs. BDB uses page-level locking.

◦ It's possible for deadlock to occur.

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Other Storage Engines Contd..

• The ARCHIVE storage engine provides an efficient way to store large amounts of data when you don't need indexes and need to minimize the amount of disk space used.

◦ This engine supports only SELECT and INSERT operations. SELECT scans the entire table, and INSERT performs compressed inserts.

◦ Each ARCHIVE table is represented on disk by an .frm format file, an .ARZ data file, and an .ARM metadata file.

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Other Storage Engines Contd..

• The CSV storage engine stores records as text in the well-known comma-separated valuesformat.

◦ It does not support indexing. Each CSV table is represented on disk by an .frm format file and a .CSV plain text file that contains data rows.

• The BLACKHOLE storage engine creates tables that act as "black holes."

◦ That is, what goes in does not come out.

◦ Data stored in a BLACKHOLE table disappears because the engine simply discards it.

◦ The only disk file associated with a BLACKHOLE table is its .frm format file.

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Other Storage Engines Contd..

• The EXAMPLE storage engine does nothing except create tables.

◦ You can't even store any rows in an EXAMPLE table, although this is by design.

◦ The purpose of this engine is to provide simple example code in MySQL source todemonstrates how to get started writing a new storage engine.

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Thank You

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