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MySQL Storage Engines - page 4

What is a Storage Engine?

  • October 7, 2005
  • By Martin C. Brown

In order to make a decision about which engine to choose, we first need to think about the different core functionality provided in each engine that allow us to differentiate between them. We can generally divide up the core functionality into four areas; the supported field and data types, locking types, indexing and transactions. Some engines have unique functionality that can also drive your decision, we'll be taking a closer look at these specifics in a moment.

Field and Data Types

Although all of the engines support the common data types, i.e., integers, reals and character based storage, not all engines support other field types, particularly the BLOB (binary large object) or TEXT types. Other engines may support only limited character widths and data sizes.

These limitations, while directly affecting the information you store may also have a related effect to the types of searches you perform, or the indexes you create on that information. In turn, these differences can affect the performance and functionality of your application as you may have to make decisions about functionality based on the storange engine choice you make for the type of data you are storing.

Locking

Locking within database engines defines how access and updates to information are controlled. When an object in the database is locked for updating, other processes cannot modify (or in some cases read) the data until the update has completed.

Locking not only affects how many different applications can update the information in the database, it can also affect queries on that data. The reason for this is that the queries may be accessing data that may be being altered or updated. In general, such delays are minimal. The bulk of the locking mechanism is devoted to preventing multiple processes updating the same data. Since both additions (INSERT statements) and alterations (UPDATE statements) to the data require locking, you can imagine that multiple applications using the ame database can have a significant impact.

Locks are supported by different storage engines at different object levels, and these levels affect the concurrency of access to the information. Three different levels are supported, table locking, block locking and row locking. Table locking is most commonly supported and is the locking provided in MyISAM. It locks an entire table during an update. This will limit the number of applications that are updating a specific table to just one, and this can affect heavily used multi-user databases because it introduces delays into the update process.

Page level locking is used by the Berkeley DB storage engine and locks data according to the page (8Kb) of information that is being uploaded. When performing updates across a range of locations within the database, the locking is not a problem, but because adding rows involves locking the final 8Kb of the data structure, adding large numbers of rows, particularly of small data, can be a problem.

Row level locking provides the best concurrency; only individual rows within a table are locked, which means that many applications can be updating different rows of the same table without causing a lock situation. Only the InnoDB storage engine supports row level locking.

Indexing

Indexing can dramatically increase the performance when searching and recovering data from the database. Different storage engines provide different indexing techniques and some may be better suited for the type of data you are storing.

Some storage engines simply do not support indexing at all either because they use the indexing of the underlying tables (in the MERGE engine for example) or because the data storage method does not allow indexing (FEDERATED or BLACKHOLE engines).

Transactions

Transactions provide data reliability during the update or insert of information by enabling you to add data to the database, but only to commit that data when other condititions and stages in the application execution have completed successfully. For example, when transferring information from one account to another you would use transactions to ensure that both the debit from one account and the credit to the other completed successfully. If either process failed, you could cancel the transaction and the changes would be lost. If the process completed,then we would confirm it by committing the changes.

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