Features and Limitations

特性和局限性

In this section we will look at the key features of SQLite and some of its limitations. The nature of SQLite makes it an ideal choice for quite a number of tasks, but it's not suitable for everything.

在本部分我們將要看一下SQLite關(guān)鍵特性和它的一些局限性。SQLite的本質(zhì)使它成為許多任務(wù)的理想選擇，但它并不適合所有的方面。

It is important to decide whether SQLite or any other database engine for that matteris the right choice for your application before committing to a particular technology.

在為你的應(yīng)用確定一個(gè)特殊的技術(shù)之前，決定選擇使用SQLite或者其它的任何數(shù)據(jù)庫(kù)引擎作是非常重要的。

Speed

速度

SQLite is extremely efficient, benefiting from a highly optimized internal architecture and a small memory footprint. Because SQLite is not a client/server database, the overheads of running a database daemon and socket communication are eliminated.

SQLite 非常高效，這得益于高度優(yōu)化的內(nèi)部結(jié)構(gòu)和很小的內(nèi)存需求。因?yàn)镾QLite 不是一個(gè)客戶端/服務(wù)器類型的數(shù)據(jù)庫(kù)，運(yùn)行一個(gè)數(shù)據(jù)庫(kù)進(jìn)程和套接字通信的開(kāi)銷被消除了。

The published speed comparison at http://www.sqlite.org/speed.html compares SQLite to both MySQL and PostgreSQL. It finds that SQLite can perform up to 20 times faster than PostgreSQL and more than twice as fast as MySQL for common operations.

在 http://www.sqlite.org/speed.html 上的發(fā)表了它和MySQL和PostgreSQL速度對(duì)比。對(duì)比表明，SQLite在執(zhí)行普通操作的時(shí)候速度比PostgreSQL快20倍，是MySQL的兩倍。


These tests were performed with default installations of each database, and although it is possible to tune the MySQL and PostgreSQL servers for slightly better performance in a given environment, SQLite does not require any such optimization.

這個(gè)是在每個(gè)數(shù)據(jù)庫(kù)的默認(rèn)安裝情況下測(cè)試的，盡管在給定的環(huán)境中調(diào)整之后，MySQL和 PostgreSQL 的執(zhí)行情況會(huì)略有提高，但是SQLite是不需要任何優(yōu)化的。


The tests found that SQLite is significantly slower than the other databases only on the operations to create an index and to drop a table. However, slowness in these areas will not affect performance on a production database.

這個(gè)測(cè)試發(fā)現(xiàn)，SQLite僅僅在執(zhí)行創(chuàng)建索引和刪除表的操作時(shí)速度要比其他數(shù)據(jù)庫(kù)慢得多。然而，這些方面得低速并不會(huì)影響到作為一個(gè)生產(chǎn)數(shù)據(jù)庫(kù)。

Portability

移植性

Because SQLite databases are stored as single files on the filesystem, they are very portable indeed. A database can be copied from one file to another, even across different operating systems. This means that for a cross-platform distribution you just need to concentrate on making your code portable even when a populated database is to be shipped with the application.

因?yàn)镾QLite數(shù)據(jù)庫(kù)在文件系統(tǒng)上是作為單個(gè)文件存儲(chǔ)的，實(shí)際上它們是非常容易移植的。一個(gè)數(shù)據(jù)庫(kù)可被從一個(gè)文件拷貝到另一個(gè)文件，甚至是跨越不同的操作系統(tǒng)。這意味著在有一個(gè)交叉平臺(tái)上發(fā)布的時(shí)候，你只需要關(guān)注于使你的代碼,使其可以移植，甚至是當(dāng)一個(gè)可移植的數(shù)據(jù)庫(kù)是和應(yīng)用一起發(fā)布的。

SQLite has no external dependencies. The SQLite library is self-contained, so the only system requirement to run an application with an embedded SQLite database is the SQLite library itself. Because SQLite can be freely distributed, you can always ensure that this is present.

SQLite沒(méi)有額外的依賴性。SQLite 庫(kù)是自持的，因此在運(yùn)行嵌入式SQLite數(shù)據(jù)庫(kù)的時(shí)候，僅僅需要的是SQLite的庫(kù)本身。因?yàn)镾QLite能夠自由的發(fā)布，你總是可以確保這是可行的。

Security
安全

SQLite databases are stored to the filesystem and access control is performed by the underlying operating system based on that file's permission settings.
?SQLite 數(shù)據(jù)庫(kù)存儲(chǔ)在文件系統(tǒng)之上，訪問(wèn)控制是由基于操作系統(tǒng)之下的的文件權(quán)限設(shè)定來(lái)實(shí)現(xiàn)的。

Though SQLite can be accessed by processes running as different users if the correct file permissions are set, the database engine does not detect which user is performing a particular operation.

如果正確的文件訪問(wèn)權(quán)限設(shè)定了，盡管SQLite能夠通過(guò)進(jìn)程運(yùn)行作為不同的用戶來(lái)被訪問(wèn)，但是數(shù)據(jù)庫(kù)引擎并不檢測(cè)哪個(gè)用戶正在執(zhí)行一個(gè)特別的操作。

The advantage of this is one of administrative simplicitythere is no need to set up a complex user grants scheme. Any user who has access to read the database file is able to access the database tables and records. Likewise, in a shared environment, users are able to create their own SQLite databases to their file space without any involvement from the system administrators.

它的優(yōu)點(diǎn)之一就是管理簡(jiǎn)單，不需要設(shè)定復(fù)雜的用戶授權(quán)模式。任何擁有訪問(wèn)權(quán)利的用戶都可以訪問(wèn)數(shù)據(jù)庫(kù)表格和記錄。同樣，在一個(gè)共享環(huán)境中，用戶可以在他們的自己的文件空間上創(chuàng)建他們自己的SQLite數(shù)據(jù)庫(kù)而不用涉及系統(tǒng)管理。

The disadvantage comes when you want to control permissions at a more finely grained level. There is no GRANT operation that would allow access to particular tables to one set of users but not others. If users have read access, they are able to read the entire database, and if they have write access, you have to be sure of their competence and trustworthiness with the data!
缺點(diǎn)是，當(dāng)你需要在更加細(xì)致粒度水平的控制權(quán)限的時(shí)候，它沒(méi)有GRANT操作，這個(gè)操作允許一部分用戶可訪問(wèn)特定的表而另一部分用戶不行。當(dāng)用戶擁有訪問(wèn)許可，他們能夠讀整個(gè)數(shù)據(jù)庫(kù)，如果他們有寫許可，你必須要確定他們?cè)跀?shù)據(jù)上的的權(quán)限和信賴度。

SQL Implementation

SQL工具

SQLite supports a large subset of the ANSI SQL-92 standard. Some features have a limited implementation and a few features are not supported at all.

SQLite支持ANSI SQL-92標(biāo)準(zhǔn)的大的子集。某些特性有一個(gè)有限的工具，其他一些特性并不都被支持。
For example, atomic transactions are available but cannot be nested; simple subqueries are possible but correlated subqueries are not; triggers can fire for each row but not for each statement; views are available but are read-only.

例如，原子事務(wù)是可用的，但不能嵌套；簡(jiǎn)單的子查詢是可以的，但相互關(guān)聯(lián)的子查詢是不行的；觸發(fā)器可以在每一行觸發(fā)，但不能在每個(gè)語(yǔ)句上觸發(fā)；視圖是可見(jiàn)但是只讀的。
The list of current limitations is maintained at http://www.sqlite.org/omitted.html with the items at the top of the list indicating which items are most likely to be added to SQLite first.

目前被維護(hù)的局限性列表在 http://www.sqlite.org/omitted.html 上，列表的上部的選項(xiàng)指示了哪些選項(xiàng)最有可能被先加入到SQLite
In the vast majority of cases, none of the limitations of SQLite will cause problems when developing your application. For those that you need to work around, the benefits of using a fast, portable embedded database will almost certainly outweigh the cost of the workaround.
在大部分的情況下，SQLite的局限性不會(huì)給你的應(yīng)用開(kāi)發(fā)造成任何問(wèn)題。因此，你需要變通的方法，使用一個(gè)快速的，可移植的嵌入式數(shù)據(jù)庫(kù)的好處將比變通方法的費(fèi)用好的多。

Customization

定制

The SQLite library includes a very powerful mechanism for adding user-defined functions to the SQL command set. Custom functions can even be written in many of the supported language APIs, not just C/C++.

Supported APIs

SQLite now has extensive support for other programming languages through APIs that use the underlying C/C++ interface to communicate with SQLite database files.

C/C++

The core interface is implemented as a single library called libsqlite.so on Linux/Unix systems and sqlite.dll on Windows.

Only the SQLite library is required to allow all users to create their own databases, so very little administration is required to add SQL database capability to a shared system.

PHP

Support for SQLite in PHP has been available for a while, but since the release of PHP 5, it has been shipped with the standard distribution.

Traditionally, PHP and MySQL have gone hand in hand as the interface and back end for dynamic web sites, but it is expected that many more web hosting providers will offer SQLite as PHP 5 gains popularity. From the host's point of view, it is much simpler to administer than a client/server database as there are no complex permissions to manage and database files will be already counted in the disk quota.

Perl

Perl allows access to SQLite through the Database Interface (DBI) module. This makes it very easy for existing Perl developers to use SQLite within their scripts. The DBI provides an abstraction layer to the Database Driver (DBD), so the same command set is used to access many different types of underlying databases.

The DBD::SQLite driver further allows access to the capabilities of SQLite that the Perl DBI does not allow. Although this does not create a Perl application that can be easily ported to another database back end, it does allow access to the powerful user-defined functions feature.

Tcl

The Tcl interface for SQLite is shipped as part of the SQLite distribution as a library that is imported into Tcl to activate the extensions.

Using Tcl and Tk together with SQLite, you have a platform that is ideal for rapid development of portable graphical user applications.

Other Programming Languages

SQLite has APIs for many other programming languages, including Java, .NET, Smalltalk, and Ruby. As more languages become supported, they are added to the list at http://www.sqlite.org/cvstrac/wiki?p=SqliteWrappers .

Scalability

The downside of using a single file to store databases is that SQLite is not as scalable as many client/server database systems.

The SQLite engine can address database files of up to 2TB in size; however, the restriction on the size of a database is more likely to be enforced by your operating system. In many cases, the size limit on a single file is 2GB.

File locking in SQLite is very coarse-grained. When a write operation takes place, the entire file is locked so that no other process can open it for reading or writing. Larger RDBMSs implement locking at the table or row level so that other processes are able to carry on working unless they are trying to access a specific piece of locked data.

Therefore, if you have a database that is likely to involve massive database files or a high frequency of slow write operations, SQLite may not be suitable and you should consider an RDBMS that is designed and tuned for multiple-user access.