1、附录 附录 A：外文资料翻译 原文部分 Distinctive Features of SQLite (From SQLite.Org (http:/www.sqlite.org/different.html) This passage highlights some of the characteristics of SQLite that are unusual and which make SQLite different from many other SQL database engines. Zero-Configuration SQLite does not need to be
2、 installed before it is used. There is no setup procedure. There is no server process that needs to be started, stopped, or configured. There is no need for an administrator to create a new database instance or assign access permissions to users. SQLite uses no configuration files. Nothing needs to
3、be done to tell the system that SQLite is running. No actions are required to recover after a system crash or power failure. There is nothing to troubleshoot. SQLite just works. Other more familiar database engines run great once you get them going. But doing the initial installation and configurati
4、on can be intimidatingly complex. Serverless Most SQL database engines are implemented as a separate server process. Programs that want to access the database communicate with the server using some kind of interprocess communication (typically TCP/IP) to send requests to the server and to receive ba
5、ck results. SQLite does not work this way. With SQLite, the process that wants to access the database reads and writes directly from the database files on disk. There is no intermediary server process. There are advantages and disadvantages to being serverless. The main advantage is that there is no
6、 separate server process to install, setup, configure, initialize, manage, and troubleshoot. This is one reason why SQLite is a zero-configuration database engine. Programs that use SQLite require no administrative support for setting up the database engine before they are run. Any program that is a
7、ble to access the disk is able to use an SQLite database. On the other hand, a database engine that uses a server can provide better protection from bugs in the client application - stray pointers in a client cannot corrupt memory on the server. And because a server is a single persistent process, i
8、t is able control database access with more precision, allowing for finer grain locking and better concurrency. Most SQL database engines are client/server based. Of those that are serverless, SQLite is the only one that this author knows of that allows multiple applications to access the same datab
9、ase at the same time. Single Database File An SQLite database is a single ordinary disk file that can be located anywhere in the directory hierarchy. If SQLite can read the disk file then it can read anything in the database. If the disk file and its directory are writable, then SQLite can change an
10、ything in the database. Database files can easily be copied onto a USB memory stick or emailed for sharing. Other SQL database engines tend to store data as a large collection of files. Often these files are in a standard location that only the database engine itself can access. This makes the data
11、more secure, but also makes it harder to access. Some SQL database engines provide the option of writing directly to disk and bypassing the filesystemall together. This provides added performance, but at the cost of considerable setup and maintenance complexity. Stable Cross-Platform Database File T
12、he SQLite file format is cross-platform. A database file written on one machine can be copied to and used on a different machine with a different architecture. Big-endian or little-endian, 32-bit or 64-bit does not matter. All machines use the same file format. Furthermore, the developers have pledg
13、ed to keep the file format stable and backwards compatible, so newer versions of SQLite can read and write older database files. Most other SQL database engines require you to dump and restore the database when moving from one platform to another and often when upgrading to a newer version of the so
14、ftware. Compact When optimized for size, the whole SQLite library with everything enabled is less than 225KiB in size (as measured on an ix86 using the size utility from the GNU compiler suite.) Unneeded features can be disabled at compile-time to further reduce the size of the library to under 170K
15、iB if desired. Most other SQL database engines are much larger than this. IBM boasts that its recently released CloudScape database engine is only a 2MiB jar file - 10 times larger than SQLite even after it is compressed! Firebird boasts that its client-side library is only 350KiB. Thats 50% larger
16、than SQLite and does not even contain the database engine. The Berkeley DB library from Sleepycat is 450KiB and it omits SQL support, providing the programmer with only simple key/value pairs. Manifest typing Most SQL database engines use static typing. A datatype is associated with each column in a
17、 table and only values of that particular datatype are allowed to be stored in that column. SQLite relaxes this restriction by using manifest typing. In manifest typing, the datatype is a property of the value itself, not of the column in which the value is stored. SQLite thus allows the user to sto
18、re any value of any datatype into any column regardless of the declared type of that column. (There are some exceptions to this rule: An INTEGER PRIMARY KEY column may only store integers. And SQLite attempts to coerce values into the declared datatype of the column when it can.) As far as we can te
19、ll, the SQL language specification allows the use of manifest typing. Nevertheless, most other SQL database engines are statically typed and so some people feel that the use of manifest typing is a bug in SQLite. But the authors of SQLite feel very strongly that this is a feature. The use of manifes
20、t typing in SQLite is a deliberate design decision which has proven in practice to make SQLite more reliable and easier to use, especially when used in combination with dynamically typed programming languages such as Tcl and Python. Variable-length records Most other SQL database engines allocated a
21、 fixed amount of disk space for each row in most tables. They play special tricks for handling BLOBs and CLOBs which can be of wildly varying length. But for most tables, if you declare a column to be a VARCHAR(100) then the database engine will allocate 100 bytes of disk space regardless of how muc
22、h information you actually store in that column. SQLite, in contrast, use only the amount of disk space actually needed to store the information in a row. If you store a single character in a VARCHAR(100) column, then only a single byte of disk space is consumed. (Actually two bytes - there is some
23、overhead at the beginning of each column to record its datatype and length.) The use of variable-length records by SQLite has a number of advantages. It results in smaller database files, obviously. It also makes the database run faster, since there is less information to move to and from disk. And,
24、 the use of variable-length records makes it possible for SQLite to employ manifest typing instead of static typing. Readable source code The source code to SQLite is designed to be readable and accessible to the average programmer. All procedures and data structures and many automatic variables are
25、 carefully commented with useful information about what they do. Boilerplate commenting is omitted. SQL statements compile into virtual machine code Every SQL database engine compiles each SQL statement into some kind of internal data structure which is then used to carry out the work of the stateme
26、nt. But in most SQL engines that internal data structure is a complex web of interlinked structures and objects. In SQLite, the compiled form of statements is a short program in a machine-language like representation. Users of the database can view this virtual machine language by prepending the EXP
27、LAIN keyword to a query. The use of a virtual machine in SQLite has been a great benefit to the librarys development. The virtual machine provides a crisp, well-defined junction between the front-end of SQLite (the part that parses SQL statements and generates virtual machine code) and the back-end (the part that executes the virtual machine code and computes a result.) The virtual machine allows the
