Access Method

Description

Choosing Occasion

B+樹

關(guān)鍵字有序存儲，并且其結(jié)構(gòu)能隨數(shù)據(jù)的插入和刪除進(jìn)行動(dòng)態(tài)調(diào)整。為了代碼的簡單，Berkeley DB沒有實(shí)現(xiàn)對關(guān)鍵字的前綴碼壓縮。B+樹支持對數(shù)據(jù)查詢、插入、刪除的常數(shù)級速度。關(guān)鍵字可以為任意的數(shù)據(jù)結(jié)構(gòu)。

1、? 當(dāng)Key為復(fù)雜類型時(shí)。

2、? 當(dāng)Key有序時(shí)。

Hash

DB中實(shí)際使用的是擴(kuò)展線性HASH算法（extended linear hashing），可以根據(jù)HASH表的增長進(jìn)行適當(dāng)?shù)恼{(diào)整。關(guān)鍵字可以為任意的數(shù)據(jù)結(jié)構(gòu)。

1、? 當(dāng)Key為復(fù)雜類型。

2、? 當(dāng)數(shù)據(jù)較大且key隨機(jī)分布時(shí)。

?

Recno

要求每一個(gè)記錄都有一個(gè)邏輯紀(jì)錄號，邏輯紀(jì)錄號由算法本身生成。相當(dāng)于關(guān)系數(shù)據(jù)庫中的自動(dòng)增長字段。Recho建立在B+樹算法之上，提供了一個(gè)存儲有序數(shù)據(jù)的接口。記錄的長度可以為定長或不定長。

1、? 當(dāng)key為邏輯記錄號時(shí)。

2、? 當(dāng)非高并發(fā)的情況下。

Queue

和Recno方式接近, 只不過記錄的長度為定長。數(shù)據(jù)以定長記錄方式存儲在隊(duì)列中，插入操作把記錄插入到隊(duì)列的尾部，相比之下插入速度是最快的。

1、? 1、當(dāng)key為邏輯記錄號時(shí)。

2、? 定長記錄。

3、? 高并發(fā)的情況下。

?

Open Flag

Description

DB_CREATE

如果數(shù)據(jù)庫不存在，則創(chuàng)建之。

DB_EXCL

如果數(shù)據(jù)庫存在，則返回失敗。只和DB_CREATE一起使用才有意義。

DB_RDONLY

以只讀的方式打開數(shù)據(jù)庫。

#include <db_cxx.h>

?

...

?

Db db(NULL, 0);?????????????? // Instantiate the Db object

?

u_int32_t oFlags = DB_CREATE; // Open flags;

?

try {

??? // Open the database

??? db.open(NULL,??????????????? // Transaction pointer

??????????? "my_db.db",????????? // Database file name

??????????? NULL,??????????????? // Optional logical database name

??????????? DB_BTREE,??????????? // Database access method

??????????? oFlags,????????????? // Open flags

??????????? 0);????????????????? // File mode (using defaults)

// DbException is not subclassed from std::exception, so

// need to catch both of these.

} catch(DbException &e) {

??? // Error handling code goes here???

} catch(std::exception &e) {

??? // Error handling code goes here

}

#include <db_cxx.h>

...

u_int32_t env_flags = DB_CREATE;? // If the environment does not

????????????????????????????????? // exist, create it.

u_int32_t db_flags = DB_CREATE;?? // If the database does not

????????????????????????????????? // exist, create it.

std::string envHome("/export1/testEnv");

std::string dbName("mydb.db");

DbEnv myEnv(0);

Db *myDb;

?

try {

??? myEnv.open(envHome.c_str(), env_flags, 0);

??? myDb = new Db(&myEnv, 0);

??? myDb->open(NULL,

?????????????? dbName.c_str(),

?????????????? NULL,

?????????????? DB_BTREE,

?????????????? db_flags,

???????? ??????0);

} catch(DbException &e) {

??? std::cerr << "Error opening database environment: "

????????????? << envHome

????????????? << " and database "

????????????? << dbName << std::endl;

??? std::cerr << e.what() << std::endl;

??? exit( -1 );

} catch(std::exception &e) {

??? std::cerr << "Error opening database environment: "

????????????? << envHome

????????????? << " and database "

????????????? << dbName << std::endl;

??? std::cerr << e.what() << std::endl;

??? exit( -1 );

}

#include <db_cxx.h>

?

...

?

Db db(NULL, 0);

?

?// Database open and access operations happen here.

?

try {

??? // Close the database

??? db.close(0);

// DbException is not subclassed from std::exception, so

// need to catch both of these.

} catch(DbException &e) {

??? // Error handling code goes here???

} catch(std::exception &e) {

??? // Error handling code goes here

}

try {

??? if (myDb != NULL) {

??????? myDb->close(0);

??? }

??? myEnv.close(0);

???

} catch(DbException &e) {

??? std::cerr << "Error closing database environment: "

????????????? << envHome

????????????? << " or database "

????????????? << dbName << std::endl;

??? std::cerr << e.what() << std::endl;

??? exit( -1 );

} catch(std::exception &e) {

??? std::cerr << "Error closing database environment: "

????????????? << envHome

????????????? << " or database "

????????????? << dbName << std::endl;

??? std::cerr << e.what() << std::endl;

??? exit( -1 );

}