Berkeley DB——Cursor
Introduction
Berkeley DB的游标（Dbc）和关系数据库的游标是类似的——一种可以迭代数据库中的记录的装置。对于重复记录，使用游标来访问他们会更加方便（使用Db的bulk get来得到多条记录是性能最高的做法）。另外，通过游标可以一条条地操作（修改和删除）记录。
在使用游标之前，必须先使用Db::cursor方法打开游标：
int Db::cursor(DbTxn *txnid, Dbc **cursorp, u_int32_t flags);
游标打开后可以通过游标search和write记录。使用完毕后，必须关闭之：
int Dbc::close(void);
例子：
|
#include <db_cxx.h>
...
Dbc *cursorp;
Db my_database(NULL, 0);
// Database open omitted for clarity
// Get a cursor
my_database.cursor(NULL, &cursorp, 0);
// Database and cursor open omitted for clarity
// Do something…
if (cursorp != NULL)
cursorp->close();
|
Getting Records from DB Using Cursor
打开数据库，并打开此数据库的游标后，就可以使用该游标来search DB来遍历符合条件的记录了。
Dbc::get方法的flag参数可以取多个值，常用的有：
|
Flag
|
Description
|
|
DB_CURRENT
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返回游标当前所指的key/data
|
|
DB_FIRST
|
游标指向第一条记录，并返回该key/data
|
|
DB_GET_BOTH
|
只有key/data都匹配才返回该key/data
|
|
DB_GET_RECNO
|
返回行号。数据库必须是B树的，且其flag为DB_RECNUM。
|
|
DB_LAST
|
和DB_FIRST对应
|
|
DB_NEXT
|
游标指向下条记录，并返回下个key/data
|
|
DB_PREV
|
和DB_NEXT对应
|
|
DB_MULTIPLE
|
返回该key的所有记录。数据库是允许重复记录的。
|
例子：
|
#include <db_cxx.h>
...
Db my_database(NULL, 0);
Dbc *cursorp;
try {
// Database open omitted for clarity
// Get a cursor
my_database.cursor(NULL, &cursorp, 0);
Dbt key, data;
int ret;
// Iterate over the database, retrieving each record in turn.
while ((ret = cursorp->get(&key, &data, DB_NEXT)) == 0) {
// Do interesting things with the Dbts here.
}
if (ret != DB_NOTFOUND) {
// ret should be DB_NOTFOUND upon exiting the loop.
// Dbc::get() will by default throw an exception if any
// significant errors occur, so by default this if block
// can never be reached.
}
} catch(DbException &e) {
my_database.err(e.get_errno(), "Error!");
} catch(std::exception &e) {
my_database.errx("Error! %s", e.what());
}
// Cursors must be closed
if (cursorp != NULL)
cursorp->close();
my_database.close(0);
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Operating Records Using Cursor
Dbc对象提供了put、del和get方法，通过这些方法我们可以写入、删除和获取记录。
例子：
|
#include "stdafx.h"
#include <iostream>
#include <db_cxx.h>
#include <string.h>
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
Db db(NULL,0);
u_int32_t oFlags = DB_CREATE; // Open flags;
try
{
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)
db.truncate(NULL,0,0);
float money = 122.45;
char *description = "Grocery bill.";
Dbt key(&money, sizeof(float));
Dbt data(description, strlen(description)+1);
int ret = db.put(NULL, &key, &data, DB_NOOVERWRITE);
cout<<"put data--"<<description<<endl;
ret = db.get(NULL, &key, &data, DB_GET_BOTH);
cout<<"get key--"<<*((float*)key.get_data())<<endl;
cout<<"get data--"<<(char *)data.get_data()<<endl;
money = 111;
description = "James--------------------";
data.set_data(description);
data.set_size(strlen(description)+1);
db.put(NULL,&key,&data,DB_NOOVERWRITE);
ret = db.get(NULL, &key, &data, DB_GET_BOTH);
cout<<"get key--"<<*((float*)key.get_data())<<endl;
cout<<"get data--"<<(char *)data.get_data()<<endl;
money = 191;
description = "Mike";
data.set_data(description);
data.set_size(strlen(description)+1);
db.put(NULL,&key,&data,DB_NOOVERWRITE);
ret = db.get(NULL, &key, &data, DB_GET_BOTH);
cout<<"get key--"<<*((float*)key.get_data())<<endl;
cout<<"get data--"<<(char *)data.get_data()<<endl;
Dbc* cursor;
db.cursor(NULL,&cursor,0);
cout<<"open cursor"<<endl;
while((ret = cursor->get(&key,&data,DB_PREV)) != DB_NOTFOUND)
{
cout<<"get key--"<<*((float*)key.get_data())<<endl;
cout<<"get data--"<<(char *)data.get_data()<<endl;
}
if (cursor != NULL)
{
cursor->close();
}
money = 191;
description = "Mike";
data.set_data(description);
data.set_size(strlen(description)+1);
db.cursor(NULL,&cursor,0);
cout<<"delete 191..."<<endl;
while((ret = cursor->get(&key,&data,DB_SET)) == 0 )
{
cursor->del(0);
}
if (cursor != NULL)
{
cursor->close();
}
cout<<"after delete 191..."<<endl;
db.cursor(NULL,&cursor,0);
while((ret = cursor->get(&key,&data,DB_PREV)) != DB_NOTFOUND)
{
cout<<"get key--"<<*((float*)key.get_data())<<endl;
cout<<"get data--"<<(char *)data.get_data()<<endl;
}
if (cursor != NULL)
{
cursor->close();
}
}
catch(DbException &e)
{
cerr<<"DBException:"<<e.what();
}
catch(std::exception &e)
{
cerr<<"DBException:"<<e.what();
}
system("pause");
return 0;
}
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Joining Cursors
Berkeley DB支持两个数据库之间的“对等”连接，即当数据库A的cursor和数据库B的cursor进行join时，返回数据库A中key和数据库B的data相等的记录。也就是说，实际的数据是存在A中的data里面，而通过数据库B的key来获得A的data。比如：
数据库A：
|
A-Key
|
A-Data
|
|
apple
|
Convenience Store
|
|
blueberry
|
Farmer's Market
|
|
peach
|
Shopway
|
|
pear
|
Farmer's Market
|
|
raspberry
|
Shopway
|
|
strawberry
|
Farmer's Market
|
数据库B：
|
B-Key
|
B-Data:
|
|
Blue
|
blueberry
|
|
red
|
apple
|
|
red
|
raspberry
|
|
red
|
strawberry
|
|
yellow
|
peach
|
|
yellow
|
pear
|
则根据B的一条记录（key为Blue），join后应该返回的是数据库A的key为blueberry的记录，即data为Farmer's Market。
例子：
|
#include <db_cxx.h>
#include <string.h>
...
// Exception handling omitted
int ret;
Db automotiveDB(NULL, 0);
Db automotiveColorDB(NULL, 0);
Db automotiveMakeDB(NULL, 0);
Db automotiveTypeDB(NULL, 0);
// Database and secondary database opens omitted for brevity.
// Assume a primary database:
// automotiveDB
// Assume 3 secondary databases:
// automotiveColorDB -- secondary database based on automobile color
// automotiveMakeDB -- secondary database based on the manufacturer
// automotiveTypeDB -- secondary database based on automobile type
// Position the cursors
Dbc *color_curs;
automotiveColorDB.cursor(NULL, &color_curs, 0);
char *the_color = "red";
Dbt key(the_color, strlen(the_color) + 1);
Dbt data;
if ((ret = color_curs->get(&key, &data, DB_SET)) != 0) {
// Error handling goes here
}
Dbc *make_curs;
automotiveMakeDB.cursor(NULL, &make_curs, 0);
char *the_make = "Toyota";
key.set_data(the_make);
key.set_size(strlen(the_make) + 1);
if ((ret = make_curs->get(&key, &data, DB_SET)) != 0) {
// Error handling goes here
}
Dbc *type_curs;
automotiveTypeDB.cursor(NULL, &type_curs, 0);
char *the_type = "minivan";
key.set_data(the_type);
key.set_size(strlen(the_type) + 1);
if ((ret = type_curs->get(&key, &data, DB_SET)) != 0) {
// Error handling goes here
}
// Set up the cursor array
Dbc *carray[4];
carray[0] = color_curs;
carray[1] = make_curs;
carray[2] = type_curs;
carray[3] = NULL;
// Create the join
Dbc *join_curs;
if ((ret = automotiveDB.join(carray, &join_curs, 0)) != 0) {
// Error handling goes here
}
// Iterate using the join cursor
while ((ret = join_curs->get(&key, &data, 0)) == 0) {
// Do interesting things with the key and data
}
// If we exited the loop because we ran out of records,
// then it has completed successfully.
if (ret == DB_NOTFOUND) {
// Close all our cursors and databases as is appropriate, and
// then exit with a normal exit status (0).
}
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从上面的例子我们可以看到，需要构建一个游标的array，并且最后一个元素是NULL。Berkeley DB用这个NULL来标志array的结尾，从而知道array中有几个cursor。
Join游标的步骤为：
1. 在第二个数据库上打开两个或者多个游标。
2. 将这些游标移动到满足你指定的条件的记录上。
3. 创建一个游标数组，大小为游标数量+1，最后一个元素放入NULL。
4. 声明一个新的cursor，以接收join的结果。
5. 迭代记录。
6. 关闭这些游标。