Berkeley DB——Cursor
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 |
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); |
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：
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#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-> } 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; } |
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：
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#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 = " 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). } |
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。
所有Berkeley DB相关的随笔
