1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
|
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Reading Secondary Databases</title>
<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
<link rel="start" href="index.html" title="Getting Started with Berkeley DB" />
<link rel="up" href="indexes.html" title="Chapter 5. Secondary Databases" />
<link rel="prev" href="keyCreator.html" title="Implementing Key Extractors" />
<link rel="next" href="secondaryDelete.html" title="Deleting Secondary Database Records" />
</head>
<body>
<div xmlns="" class="navheader">
<div class="libver">
<p>Library Version 11.2.5.3</p>
</div>
<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">Reading Secondary Databases</th>
</tr>
<tr>
<td width="20%" align="left"><a accesskey="p" href="keyCreator.html">Prev</a> </td>
<th width="60%" align="center">Chapter 5. Secondary Databases</th>
<td width="20%" align="right"> <a accesskey="n" href="secondaryDelete.html">Next</a></td>
</tr>
</table>
<hr />
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="readSecondary"></a>Reading Secondary Databases</h2>
</div>
</div>
</div>
<p>
Like a primary database, you can read records from your secondary
database either by using the
<span>
<code class="methodname">DB->get()</code>
or
<code class="methodname">DB->pget()</code>
methods,
</span>
or by using
<span>a cursor on the secondary database.</span>
The main difference between reading secondary and primary databases is that when
you read a secondary database record, the secondary record's data is not
returned to you. Instead, the primary key and data corresponding to the
secondary key are returned to you.
</p>
<p>
For example, assuming your secondary database contains keys related
to a person's full name:
</p>
<a id="c_index6"></a>
<pre class="programlisting">#include <db.h>
#include <string.h>
...
DB *my_secondary_database;
DBT key; /* Used for the search key */
DBT pkey, pdata; /* Used to return the primary key and data */
char *search_name = "John Doe";
/* Primary and secondary database opens omitted for brevity */
/* Zero out the DBTs before using them. */
memset(&key, 0, sizeof(DBT));
memset(&pkey, 0, sizeof(DBT));
memset(&pdata, 0, sizeof(DBT));
key.data = search_name;
key.size = strlen(search_name) + 1;
/* Returns the key from the secondary database, and the data from the
* associated primary database entry.
*/
my_secondary_database->get(my_secondary_database, NULL,
&key, &pdata, 0);
/* Returns the key from the secondary database, and the key and data
* from the associated primary database entry.
*/
my_secondary_database->pget(my_secondary_database, NULL,
&key, &pkey, &pdata, 0); </pre>
<p>
Note that, just like
<span>a primary database,</span>
if your secondary database supports duplicate records then
<span>
<code class="methodname">DB->get()</code>
and
<code class="methodname">DB->pget()</code>
</span>
only return the first record found in a matching duplicates set. If you
want to see all the records related to a specific secondary key, then use a
<span>
cursor opened on the secondary database. Cursors are described in
<a class="xref" href="Cursors.html" title="Chapter 4. Using Cursors">Using Cursors</a>.
</span>
</p>
</div>
<div class="navfooter">
<hr />
<table width="100%" summary="Navigation footer">
<tr>
<td width="40%" align="left"><a accesskey="p" href="keyCreator.html">Prev</a> </td>
<td width="20%" align="center">
<a accesskey="u" href="indexes.html">Up</a>
</td>
<td width="40%" align="right"> <a accesskey="n" href="secondaryDelete.html">Next</a></td>
</tr>
<tr>
<td width="40%" align="left" valign="top">Implementing Key
<span>Extractors</span>
</td>
<td width="20%" align="center">
<a accesskey="h" href="index.html">Home</a>
</td>
<td width="40%" align="right" valign="top"> Deleting Secondary Database Records</td>
</tr>
</table>
</div>
</body>
</html>
|
