2011-09-13 17:44:24 +00:00
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2012-11-14 21:35:20 +00:00
< p > Library Version 11.2.5.3< / p >
2011-09-13 17:44:24 +00:00
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< th colspan = "3" align = "center" > Cursor operations< / th >
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< tr >
< td width = "20%" align = "left" > < a accesskey = "p" href = "am_foreign.html" > Prev< / a > < / td >
< th width = "60%" align = "center" > Chapter
Access Method Operations
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< div class = "sect1" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
< h2 class = "title" style = "clear: both" > < a id = "am_cursor" > < / a > Cursor operations< / h2 >
< / div >
< / div >
< / div >
< div class = "toc" >
< dl >
< dt >
< span class = "sect2" >
< a href = "am_cursor.html#am_curget" > Retrieving records with a cursor< / a >
< / span >
< / dt >
< dt >
< span class = "sect2" >
< a href = "am_cursor.html#am_curput" > Storing records with a cursor< / a >
< / span >
< / dt >
< dt >
< span class = "sect2" >
< a href = "am_cursor.html#am_curdel" > Deleting records with a cursor< / a >
< / span >
< / dt >
< dt >
< span class = "sect2" >
< a href = "am_cursor.html#am_curdup" > Duplicating a cursor< / a >
< / span >
< / dt >
< dt >
< span class = "sect2" >
< a href = "am_cursor.html#am_join" > Equality Join< / a >
< / span >
< / dt >
< dt >
< span class = "sect2" >
< a href = "am_cursor.html#am_count" > Data item count< / a >
< / span >
< / dt >
< dt >
< span class = "sect2" >
< a href = "am_cursor.html#am_curclose" > Cursor close< / a >
< / span >
< / dt >
< / dl >
< / div >
< p >
A database cursor refers to a single key/data pair in the database.
It supports traversal of the database and is the only way to access
individual duplicate data items. Cursors are used for operating on
collections of records, for iterating over a database, and for
saving handles to individual records, so that they can be modified
after they have been read.
< / p >
< p >
The < a href = "../api_reference/C/dbcursor.html" class = "olink" > DB-> cursor()< / a > method opens a cursor into a database. Upon return
the cursor is uninitialized, cursor positioning occurs as part of
the first cursor operation.
< / p >
< p >
Once a database cursor has been opened, records may be retrieved
(< a href = "../api_reference/C/dbcget.html" class = "olink" > DBC-> get()< / a > ), stored (< a href = "../api_reference/C/dbcput.html" class = "olink" > DBC-> put()< / a > ), and deleted (< a href = "../api_reference/C/dbcdel.html" class = "olink" > DBC-> del()< / a > ).
< / p >
< p >
Additional operations supported by the cursor handle include
duplication (< a href = "../api_reference/C/dbcdup.html" class = "olink" > DBC-> dup()< / a > ), equality join (< a href = "../api_reference/C/dbjoin.html" class = "olink" > DB-> join()< / a > ), and a count of
duplicate data items (< a href = "../api_reference/C/dbccount.html" class = "olink" > DBC-> count()< / a > ). Cursors are eventually closed
using < a href = "../api_reference/C/dbcclose.html" class = "olink" > DBC-> close()< / a > .
< / p >
< p >
For more information on the operations supported by the cursor
handle, see the
< a href = "../api_reference/C/dbc.html#dbclist" class = "olink" > Database Cursors and Related Methods< / a >
section in the < em class = "citetitle" > Berkeley DB C API Reference Guide.< / em >
< / p >
< div class = "sect2" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
< h3 class = "title" > < a id = "am_curget" > < / a > Retrieving records with a cursor< / h3 >
< / div >
< / div >
< / div >
< p >
The < a href = "../api_reference/C/dbcget.html" class = "olink" > DBC-> get()< / a > method retrieves records from the database using a
cursor. The < a href = "../api_reference/C/dbcget.html" class = "olink" > DBC-> get()< / a > method takes a flag which controls how the
cursor is positioned within the database and returns the key/data
item associated with that positioning. Similar to < a href = "../api_reference/C/dbget.html" class = "olink" > DB-> get()< / a > ,
< a href = "../api_reference/C/dbcget.html" class = "olink" > DBC-> get()< / a > may also take a supplied key and retrieve the data
associated with that key from the database. There are several
flags that you can set to customize retrieval.
< / p >
< div class = "sect3" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
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< h4 class = "title" > < a id = "idp894136" > < / a > Cursor position flags< / h4 >
2011-09-13 17:44:24 +00:00
< / div >
< / div >
< / div >
< div class = "variablelist" >
< dl >
< dt >
< span class = "term" > < a href = "../api_reference/C/dbcget.html#dbcget_DB_FIRST" class = "olink" > DB_FIRST< / a > , < a href = "../api_reference/C/dbcget.html#dbcget_DB_LAST" class = "olink" > DB_LAST< / a > < / span >
< / dt >
< dd >
Return the first (last) record in the database.
< / dd >
< dt >
< span class = "term" > < a href = "../api_reference/C/dbcget.html#dbcget_DB_NEXT" class = "olink" > DB_NEXT< / a > , < a href = "../api_reference/C/dbcget.html#dbcget_DB_PREV" class = "olink" > DB_PREV< / a > < / span >
< / dt >
< dd >
Return the next (previous) record in the database.
< / dd >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbcget.html#dbcget_DB_NEXT_DUP" class = "olink" > DB_NEXT_DUP< / a >
< / span >
< / dt >
< dd >
Return the next record in the database, if it is a
duplicate data item for the current key. For Heap
databases, this flag always results in the cursor
returning the < code class = "literal" > DB_NOTFOUND< / code >
error.
< / dd >
< dt >
< span class = "term" > < a href = "../api_reference/C/dbcget.html#dbcget_DB_NEXT_NODUP" class = "olink" > DB_NEXT_NODUP< / a > , < a href = "../api_reference/C/dbcget.html#dbcget_DB_PREV_NODUP" class = "olink" > DB_PREV_NODUP< / a > < / span >
< / dt >
< dd >
Return the next (previous) record in the database that
is not a duplicate data item for the current
key.
< / dd >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbcget.html#dbcget_DB_CURRENT" class = "olink" > DB_CURRENT< / a >
< / span >
< / dt >
< dd >
Return the record from the database to which the cursor
currently refers.
< / dd >
< / dl >
< / div >
< / div >
< div class = "sect3" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
2012-11-14 21:35:20 +00:00
< h4 class = "title" > < a id = "idp900744" > < / a > Retrieving specific key/data pairs< / h4 >
2011-09-13 17:44:24 +00:00
< / div >
< / div >
< / div >
< div class = "variablelist" >
< dl >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbcget.html#dbcget_DB_SET" class = "olink" > DB_SET< / a >
< / span >
< / dt >
< dd >
Return the record from the database that matches the
supplied key. In the case of duplicates the first
duplicate is returned and the cursor is positioned at
the beginning of the duplicate list. The user can then
traverse the duplicate entries for the
key.
< / dd >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbcget.html#dbcget_DB_SET_RANGE" class = "olink" > DB_SET_RANGE< / a >
< / span >
< / dt >
< dd >
Return the smallest record in the database greater than
or equal to the supplied key. This functionality
permits partial key matches and range searches in the
Btree access method.
< / dd >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbcget.html#dbcget_DB_GET_BOTH" class = "olink" > DB_GET_BOTH< / a >
< / span >
< / dt >
< dd >
Return the record from the database that matches both
the supplied key and data items. This is particularly
useful when there are large numbers of duplicate
records for a key, as it allows the cursor to easily be
positioned at the correct place for traversal of some
part of a large set of duplicate records.
< / dd >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbcget.html#dbcget_DB_GET_BOTH_RANGE" class = "olink" > DB_GET_BOTH_RANGE< / a >
< / span >
< / dt >
< dd >
If used on a database configured for sorted duplicates, this
returns the smallest record in the database greater than or equal
to the supplied key and data items. If used on a database that is
< span class = "emphasis" > < em > not< / em > < / span > configured for sorted duplicates, this
flag behaves identically to < code class = "literal" > DB_GET_BOTH< / code > .
< / dd >
< / dl >
< / div >
< / div >
< div class = "sect3" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
2012-11-14 21:35:20 +00:00
< h4 class = "title" > < a id = "idp889792" > < / a > Retrieving based on record numbers< / h4 >
2011-09-13 17:44:24 +00:00
< / div >
< / div >
< / div >
< div class = "variablelist" >
< dl >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbcget.html#dbcget_DB_SET_RECNO" class = "olink" > DB_SET_RECNO< / a >
< / span >
< / dt >
< dd >
If the underlying database is a Btree, and was
configured so that it is possible to search it by
logical record number, retrieve a specific record based
on a record number argument.
< / dd >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbcget.html#dbcget_DB_GET_RECNO" class = "olink" > DB_GET_RECNO< / a >
< / span >
< / dt >
< dd >
If the underlying database is a Btree, and was
configured so that it is possible to search it by
logical record number, return the record number for the
record to which the cursor refers.
< / dd >
< / dl >
< / div >
< / div >
< div class = "sect3" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
2012-11-14 21:35:20 +00:00
< h4 class = "title" > < a id = "idp899384" > < / a > Special-purpose flags< / h4 >
2011-09-13 17:44:24 +00:00
< / div >
< / div >
< / div >
< div class = "variablelist" >
< dl >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbget.html#dbget_DB_CONSUME" class = "olink" > DB_CONSUME< / a >
< / span >
< / dt >
< dd >
Read-and-delete: the first record (the head) of the
queue is returned and deleted. The underlying database
must be a Queue.
< / dd >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbcget.html#dbcget_DB_RMW" class = "olink" > DB_RMW< / a >
< / span >
< / dt >
< dd >
Read-modify-write: acquire write locks instead of read
locks during retrieval. This can enhance performance in
threaded applications by reducing the chance of
deadlock.
< / dd >
< / dl >
< / div >
< p >
In all cases, the cursor is repositioned by a < a href = "../api_reference/C/dbcget.html" class = "olink" > DBC-> get()< / a >
operation to point to the newly-returned key/data pair in the
database.
< / p >
< p >
The following is a code example showing a cursor walking
through a database and displaying the records it contains to
the standard output:
< / p >
< a id = "prog_am19" > < / a >
< pre class = "programlisting" > int
display(char *database)
{
DB *dbp;
DBC *dbcp;
DBT key, data;
int close_db, close_dbc, ret;
close_db = close_dbc = 0;
/* Open the database. */
if ((ret = db_create(& dbp, NULL, 0)) != 0) {
fprintf(stderr,
"%s: db_create: %s\n", progname, db_strerror(ret));
return (1);
}
close_db = 1;
/* Turn on additional error output. */
dbp-> set_errfile(dbp, stderr);
dbp-> set_errpfx(dbp, progname);
/* Open the database. */
if ((ret = dbp-> open(dbp, NULL, database, NULL,
DB_UNKNOWN, DB_RDONLY, 0)) != 0) {
dbp-> err(dbp, ret, "%s: DB-> open", database);
goto err;
}
/* Acquire a cursor for the database. */
if ((ret = dbp-> cursor(dbp, NULL, & dbcp, 0)) != 0) {
dbp-> err(dbp, ret, "DB-> cursor");
goto err;
}
close_dbc = 1;
/* Initialize the key/data return pair. */
memset(& key, 0, sizeof(key));
memset(& data, 0, sizeof(data));
/* Walk through the database and print out the key/data pairs. */
while ((ret = dbcp-> get(dbcp, & key, & data, DB_NEXT)) == 0)
printf("%.*s : %.*s\n",
(int)key.size, (char *)key.data,
(int)data.size, (char *)data.data);
if (ret != DB_NOTFOUND) {
dbp-> err(dbp, ret, "DBcursor-> get");
goto err;
}
err: if (close_dbc & & (ret = dbcp-> close(dbcp)) != 0)
dbp-> err(dbp, ret, "DBcursor-> close");
if (close_db & & (ret = dbp-> close(dbp, 0)) != 0)
fprintf(stderr,
"%s: DB-> close: %s\n", progname, db_strerror(ret));
return (0);
}< / pre >
< / div >
< / div >
< div class = "sect2" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
< h3 class = "title" > < a id = "am_curput" > < / a > Storing records with a cursor< / h3 >
< / div >
< / div >
< / div >
< p >
The < a href = "../api_reference/C/dbcput.html" class = "olink" > DBC-> put()< / a > method stores records into the database using a
cursor. In general, < a href = "../api_reference/C/dbcput.html" class = "olink" > DBC-> put()< / a > takes a key and inserts the
associated data into the database, at a location controlled by a
specified flag.
< / p >
< p >
There are several flags that you can set to customize storage:
< / p >
< div class = "variablelist" >
< dl >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbcput.html#dbcput_DB_AFTER" class = "olink" > DB_AFTER< / a >
< / span >
< / dt >
< dd >
Create a new record, immediately after the record to which
the cursor refers.
< / dd >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbcput.html#dbcput_DB_BEFORE" class = "olink" > DB_BEFORE< / a >
< / span >
< / dt >
< dd >
Create a new record, immediately before the record to
which the cursor refers.
< / dd >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbcget.html#dbcget_DB_CURRENT" class = "olink" > DB_CURRENT< / a >
< / span >
< / dt >
< dd >
Replace the data part of the record to which the cursor
refers.
< / dd >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbcput.html#dbcput_DB_KEYFIRST" class = "olink" > DB_KEYFIRST< / a >
< / span >
< / dt >
< dd >
Create a new record as the first of the duplicate records
for the supplied key.
< / dd >
< dt >
< span class = "term" >
< a href = "../api_reference/C/dbcput.html#dbcput_DB_KEYLAST" class = "olink" > DB_KEYLAST< / a >
< / span >
< / dt >
< dd >
Create a new record, as the last of the duplicate records
for the supplied key.
< / dd >
< / dl >
< / div >
< p >
In all cases, the cursor is repositioned by a < a href = "../api_reference/C/dbcput.html" class = "olink" > DBC-> put()< / a > operation to
point to the newly inserted key/data pair in the database.
< / p >
< p >
The following is a code example showing a cursor storing two data
items in a database that supports duplicate data items:
< / p >
< a id = "prog_am20" > < / a >
< pre class = "programlisting" > int
store(DB *dbp)
{
DBC *dbcp;
DBT key, data;
int ret;
/*
* The DB handle for a Btree database supporting duplicate data
* items is the argument; acquire a cursor for the database.
*/
if ((ret = dbp-> cursor(dbp, NULL, & dbcp, 0)) != 0) {
dbp-> err(dbp, ret, "DB-> cursor");
goto err;
}
/* Initialize the key. */
memset(& key, 0, sizeof(key));
key.data = "new key";
key.size = strlen(key.data) + 1;
/* Initialize the data to be the first of two duplicate records. */
memset(& data, 0, sizeof(data));
data.data = "new key's data: entry #1";
data.size = strlen(data.data) + 1;
/* Store the first of the two duplicate records. */
if ((ret = dbcp-> put(dbcp, & key, & data, DB_KEYFIRST)) != 0)
dbp-> err(dbp, ret, "DB-> cursor");
/* Initialize the data to be the second of two duplicate records. */
data.data = "new key's data: entry #2";
data.size = strlen(data.data) + 1;
/*
* Store the second of the two duplicate records. No duplicate
* record sort function has been specified, so we explicitly
* store the record as the last of the duplicate set.
*/
if ((ret = dbcp-> put(dbcp, & key, & data, DB_KEYLAST)) != 0)
dbp-> err(dbp, ret, "DB-> cursor");
err: if ((ret = dbcp-> close(dbcp)) != 0)
dbp-> err(dbp, ret, "DBcursor-> close");
return (0);
}< / pre >
< div class = "note" style = "margin-left: 0.5in; margin-right: 0.5in;" >
< h3 class = "title" > Note< / h3 >
< p >
If you are using the Heap access method and you are creating a
new record in the database, then the key that you provide to
the < a href = "../api_reference/C/dbcput.html" class = "olink" > DBC-> put()< / a > method should be empty. The < a href = "../api_reference/C/dbcput.html" class = "olink" > DBC-> put()< / a > method will
return the record's ID (RID) in the key. The RID is
automatically created for you when Heap database records are
created.
< / p >
< / div >
< / div >
< div class = "sect2" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
< h3 class = "title" > < a id = "am_curdel" > < / a > Deleting records with a cursor< / h3 >
< / div >
< / div >
< / div >
< p > The < a href = "../api_reference/C/dbcdel.html" class = "olink" > DBC-> del()< / a > method deletes records from the database using a cursor.
The < a href = "../api_reference/C/dbcdel.html" class = "olink" > DBC-> del()< / a > method deletes the record to which the cursor currently
refers. In all cases, the cursor position is unchanged after a
delete.< / p >
< / div >
< div class = "sect2" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
< h3 class = "title" > < a id = "am_curdup" > < / a > Duplicating a cursor< / h3 >
< / div >
< / div >
< / div >
< p > Once a cursor has been initialized (for example, by a call to
< a href = "../api_reference/C/dbcget.html" class = "olink" > DBC-> get()< / a > ), it can be thought of as identifying a particular
location in a database. The < a href = "../api_reference/C/dbcdup.html" class = "olink" > DBC-> dup()< / a > method permits an application to
create a new cursor that has the same locking and transactional
information as the cursor from which it is copied, and which optionally
refers to the same position in the database.< / p >
< p > In order to maintain a cursor position when an application is using
locking, locks are maintained on behalf of the cursor until the cursor is
closed. In cases when an application is using locking without
transactions, cursor duplication is often required to avoid
self-deadlocks. For further details, refer to
< a class = "xref" href = "lock_am_conv.html" title = "Berkeley DB Transactional Data Store locking conventions" > Berkeley DB Transactional Data Store locking conventions< / a > .< / p >
< / div >
< div class = "sect2" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
< h3 class = "title" > < a id = "am_join" > < / a > Equality Join< / h3 >
< / div >
< / div >
< / div >
< p > Berkeley DB supports "equality" (also known as "natural"), joins on secondary
indices. An equality join is a method of retrieving data from a primary
database using criteria stored in a set of secondary indices. It
requires the data be organized as a primary database which contains the
primary key and primary data field, and a set of secondary indices.
Each of the secondary indices is indexed by a different secondary key,
and, for each key in a secondary index, there is a set of duplicate data
items that match the primary keys in the primary database.< / p >
< p > For example, let's assume the need for an application that will return
the names of stores in which one can buy fruit of a given color. We
would first construct a primary database that lists types of fruit as
the key item, and the store where you can buy them as the data item:< / p >
< div class = "informaltable" >
< table border = "1" width = "80%" >
< colgroup >
< col / >
< col / >
< / colgroup >
< thead >
< tr >
< th > Primary key:< / th >
< th > Primary data:< / th >
< / tr >
< / thead >
< tbody >
< tr >
< td align = "left" > apple< / td >
< td align = "left" > Convenience Store< / td >
< / tr >
< tr >
< td align = "left" > blueberry< / td >
< td align = "left" > Farmer's Market< / td >
< / tr >
< tr >
< td align = "left" > peach< / td >
< td align = "left" > Shopway< / td >
< / tr >
< tr >
< td align = "left" > pear< / td >
< td align = "left" > Farmer's Market< / td >
< / tr >
< tr >
< td align = "left" > raspberry< / td >
< td align = "left" > Shopway< / td >
< / tr >
< tr >
< td align = "left" > strawberry< / td >
< td align = "left" > Farmer's Market< / td >
< / tr >
< / tbody >
< / table >
< / div >
< p > We would then create a secondary index with the key < span class = "bold" > < strong > color< / strong > < / span > , and,
as the data items, the names of fruits of different colors.< / p >
< div class = "informaltable" >
< table border = "1" width = "80%" >
< colgroup >
< col / >
< col / >
< / colgroup >
< thead >
< tr >
< th > Secondary key:< / th >
< th > Secondary data:< / th >
< / tr >
< / thead >
< tbody >
< tr >
< td align = "left" > blue< / td >
< td align = "left" > blueberry< / td >
< / tr >
< tr >
< td align = "left" > red< / td >
< td align = "left" > apple< / td >
< / tr >
< tr >
< td align = "left" > red< / td >
< td align = "left" > raspberry< / td >
< / tr >
< tr >
< td align = "left" > red< / td >
< td align = "left" > strawberry< / td >
< / tr >
< tr >
< td align = "left" > yellow< / td >
< td align = "left" > peach< / td >
< / tr >
< tr >
< td align = "left" > yellow< / td >
< td align = "left" > pear< / td >
< / tr >
< / tbody >
< / table >
< / div >
< p > This secondary index would allow an application to look up a color, and
then use the data items to look up the stores where the colored fruit
could be purchased. For example, by first looking up < span class = "bold" > < strong > blue< / strong > < / span > ,
the data item < span class = "bold" > < strong > blueberry< / strong > < / span > could be used as the lookup key in the
primary database, returning < span class = "bold" > < strong > Farmer's Market< / strong > < / span > .< / p >
< p > Your data must be organized in the following manner in order to use the
< a href = "../api_reference/C/dbjoin.html" class = "olink" > DB-> join()< / a > method:< / p >
< div class = "orderedlist" >
< ol type = "1" >
< li > The actual data should be stored in the database represented by the
< a href = "../api_reference/C/db.html" class = "olink" > DB< / a > object used to invoke this method. Generally, this
< a href = "../api_reference/C/db.html" class = "olink" > DB< / a > object is called the < span class = "emphasis" > < em > primary< / em > < / span > .< / li >
< li > Secondary indices should be stored in separate databases, whose keys
are the values of the secondary indices and whose data items are the
primary keys corresponding to the records having the designated
secondary key value. It is acceptable (and expected) that there may be
duplicate entries in the secondary indices.
< p > These duplicate entries should be sorted for performance reasons, although
it is not required. For more information see the < a href = "../api_reference/C/dbset_flags.html#dbset_flags_DB_DUPSORT" class = "olink" > DB_DUPSORT< / a > flag
to the < a href = "../api_reference/C/dbset_flags.html" class = "olink" > DB-> set_flags()< / a > method.< / p > < / li >
< / ol >
< / div >
< p > What the < a href = "../api_reference/C/dbjoin.html" class = "olink" > DB-> join()< / a > method does is review a list of secondary keys, and,
when it finds a data item that appears as a data item for all of the
secondary keys, it uses that data item as a lookup into the primary
database, and returns the associated data item.< / p >
< p > If there were another secondary index that had as its key the < span class = "bold" > < strong > cost< / strong > < / span >
of the fruit, a similar lookup could be done on stores where inexpensive
fruit could be purchased:< / p >
< div class = "informaltable" >
< table border = "1" width = "80%" >
< colgroup >
< col / >
< col / >
< / colgroup >
< thead >
< tr >
< th > Secondary key:< / th >
< th > Secondary data:< / th >
< / tr >
< / thead >
< tbody >
< tr >
< td align = "left" > expensive< / td >
< td align = "left" > blueberry< / td >
< / tr >
< tr >
< td align = "left" > expensive< / td >
< td align = "left" > peach< / td >
< / tr >
< tr >
< td align = "left" > expensive< / td >
< td align = "left" > pear< / td >
< / tr >
< tr >
< td align = "left" > expensive< / td >
< td align = "left" > strawberry< / td >
< / tr >
< tr >
< td align = "left" > inexpensive< / td >
< td align = "left" > apple< / td >
< / tr >
< tr >
< td align = "left" > inexpensive< / td >
< td align = "left" > pear< / td >
< / tr >
< tr >
< td align = "left" > inexpensive< / td >
< td align = "left" > raspberry< / td >
< / tr >
< / tbody >
< / table >
< / div >
< p > The < a href = "../api_reference/C/dbjoin.html" class = "olink" > DB-> join()< / a > method provides equality join functionality. While not
strictly cursor functionality, in that it is not a method off a cursor
handle, it is more closely related to the cursor operations than to the
standard < a href = "../api_reference/C/db.html" class = "olink" > DB< / a > operations.< / p >
< p > It is also possible to do lookups based on multiple criteria in a single
operation. For example, it is possible to look up fruits that are both
red and expensive in a single operation. If the same fruit appeared as
a data item in both the color and expense indices, then that fruit name
would be used as the key for retrieval from the primary index, and would
then return the store where expensive, red fruit could be purchased.< / p >
< div class = "sect3" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
2012-11-14 21:35:20 +00:00
< h4 class = "title" > < a id = "idp942824" > < / a > Example< / h4 >
2011-09-13 17:44:24 +00:00
< / div >
< / div >
< / div >
< p > Consider the following three databases:< / p >
< div class = "variablelist" >
< dl >
< dt >
< span class = "term" > personnel< / span >
< / dt >
< dd >
< div class = "itemizedlist" >
< ul type = "disc" >
< li > key = SSN< / li >
< li > data = record containing name, address, phone number, job title< / li >
< / ul >
< / div >
< / dd >
< dt >
< span class = "term" > lastname< / span >
< / dt >
< dd >
< div class = "itemizedlist" >
< ul type = "disc" >
< li > key = lastname< / li >
< li > data = SSN< / li >
< / ul >
< / div >
< / dd >
< dt >
< span class = "term" > jobs< / span >
< / dt >
< dd >
< div class = "itemizedlist" >
< ul type = "disc" >
< li > key = job title< / li >
< li > data = SSN< / li >
< / ul >
< / div >
< / dd >
< / dl >
< / div >
< p > Consider the following query:< / p >
< pre class = "programlisting" > Return the personnel records of all people named smith with the job
title manager.< / pre >
< p > This query finds are all the records in the primary database (personnel)
for whom the criteria < span class = "bold" > < strong > lastname=smith and job title=manager< / strong > < / span > is
true.< / p >
< p > Assume that all databases have been properly opened and have the
handles: pers_db, name_db, job_db. We also assume that we have an
active transaction to which the handle txn refers.< / p >
< a id = "prog_am21" > < / a >
< pre class = "programlisting" > DBC *name_curs, *job_curs, *join_curs;
DBC *carray[3];
DBT key, data;
int ret, tret;
name_curs = NULL;
job_curs = NULL;
memset(& key, 0, sizeof(key));
memset(& data, 0, sizeof(data));
if ((ret =
name_db-> cursor(name_db, txn, & name_curs, 0)) != 0)
goto err;
key.data = "smith";
key.size = sizeof("smith");
if ((ret =
name_curs-> get(name_curs, & key, & data, DB_SET)) != 0)
goto err;
if ((ret = job_db-> cursor(job_db, txn, & job_curs, 0)) != 0)
goto err;
key.data = "manager";
key.size = sizeof("manager");
if ((ret =
job_curs-> get(job_curs, & key, & data, DB_SET)) != 0)
goto err;
carray[0] = name_curs;
carray[1] = job_curs;
carray[2] = NULL;
if ((ret =
pers_db-> join(pers_db, carray, & join_curs, 0)) != 0)
goto err;
while ((ret =
join_curs-> get(join_curs, & key, & data, 0)) == 0) {
/* Process record returned in key/data. */
}
/*
* If we exited the loop because we ran out of records,
* then it has completed successfully.
*/
if (ret == DB_NOTFOUND)
ret = 0;
err:
if (join_curs != NULL & &
(tret = join_curs-> close(join_curs)) != 0 & & ret == 0)
ret = tret;
if (name_curs != NULL & &
(tret = name_curs-> close(name_curs)) != 0 & & ret == 0)
ret = tret;
if (job_curs != NULL & &
(tret = job_curs-> close(job_curs)) != 0 & & ret == 0)
ret = tret;
return (ret);< / pre >
< p > The name cursor is positioned at the beginning of the duplicate list
for < span class = "bold" > < strong > smith< / strong > < / span > and the job cursor is placed at the beginning of
the duplicate list for < span class = "bold" > < strong > manager< / strong > < / span > . The join cursor is returned
from the join method. This code then loops over the join cursor getting
the personnel records of each one until there are no more.< / p >
< / div >
< / div >
< div class = "sect2" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
< h3 class = "title" > < a id = "am_count" > < / a > Data item count< / h3 >
< / div >
< / div >
< / div >
< p > Once a cursor has been initialized to refer to a particular key in the
database, it can be used to determine the number of data items that are
stored for any particular key. The < a href = "../api_reference/C/dbccount.html" class = "olink" > DBC-> count()< / a > method returns
this number of data items. The returned value is always one, unless
the database supports duplicate data items, in which case it may be any
number of items.< / p >
< / div >
< div class = "sect2" lang = "en" xml:lang = "en" >
< div class = "titlepage" >
< div >
< div >
< h3 class = "title" > < a id = "am_curclose" > < / a > Cursor close< / h3 >
< / div >
< / div >
< / div >
< p > The < a href = "../api_reference/C/dbcclose.html" class = "olink" > DBC-> close()< / a > method closes the < a href = "../api_reference/C/dbc.html" class = "olink" > DBC< / a > cursor, after which the
cursor may no longer be used. Although cursors are implicitly closed
when the database they point to are closed, it is good programming
practice to explicitly close cursors. In addition, in transactional
systems, cursors may not exist outside of a transaction and so must be
explicitly closed.< / p >
< / div >
< / div >
< div class = "navfooter" >
< hr / >
< table width = "100%" summary = "Navigation footer" >
< tr >
< td width = "40%" align = "left" > < a accesskey = "p" href = "am_foreign.html" > Prev< / a > < / td >
< td width = "20%" align = "center" >
< a accesskey = "u" href = "am.html" > Up< / a >
< / td >
< td width = "40%" align = "right" > < a accesskey = "n" href = "am_misc.html" > Next< / a > < / td >
< / tr >
< tr >
< td width = "40%" align = "left" valign = "top" > Foreign key indexes < / td >
< td width = "20%" align = "center" >
< a accesskey = "h" href = "index.html" > Home< / a >
< / td >
< td width = "40%" align = "right" valign = "top" >
Access Method Wrapup
< / td >
< / tr >
< / table >
< / div >
< / body >
< / html >
