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| author | Lorry Tar Creator <lorry-tar-importer@baserock.org> | 2015-02-17 17:25:57 +0000 |
|---|---|---|
| committer | <> | 2015-03-17 16:26:24 +0000 |
| commit | 780b92ada9afcf1d58085a83a0b9e6bc982203d1 (patch) | |
| tree | 598f8b9fa431b228d29897e798de4ac0c1d3d970 /docs/programmer_reference/am_cursor.html | |
| parent | 7a2660ba9cc2dc03a69ddfcfd95369395cc87444 (diff) | |
| download | berkeleydb-master.tar.gz | |
Diffstat (limited to 'docs/programmer_reference/am_cursor.html')
| -rw-r--r-- | docs/programmer_reference/am_cursor.html | 521 |
1 files changed, 297 insertions, 224 deletions
diff --git a/docs/programmer_reference/am_cursor.html b/docs/programmer_reference/am_cursor.html index 0d1fe075..c43d74b3 100644 --- a/docs/programmer_reference/am_cursor.html +++ b/docs/programmer_reference/am_cursor.html @@ -14,7 +14,7 @@ <body> <div xmlns="" class="navheader"> <div class="libver"> - <p>Library Version 11.2.5.3</p> + <p>Library Version 12.1.6.1</p> </div> <table width="100%" summary="Navigation header"> <tr> @@ -22,9 +22,7 @@ </tr> <tr> <td width="20%" align="left"><a accesskey="p" href="am_foreign.html">Prev</a> </td> - <th width="60%" align="center">Chapter 3. - Access Method Operations - </th> + <th width="60%" align="center">Chapter 3. Access Method Operations </th> <td width="20%" align="right"> <a accesskey="n" href="am_misc.html">Next</a></td> </tr> </table> @@ -42,7 +40,8 @@ <dl> <dt> <span class="sect2"> - <a href="am_cursor.html#am_curget">Retrieving records with a cursor</a> + <a href="am_cursor.html#am_curget">Retrieving records with a + cursor</a> </span> </dt> <dt> @@ -77,57 +76,61 @@ </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> + 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. + 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>). + 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> + 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> + 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> + <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. + 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> - <h4 class="title"><a id="idp894136"></a>Cursor position flags</h4> + <h4 class="title"><a id="idp413624"></a>Cursor position flags</h4> </div> </div> </div> @@ -136,43 +139,45 @@ <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> + 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> + 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> + 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. + 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> + Return the record from the database to + which the cursor currently refers. </dd> </dl> </div> @@ -181,7 +186,7 @@ <div class="titlepage"> <div> <div> - <h4 class="title"><a id="idp900744"></a>Retrieving specific key/data pairs</h4> + <h4 class="title"><a id="idp419144"></a>Retrieving specific key/data pairs</h4> </div> </div> </div> @@ -193,12 +198,12 @@ </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. + 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"> @@ -206,23 +211,24 @@ </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. + 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> + 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"> @@ -230,11 +236,13 @@ </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>. + 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> @@ -243,7 +251,7 @@ <div class="titlepage"> <div> <div> - <h4 class="title"><a id="idp889792"></a>Retrieving based on record numbers</h4> + <h4 class="title"><a id="idp409264"></a>Retrieving based on record numbers</h4> </div> </div> </div> @@ -255,21 +263,21 @@ </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. + 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> + 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> @@ -278,7 +286,7 @@ <div class="titlepage"> <div> <div> - <h4 class="title"><a id="idp899384"></a>Special-purpose flags</h4> + <h4 class="title"><a id="idp418264"></a>Special-purpose flags</h4> </div> </div> </div> @@ -289,10 +297,10 @@ <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> + 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"> @@ -300,22 +308,22 @@ </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. + 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> + <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. + 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: + 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 @@ -387,13 +395,14 @@ err: if (close_dbc && (ret = dbcp->close(dbcp)) != 0) </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. + 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: + There are several flags that you can set to customize + storage: </p> <div class="variablelist"> <dl> @@ -402,18 +411,18 @@ err: if (close_dbc && (ret = dbcp->close(dbcp)) != 0) <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> + 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> + Create a new record, immediately before the + record to which the cursor refers. </dd> <dt> <span class="term"> @@ -421,17 +430,17 @@ err: if (close_dbc && (ret = dbcp->close(dbcp)) != 0) </span> </dt> <dd> - Replace the data part of the record to which the cursor - refers. + 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> + Create a new record as the first of the + duplicate records for the supplied key. </dd> <dt> <span class="term"> @@ -439,18 +448,20 @@ err: if (close_dbc && (ret = dbcp->close(dbcp)) != 0) </span> </dt> <dd> - Create a new record, as the last of the duplicate records - for the supplied key. + 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> + 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> + 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 @@ -504,12 +515,12 @@ err: if ((ret = dbcp->close(dbcp)) != 0) <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. + 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> @@ -521,10 +532,12 @@ err: if ((ret = dbcp->close(dbcp)) != 0) </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> + <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"> @@ -534,18 +547,24 @@ delete.</p> </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> + <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"> @@ -555,18 +574,25 @@ self-deadlocks. For further details, refer to </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> + <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> @@ -607,8 +633,11 @@ the key item, and the store where you can buy them as the data item:</p> </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> + <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> @@ -649,35 +678,54 @@ as the data items, the names of fruits of different colors.</p> </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> + <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> + <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> + <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> @@ -722,25 +770,33 @@ fruit could be purchased:</p> </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> + <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> - <h4 class="title"><a id="idp942824"></a>Example</h4> + <h4 class="title"><a id="idp472064"></a>Example</h4> </div> </div> </div> - <p>Consider the following three databases:</p> + <p> + Consider the following three databases: + </p> <div class="variablelist"> <dl> <dt> @@ -750,7 +806,9 @@ then return the store where expensive, red fruit could be purchased.</p> <div class="itemizedlist"> <ul type="disc"> <li>key = SSN</li> - <li>data = record containing name, address, phone number, job title</li> + <li>data = record containing name, + address, phone number, job + title</li> </ul> </div> </dd> @@ -778,15 +836,22 @@ then return the store where expensive, red fruit could be purchased.</p> </dd> </dl> </div> - <p>Consider the following query:</p> + <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> + <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]; @@ -846,11 +911,15 @@ if (job_curs != NULL && 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> + <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"> @@ -861,12 +930,15 @@ the personnel records of each one until there are no more.</p> </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> + <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"> @@ -876,12 +948,15 @@ number of items.</p> </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> + <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"> @@ -899,9 +974,7 @@ explicitly closed.</p> <td width="20%" align="center"> <a accesskey="h" href="index.html">Home</a> </td> - <td width="40%" align="right" valign="top"> Chapter 4. - Access Method Wrapup - </td> + <td width="40%" align="right" valign="top"> Chapter 4. Access Method Wrapup </td> </tr> </table> </div> |