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<div class="sect1" lang="en" xml:lang="en">
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<h2 class="title" style="clear: both"><a id="am_second"></a>Secondary indexes</h2>
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<dt>
<span class="sect2">
<a href="am_second.html#idm140654540148112">Error Handling With Secondary Indexes</a>
</span>
</dt>
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<p>
A secondary index, put simply, is a way to efficiently
access records in a database (the primary) by means of some
piece of information other than the usual (primary) key. In
Berkeley DB, this index is simply another database whose keys
are these pieces of information (the secondary keys), and
whose data are the primary keys. Secondary indexes can be
created manually by the application; there is no disadvantage,
other than complexity, to doing so. However, when the
secondary key can be mechanically derived from the primary key
and datum that it points to, as is frequently the case,
Berkeley DB can automatically and transparently manage
secondary indexes.
</p>
<p>
As an example of how secondary indexes might be used,
consider a database containing a list of students at a
college, each of whom has a unique student ID number. A
typical database would use the student ID number as the key;
however, one might also reasonably want to be able to look up
students by last name. To do this, one would construct a
secondary index in which the secondary key was this last
name.
</p>
<p>
In SQL, this would be done by executing something like the
following:
</p>
<pre class="programlisting">CREATE TABLE students(student_id CHAR(4) NOT NULL,
lastname CHAR(15), firstname CHAR(15), PRIMARY KEY(student_id));
CREATE INDEX lname ON students(lastname);</pre>
<p>
In Berkeley DB, this would work as follows (a <a class="ulink" href="second.javas" target="_top">Java API example is also
available</a>):
</p>
<a id="prog_am13"></a>
<pre class="programlisting">struct student_record {
char student_id[4];
char last_name[15];
char first_name[15];
};
....
void
second()
{
DB *dbp, *sdbp;
int ret;
/* Open/create primary */
if ((ret = db_create(&dbp, dbenv, 0)) != 0)
handle_error(ret);
if ((ret = dbp->open(dbp, NULL,
"students.db", NULL, DB_BTREE, DB_CREATE, 0600)) != 0)
handle_error(ret);
/*
* Open/create secondary. Note that it supports duplicate data
* items, since last names might not be unique.
*/
if ((ret = db_create(&sdbp, dbenv, 0)) != 0)
handle_error(ret);
if ((ret = sdbp->set_flags(sdbp, DB_DUP | DB_DUPSORT)) != 0)
handle_error(ret);
if ((ret = sdbp->open(sdbp, NULL,
"lastname.db", NULL, DB_BTREE, DB_CREATE, 0600)) != 0)
handle_error(ret);
/* Associate the secondary with the primary. */
if ((ret = dbp->associate(dbp, NULL, sdbp, getname, 0)) != 0)
handle_error(ret);
}
/*
* getname -- extracts a secondary key (the last name) from a primary
* key/data pair
*/
int
getname(DB *secondary, const DBT *pkey, const DBT *pdata, DBT *skey)
{
/*
* Since the secondary key is a simple structure member of the
* record, we don't have to do anything fancy to return it. If
* we have composite keys that need to be constructed from the
* record, rather than simply pointing into it, then the user's
* function might need to allocate space and copy data. In
* this case, the DB_DBT_APPMALLOC flag should be set in the
* secondary key DBT.
*/
memset(skey, 0, sizeof(DBT));
skey->data = ((struct student_record *)pdata->data)->last_name;
skey->size = sizeof(((struct student_record *)pdata->data)->last_name);
return (0);
}</pre>
<p>
From the application's perspective, putting things into the
database works exactly as it does without a secondary index;
one can simply insert records into the primary database. In
SQL one would do the following:
</p>
<pre class="programlisting">INSERT INTO student
VALUES ("WC42", "Churchill ", "Winston ");</pre>
<p>
and in Berkeley DB, one does:
</p>
<a id="prog_am14"></a>
<pre class="programlisting">struct student_record s;
DBT data, key;
memset(&key, 0, sizeof(DBT));
memset(&data, 0, sizeof(DBT));
memset(&s, 0, sizeof(struct student_record));
key.data = "WC42";
key.size = 4;
memcpy(&s.student_id, "WC42", sizeof(s.student_id));
memcpy(&s.last_name, "Churchill ", sizeof(s.last_name));
memcpy(&s.first_name, "Winston ", sizeof(s.first_name));
data.data = &s;
data.size = sizeof(s);
if ((ret = dbp->put(dbp, txn, &key, &data, 0)) != 0)
handle_error(ret);
</pre>
<p>Internally, a record with secondary key "Churchill" is
inserted into the secondary database (in addition to the
insertion of "WC42" into the primary, of course).</p>
<p>Deletes are similar. The SQL clause:</p>
<pre class="programlisting">DELETE FROM student WHERE (student_id = "WC42");</pre>
<p>looks like:</p>
<a id="prog_am15"></a>
<pre class="programlisting">DBT key;
memset(&key, 0, sizeof(DBT));
key.data = "WC42";
key.size = 4;
if ((ret = dbp->del(dbp, txn, &key, 0)) != 0)
handle_error(ret);</pre>
<p>
Deletes can also be performed on the secondary index
directly; a delete done this way will delete the "real" record
in the primary as well. If the secondary supports duplicates
and there are duplicate occurrences of the secondary key, then
all records with that secondary key are removed from both the
secondary index and the primary database. In SQL:
</p>
<pre class="programlisting">DELETE FROM lname WHERE (lastname = "Churchill ");</pre>
<p>
In Berkeley DB:
</p>
<a id="prog_am16"></a>
<pre class="programlisting">DBT skey;
memset(&skey, 0, sizeof(DBT));
skey.data = "Churchill ";
skey.size = 15;
if ((ret = sdbp->del(sdbp, txn, &skey, 0)) != 0)
handle_error(ret);</pre>
<p>
Gets on a secondary automatically return the primary datum.
If <a href="../api_reference/C/dbget.html" class="olink">DB->pget()</a> or <a href="../api_reference/C/dbcget.html" class="olink">DBC->pget()</a> is used in lieu of <a href="../api_reference/C/dbget.html" class="olink">DB->get()</a> or
<a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a>, the primary key is returned as well. Thus, the
equivalent of:
</p>
<pre class="programlisting">SELECT * from lname WHERE (lastname = "Churchill ");</pre>
<p>
would be:
</p>
<a id="prog_am17"></a>
<pre class="programlisting">DBT data, pkey, skey;
memset(&skey, 0, sizeof(DBT));
memset(&pkey, 0, sizeof(DBT));
memset(&data, 0, sizeof(DBT));
skey.data = "Churchill ";
skey.size = 15;
if ((ret = sdbp->pget(sdbp, txn, &skey, &pkey, &data, 0)) != 0)
handle_error(ret);
/*
* Now pkey contains "WC42" and data contains Winston's record.
*/</pre>
<p>
To create a secondary index to a Berkeley DB database, open
the database that is to become a secondary index normally,
then pass it as the "secondary" argument to the <a href="../api_reference/C/dbassociate.html" class="olink">DB->associate()</a>
method for some primary database.
</p>
<p>
After a <a href="../api_reference/C/dbassociate.html" class="olink">DB->associate()</a> call is made, the secondary indexes
become alternate interfaces to the primary database. All
updates to the primary will be automatically reflected in each
secondary index that has been associated with it. All get
operations using the <a href="../api_reference/C/dbget.html" class="olink">DB->get()</a> or <a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a> methods on the
secondary index return the primary datum associated with the
specified (or otherwise current, in the case of cursor
operations) secondary key. The <a href="../api_reference/C/dbget.html" class="olink">DB->pget()</a> and <a href="../api_reference/C/dbcget.html" class="olink">DBC->pget()</a> methods
also become usable; these behave just like <a href="../api_reference/C/dbget.html" class="olink">DB->get()</a> and
<a href="../api_reference/C/dbcget.html" class="olink">DBC->get()</a>, but return the primary key in addition to the
primary datum, for those applications that need it as
well.
</p>
<p>
Cursor get operations on a secondary index perform as
expected; although the data returned will by default be those
of the primary database, a position in the secondary index is
maintained normally, and records will appear in the order
determined by the secondary key and the comparison function or
other structure of the secondary database.
</p>
<p>
Delete operations on a secondary index delete the item from
the primary database and all relevant secondaries, including
the current one.
</p>
<p>
Put operations of any kind are forbidden on secondary
indexes, as there is no way to specify a primary key for a
newly put item. Instead, the application should use the
<a href="../api_reference/C/dbput.html" class="olink">DB->put()</a> or <a href="../api_reference/C/dbcput.html" class="olink">DBC->put()</a> methods on the primary database.
</p>
<p>
Any number of secondary indexes may be associated with a
given primary database, up to limitations on available memory
and the number of open file descriptors.
</p>
<p>
Note that although Berkeley DB guarantees that updates made
using any <a href="../api_reference/C/db.html" class="olink">DB</a> handle with an associated secondary will be
reflected in the that secondary, associating each primary
handle with all the appropriate secondaries is the
responsibility of the application and is not enforced by
Berkeley DB. It is generally unsafe, but not forbidden by
Berkeley DB, to modify a database that has secondary indexes
without having those indexes open and associated. Similarly,
it is generally unsafe, but not forbidden, to modify a
secondary index directly. Applications that violate these
rules face the possibility of outdated or incorrect results if
the secondary indexes are later used.
</p>
<p>
If a secondary index becomes outdated for any reason, it
should be discarded using the <a href="../api_reference/C/dbremove.html" class="olink">DB->remove()</a> method and a new one
created using the <a href="../api_reference/C/dbassociate.html" class="olink">DB->associate()</a> method. If a secondary index
is no longer needed, all of its handles should be closed using
the <a href="../api_reference/C/dbclose.html" class="olink">DB->close()</a> method, and then the database should be removed
using a new database handle and the <a href="../api_reference/C/dbremove.html" class="olink">DB->remove()</a> method.
</p>
<p>
Closing a primary database handle automatically
dis-associates all secondary database handles associated with
it.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idm140654540148112"></a>Error Handling With Secondary Indexes</h3>
</div>
</div>
</div>
<p>
An error return during a secondary update in CDS or DS
(which requires an abort in TDS) may leave a secondary
index inconsistent in CDS or DS. There are a few non-error
returns:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>0 </li>
<li>DB_BUFFER_SMALL </li>
<li>DB_NOTFOUND </li>
<li>DB_KEYEMPTY </li>
<li> DB_KEYEXIST </li>
</ul>
</div>
<p>
In the case of any other error return during a
secondary update in CDS or DS, delete the secondary
indices, recreate them and set the <code class="literal">DB_CREATE
flag</code> to the <code class="literal">DB->associate</code>
method. Some examples of error returns that need to be
handled this way are:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>ENOMEM - indicating there is insufficient memory
to return the requested item</li>
<li>EINVAL - indicating that an invalid flag value
or parameter is specified</li>
</ul>
</div>
<p>
Note that <code class="literal">DB_RUNRECOVERY</code> and
<code class="literal">DB_PAGE_NOTFOUND</code> are fatal errors
which should never occur during normal use of CDS or DS.
If those errors are returned by Berkeley DB when running
without transactions, check the database integrity with
the <code class="literal">DB->verify</code> method before rebuilding
the secondary indices.
</p>
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