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<title>Berkeley DB Reference Guide: Btree prefix comparison</title>
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<td><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></td>
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Btree prefix comparison
The Berkeley DB Btree implementation maximizes the number of keys that can be
stored on an internal page by storing only as many bytes of each key as
are necessary to distinguish it from adjacent keys. The prefix
comparison routine is what determines this minimum number of bytes (that
is, the length of the unique prefix), that must be stored. A prefix
comparison function for the Btree can be specified by calling
<a href="../../api_c/db_set_bt_prefix.html">DB-&gt;set_bt_prefix</a>.
The prefix comparison routine must be compatible with the overall
comparison function of the Btree, since what distinguishes any two keys
depends entirely on the function used to compare them. This means that
if a prefix comparison routine is specified by the application, a
compatible overall comparison routine must also have been specified.
Prefix comparison routines are passed pointers to keys as arguments.
The keys are represented as <a href="../../api_c/dbt_class.html">DBT</a> structures. The only fields
the routines may examine in the <a href="../../api_c/dbt_class.html">DBT</a> structures are data
and size fields.
The prefix comparison function must return the number of bytes necessary
to distinguish the two keys. If the keys are identical (equal and equal
in length), the length should be returned. If the keys are equal up to
the smaller of the two lengths, then the length of the smaller key plus
1 should be returned.
An example prefix comparison routine follows:
<blockquote><pre>u_int32_t
compare_prefix(dbp, a, b)
	DB *dbp;
	const DBT *a, *b;
{
	size_t cnt, len;
	u_int8_t *p1, *p2;

	cnt = 1;
	len = a-&gt;size &gt; b-&gt;size ? b-&gt;size : a-&gt;size;
	for (p1 =
		a-&gt;data, p2 = b-&gt;data; len--; ++p1, ++p2, ++cnt)
			if (*p1 != *p2)
				return (cnt);
	/*
	 * They match up to the smaller of the two sizes.
	 * Collate the longer after the shorter.
	 */
	if (a-&gt;size &lt; b-&gt;size)
		return (a-&gt;size + 1);
	if (b-&gt;size &lt; a-&gt;size)
		return (b-&gt;size + 1);
	return (b-&gt;size);
}</pre></blockquote>
The usefulness of this functionality is data-dependent, but in some data
sets can produce significantly reduced tree sizes and faster search times.
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