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<title>49.3.Implementation</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="gist-implementation"></a>49.3.Implementation</h2></div></div></div>
<p> There are seven methods that an index operator class for
<acronym class="acronym">GiST</acronym> must provide:
</p>
<div class="variablelist"><dl>
<dt><span class="term">consistent</span></dt>
<dd><p> Given a predicate <code class="literal">p</code> on a tree page, and a user
query, <code class="literal">q</code>, this method will return false if it is
certain that both <code class="literal">p</code> and <code class="literal">q</code> cannot
be true for a given data item.
</p></dd>
<dt><span class="term">union</span></dt>
<dd><p> This method consolidates information in the tree. Given a set of
entries, this function generates a new predicate that is true for all
the entries.
</p></dd>
<dt><span class="term">compress</span></dt>
<dd><p> Converts the data item into a format suitable for physical storage in
an index page.
</p></dd>
<dt><span class="term">decompress</span></dt>
<dd><p> The reverse of the <code class="function">compress</code> method. Converts the
index representation of the data item into a format that can be
manipulated by the database.
</p></dd>
<dt><span class="term">penalty</span></dt>
<dd><p> Returns a value indicating the “<span class="quote">cost</span>” of inserting the new
entry into a particular branch of the tree. items will be inserted
down the path of least <code class="function">penalty</code> in the tree.
</p></dd>
<dt><span class="term">picksplit</span></dt>
<dd><p> When a page split is necessary, this function decides which entries on
the page are to stay on the old page, and which are to move to the new
page.
</p></dd>
<dt><span class="term">same</span></dt>
<dd><p> Returns true if two entries are identical, false otherwise.
</p></dd>
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