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    <p>This depends on how your table is indexed and what functions do you use.</p> <p><code>ROW_NUMBER()</code>, for instance, seems to be less efficient than <code>ROWNUM</code>, even if the indexes are used. See this article in my blog for performance comparison:</p> <ul> <li><a href="http://explainextended.com/2009/05/06/oracle-row_number-vs-rownum/" rel="nofollow noreferrer"><strong><code>Oracle</code>: <code>ROW_NUMBER</code> vs <code>ROWNUM</code></strong></a></li> </ul> <p><code>Oracle</code>'s optimizer knows about window functions and can use several tricks like <code>STOPKEY</code> and <code>PUSHED RANK</code> which make them more efficient.</p> <blockquote> <p>The explain plan indicates a full table scan is performed on the table on which the oracle analytic function was used.</p> </blockquote> <p>The table scan itself is not bad. This may be optimal indeed, if the <code>TABLE ACCESS</code> to retrieve the values missing in the index is more expensive than filtering and sorting.</p> <p>Usually, if your table is indexed, the query <code>WHERE</code> and <code>ORDER BY</code> clauses allow using this index for ordering and optimizer considers this index worth using, the <code>WINDOW BUFFER</code> method is used for <code>LAG</code> and <code>LEAD</code> functions.</p> <p>The engine just keeps a running buffer of <code>2</code> rows (or more, depending on the value of the offset) and returns the values from the first and the second row.</p> <p>However, the optimizer can consider the index not worth using at all.</p> <p>In this case, it will use <code>WINDOW SORT</code>: same thing but the sorting is done in memory or temporary tablespace.</p>
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