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POSpeed of paged queries in Oracle
 

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    <p>This is a never-ending topic for me and I'm wondering if I might be overlooking something. Essentially I use two types of SQL statements in an application:</p> <ol> <li>Regular queries with a "fallback" limit</li> <li>Sorted and paged queries</li> </ol> <p>Now, we're talking about some queries against tables with several million records, joined to 5 more tables with several million records. Clearly, we hardly want to fetch all of them, that's why we have the above two methods to limit user queries.</p> <p><strong>Case 1</strong> is really simple. We just add an additional <code>ROWNUM</code> filter:</p> <pre><code>WHERE ... AND ROWNUM &lt; ? </code></pre> <p>That's quite fast, as Oracle's CBO will take this filter into consideration for its execution plan and probably apply a <code>FIRST_ROWS</code> operation (similar to the one enforced by the <code>/*+FIRST_ROWS*/</code> hint.</p> <p><strong>Case 2</strong>, however is a bit more tricky with Oracle, as there is no <code>LIMIT ... OFFSET</code> clause as in other RDBMS. So we nest our "business" query in a technical wrapper as such:</p> <pre><code>SELECT outer.* FROM ( SELECT * FROM ( SELECT inner.*, ROWNUM as RNUM, MAX(ROWNUM) OVER(PARTITION BY 1) as TOTAL_ROWS FROM ( [... USER SORTED business query ...] ) inner ) WHERE ROWNUM &lt; ? ) outer WHERE outer.RNUM &gt; ? </code></pre> <p>Note that the <code>TOTAL_ROWS</code> field is calculated to know how many pages we will have even without fetching all data. Now this paging query is usually quite satisfying. But every now and then (as I said, when querying 5M+ records, possibly including non-indexed searches), this runs for 2-3minutes.</p> <p><strong>EDIT</strong>: Please note, that a potential bottleneck is not so easy to circumvent, because of sorting that has to be applied before paging!</p> <p>I'm wondering, is that state-of-the-art simulation of <code>LIMIT ... OFFSET</code>, including <code>TOTAL_ROWS</code> in Oracle, or is there a better solution that will be faster by design, e.g. by using the <code>ROW_NUMBER()</code> window function instead of the <code>ROWNUM</code> pseudo-column?</p>
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