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    <p><code>Oracle</code> <em>may</em> push predicate into a view if it thinks it will improve the plan.</p> <p>If you want to avoid this, you may use either of the following:</p> <ol> <li>Add a <code>/*+ NO_MERGE */</code> or <code>/*+ NO_PUSH_PRED */</code> hint into the view definition</li> <li>Add a <code>/*+ NO_MERGE (view) */</code> or <code>/*+ NO_PUSH_PRED (view) */</code> hint into the query that uses the view.</li> </ol> <p>If you want to force this, use their couterparts <code>/*+ PUSH_PRED */</code> and <code>/*+ MERGE */</code></p> <p>As long as performance is concerned, there is no difference between using a defined view (if it's not a <code>MATERIALIZED VIEW</code> of course) or an inline view (i. e. subquery).</p> <p><code>Oracle</code> compiles plans not for views, but for exact <code>SQL</code> texts.</p> <p>That is, for the following statements:</p> <pre><code>SELECT A, B, C FROM aTbl, bTbl LEFT JOIN cTbl ON bTbl.cTblID = cTbl.objectkey WHERE aTbl.objectkey = bTbl.parentkey AND aTbl.flag1 = :NUMBER SELECT * FROM ( SELECT A, B, C, flag1 FROM aTbl, bTbl LEFT JOIN cTbl ON bTbl.cTblID = cTbl.objectkey WHERE aTbl.objectkey = bTbl.parentkey ) WHERE flag1 = :NUMBER /* CREATE VIEW v_abc AS SELECT A, B, C, flag1 FROM aTbl, bTbl LEFT JOIN cTbl ON bTbl.cTblID = cTbl.objectkey WHERE aTbl.objectkey = bTbl.parentkey */ SELECT A, B, C FROM v_abc WHERE flag1 = :NUMBER </code></pre> <p>the plan will:</p> <ol> <li>Be the same (if <code>Oracle</code> will choose to push the predicate, which is more than probable); <ul> <li>Be compiled when first called;</li> <li>Be reused when called again.</li> </ul></li> </ol>
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    1. POCan using a VIEW for SELECT operations improve performance?
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    1. POCan using a VIEW for SELECT operations improve performance?
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    1. COIt seems that he wants oracle to do it, not to avoid it.
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      1. USNicolas
    2. COThank you, I am deeply sorry for forgetting to accept your input as the answer of this question.
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      1. USMaxim Veksler
 

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