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OCA Oracle Database 11g: SQL Fundamentals I: A Real-World Certification Guide by Steve Ries

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Understanding the Oracle optimizer

When we execute a query in Oracle, it is easy to forget what's happening "under the hood". If we think about it for a moment, we might say that not all queries are created equal. For instance, when we discussed indexes, we noted that an index can speed up the execution of certain statements, provided that the WHERE clause is making its selection based on an indexed column. However, we noted that even when an index is present, an index scan may not be the most efficient execution path. What if we selected every row from a particular column in a table? In such a case, a full table scan might be more efficient. There are innumerable possibilities for such choices, given the scope and complexity possible with SQL. ...

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