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While Oracle will merge two single-column indexes, it will not merge a multi-column index with another index. There is one thing to be aware of with respect to this scenario. If the single-column index is a unique or primary key index, that would cause the single-column index to take precedence over the multi-column index. Compare rank 4 with rank 9 in Table 1. What the RBO rules don't tell you 2 If all columns in an index are specified in the WHERE clause, that index will be used in preference to other indexes for which some columns are referenced.
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In this example, only Index1 is used, because the WHERE clause includes all columns for that index, but does not include all columns for Index2. If multiple indexes can be applied to a WHERE clause, and they all have an equal number of columns specified, only the index created last will be used.
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In this example, only Index2 is used, because it was created at 5 p. This behavior can pose a problem, because if you rebuild indexes in a different order than they were first created, a different index may suddenly be used for your queries. To deal with this problem, many sites have a naming standard requiring that indexes are named in alphabetical order as they are created.
Then, if a table is rebuilt, the indexes can be rebuilt in alphabetical order, preserving the correct creation order.
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You could, for example, number your indexes. Each new index added to a table would then be given the next number.
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In this example, only Index2 is utilized despite Index1 having three columns accessed and Index2 having only two column accessed See All Customer Reviews. Shop Books. Read an excerpt of this book! Add to Wishlist.
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USD 9. Sign in to Purchase Instantly. Overview One of the most important challenges faced by Oracle database administrators and Oracle developers is the need to tune SQL statements so that they execute efficiently. About the Author As a database administrator on a financial IMS project about eight years ago, Mark Gurry was asked to investigate his company's database direction for the next five years. Show More. Read an Excerpt The SQL Optimizers Whenever you execute a SQL statement, a component of the database known as the optimizer must decide how best to access the data operated on by that statement.
Understanding the Rule-Based Optimizer The rule-based optimizer RBO uses a predefined set of precedence rules to figure out which path it will use to access the database. Moreover this book contains lot of solutions to cost based and rule based optimizers. You will learn SQL optimizers such as rule based and cost based optimers, its problems and solutions and problems common to rule and cost with solutions, etc. Moreover this book explain you to use outlines for consistent executive plans such recording outlines, enabling outlines and managing outlines, etc.
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