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An index is a data structure that takes the value of one or more columns of a table (the key) and returns all rows (or the requested columns in a row) with that value of the column quickly. The efficiency of an index comes from the fact that it lets you find necessary rows without having to scan all the rows of a table. As a result, indexes are more efficient in general, because they need fewer disk I/Os than if you had to scan the table.
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For a quick summary of indexing guidelines, please refer to the section Guidelines for Creating Indexes in 7.
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Developers are content when the EXPLAIN PLAN indicates that a query was using indexes. However, there s more to query optimization than simply using an index for speeding up your queries. If you don t use good indexes, your queries could slow down the database significantly. Important things to consider are whether you have the right indexes or even if the index is necessary in a certain query. In the next sections you ll look at some of the issues you should consider regarding the use of indexes.
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A common problem is that an index that performs admirably during development and testing phases simply won t perform well on a production database. Often, this is due to the much larger amounts of data in the real system than in the development system. Ideally, you should develop and test queries on an identical version of the production database.
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When to Index
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You need to index tables only if you think your queries will be selecting a small portion of the table. If your query is retrieving rows that are greater than 10 or 15 percent of the total rows in the table, you may not need an index. Remember that using an index prevents a full table scan, so it is inherently a faster means to traverse a table s rows. However, each time you want to access a particular row in an indexed table, first Oracle has to look up the column referenced in your query in its index. From the index, Oracle obtains the ROWID of the row, which is the logical address of its location on disk. If you choose to enforce uniqueness of the rows in a table, you can use a primary index on that table. By definition, a column that serves as a primary index must be non-null and unique. In addition to the primary index, you can have several secondary indexes. For example, the attribute LAST_NAME may serve as a primary index. However, if most of your queries include the CITY column, you may choose to index the CITY column as well. Thus, the addition of secondary indexes would enhance query
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CHAPTER 19 IM PR OVING DA TA BAS E PERFORM ANC E: S QL QUE RY OPTIMIZA TION
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performance. However, a cost is associated with maintaining additional secondary indexes. In addition to the additional disk space needed for large secondary indexes, remember that all inserts and updates to the table require that the indexes also be updated. If your system involves a large number of inserts and deletes, understand that too many indexes may be detrimental, because each DML causes changes in both the table and its indexes. Therefore, an OLTP-oriented database ought to keep its indexes to a minimum. A data warehouse, on the other hand, can have a much larger number of indexes because there is no penalty to be paid. That s because the data warehouse is a purely query-oriented database, not a transactional database.
What to Index
Your goal should be to use as few indexes as possible to meet your performance criteria. There s a price to be paid for having too many indexes, especially in OLTP databases. Each INSERT, UPDATE, and DELETE statement causes changes to be made to the underlying indexes of a table, and can slow down an application in some cases. The following are some broad guidelines you can follow to make sure your indexes help the application instead of hindering it: Index columns with high selectivity. Selectivity here means the percentage of rows in a table with a certain value. High selectivity, as you learned earlier in this chapter, means that there are few rows with identical values. Index all important foreign keys. Index all predicate columns. Index columns used in table joins. Proper indexing of tables involves carefully considering the type of application you re running, the number of DML operations, and the response time expectations. Here are some additional tips that can aid you in selecting appropriate indexes for your application: Try to avoid indexing columns that consist of long character strings, unless you re using the Oracle Text feature. Wherever possible, use index-only plans, meaning a query that can be satisfied completely by just the data in the index alone. This requires that you pay attention to the most common queries and create any necessary composite indexes (indexes that include more than one column attribute). Use secondary indexes on columns frequently involved in ORDER BY and GROUP BY operations, as well as sorting operations such as UNION or DISTINCT.
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