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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

You need to turn on statistics collection by either setting the initialization parameter TIMED_STATISTICS to TRUE or setting the initialization parameter STATISTICS_LEVEL to TYPICAL or ALL.

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.

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If you want a quick instance-wide wait event status, showing which events were the biggest contributors to total wait time, you can use the query shown in Listing 22-16 (several idle events are listed in the output, but I don t show them here). Listing 22-16. Instance-Wide Waits Sorted by Total Wait Time SQL> 2 3 4 5 6* SELECT event, total_waits,time_waited FROM V$SYSTEM_EVENT WHERE event NOT IN ('pmon timer','smon timer','rdbms ipc reply','parallel deque wait ,'virtual circuit','%SQL*Net%','client message','NULL ORDER BY time_waited DESC;

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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