crystal report barcode font free download Figure 19-1. Collecting optimizer statistics through the OEM in Font

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Figure 19-1. Collecting optimizer statistics through the OEM
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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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Note Oracle strongly recommends that you just use the Oracle-created GATHER_STATS_JOB, run by the Scheduler during the scheduled maintenance window, to collect optimizer statistics. You may want to collect optimizer statistics manually under an extreme situation, such as the database not being up during the scheduled maintenance window, or if you want to analyze a newly created table right away.
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Writing Efficient SQL
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One of the trickiest and most satisfying aspects of a DBA s job is helping to improve the quality of SQL code in the application. Efficient code means fast performance, and an easy way to decrease the I/O your query requires is to try to lower the number of rows that the optimizer has to examine. The optimizer is supposed to find the optimal plan based on your query. This means the optimizer won t rewrite an inefficiently written query it only produces the execution plan for that query. Also, even if your query is efficiently written, the optimizer may not always end up producing the best execution plan. You have better knowledge of your application and data than the optimizer does, and you can, with hints, force the optimizer to use that knowledge. The following sections cover some of the best guidelines for writing good SQL.
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Efficient WHERE Clauses
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Selective criteria in your WHERE clauses can dramatically decrease the amount of data Oracle has to consider during a query. You can follow some simple principles to ensure that the structure of your SQL statements is not inherently inefficient. Your join methods may be fine, but overlooking some of these principles could doom your statement from a performance point of view. Careful specification of WHERE conditions can have a significant bearing on whether the optimizer will choose existing indexes. The principle of selectivity the number of rows returned by a query as a percentage of the total number of rows in a table is the key idea here. A low percentage means high selectivity and a high percentage means the reverse. Because more selective WHERE clauses mean fewer I/Os, the CBO tends to prefer to choose those kinds of WHERE clauses over others in the same query. The following example makes this clear: SQL> SELECT * FROM national_employees WHERE ss_no = 515086789 AND city='DALLAS'; Two WHERE clauses are in this example, but you can see that the first WHERE clause that uses ss_no requires fewer I/Os. The column ss_no is the primary key and is highly selective only one row with that ss_no is in the entire table. The optimizer determines the selectivity of each of the two columns in the query by looking at the index statistics, which tell it how many rows in the table contain each of the two column values in the query. If neither of the columns has an index, Oracle will use a full table scan to retrieve the answer to the query. If both of them have indexes, it will use the more selective (and hence more efficient) index on the ss_no column. If you think that the optimizer should have used an index instead of doing a full table scan, then perform the following steps: 1. Views in a query sometimes prevent the use of indexes. Check to make sure that the execution plan shows that the correct indexes are being used.
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CHAPTER 19 IMPROV IN G DA TA BAS E PERFORMAN CE: SQL QUERY OPTIMIZA TION
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2. If you think heavy data skew is in the table, use histograms to provide Oracle with a more accurate representation of the data distribution in the table. The CBO assumes a uniform distribution of column data. The CBO may forego the use of an index even when a column value is selective, because the column itself is unselective in nature. Histograms help by providing the CBO with an accurate picture of the column data distribution. I discuss histograms later in this chapter, in the section Using Histograms. 3. If Oracle is still refusing to use the index, force it to do so by using an index hint, as explained in the section Using Hints to Influence the Execution Plan later in this chapter.
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