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It isn t always obvious why Oracle doesn t use an index. For example, Oracle may not use an index because the indexed columns are part of an IN list, and the consequent transformation prevents the use of an index.
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If you use a WHERE clause such as WHERE last_name LIKE '%MA%', the optimizer might just decide to skip the index and do a full scan of the table because it needs to perform a pattern match of the entire LAST_NAME column to retrieve data. The optimizer correctly figures that it will go ahead and look up just the table, instead of having to read both the index and the table values. For example, if a table has 1,000 rows placed in 200 blocks, and you perform a full table scan assuming that the database has set the DB_FILE_MULTIBLOCK_READ_COUNT to 8, you ll incur a total of 25 I/Os to read in the entire table. If your index has a low selectivity, most of the index has to be read first. If your index has 40 leaf blocks and you have to read 90 percent of them to get the indexed data first, your I/O is already at 32. On top of this, you have to incur additional I/O to read the table values. However, a full table scan costs you only 25 I/Os, making that a far more efficient choice than using the index. Be aware that the mere existence of an index on a column doesn t guarantee that it will be used all the time. You ll look at some important principles to make your queries more efficient in the following sections.
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If you use SQL functions in the WHERE clause (for example, the SUBSTR, INSTR, TO_DATE, and TO_NUMBER functions), the Oracle optimizer will ignore the index on that column. Make sure you use a functionbased index if you must use a SQL function in the WHERE clause.
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Most of your SQL statements will involve multitable joins. Often, improper table-joining strategies doom a query. Here are some pointers regarding joining tables wisely: Using the equi join leads to a more efficient query path than otherwise. Try to use equi joins wherever possible. Performing filtering operations early reduces the number of rows to be joined in later steps. Fop example, a WHERE condition applied early reduces the row source that needs to be joined to another table. The goal is to use the table that has the most selective filter as the driving table, because this means fewer rows are passed to the next step. Join in the order that will produce the least number of rows as output to the parent step.
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When you need to calculate multiple aggregates from the same table, avoid writing a separate query for each aggregate. With separate queries, Oracle has to read the entire table for each query. It s
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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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more efficient to use the CASE statement in this case, as it enables you to compute multiple aggregates from the table with just a single read of the table.
Efficient Subquery Execution
Subqueries perform better when you use IN rather than EXISTS. Oracle recommends using the IN clause if the subquery has the selective WHERE clause. If the parent query contains the selective WHERE clause, use the EXISTS clause rather than the IN clause.
Using WHERE Instead of HAVING
Wherever possible, use the WHERE clause instead of the HAVING clause. The WHERE clause restricts the number of rows retrieved at the outset. The HAVING clause forces the retrieval of a lot more rows than necessary. It then also incurs the additional overhead of sorting and summing.
Minimizing Table Lookups
One of the primary mottos of query writing is Visit the data as few times as possible. This means getting rid of SQL that repeatedly accesses a table for different column values. Use multicolumn updates instead.
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