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CHAPTER 11 INDEXES
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fact deterministic (the following section contains examples of deterministic code and its meaning). If the code is, in fact, not deterministic (i.e., it returns different output given the same inputs), the resulting rows retrieved via the index may be incorrect. You must take care to ensure that a function defined as deterministic is actually deterministic. In all releases, the following points apply: Use the cost-based optimizer (CBO). The virtual columns (columns with functions applied to them) in the function-based indexes are only visible to the CBO and will not be used by the rule-based optimizer (RBO) ever. The RBO can make use of the leading-edge columns in a function-based index that have no functions applied to them. Use SUBSTR to constrain return values from user-written functions that return VARCHAR2 or RAW types. Optionally hide the SUBSTR in a view (recommended). Again, the following section contains examples of this.
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Once the preceding criteria have been satisfied, function-based indexes are as easy to use as the CREATE INDEX command. The optimizer will find and use your indexes at runtime for you.
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A Simple Function-Based Index Example
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Consider the following example. We want to perform a case-insensitive search on the ENAME column of the EMP table. Prior to function-based indexes, we would have approached this in a very different manner. We would have added an extra column to the EMP table called UPPER_ENAME, for example. This column would have been maintained by a database trigger on INSERT and UPDATE; that trigger would simply have set NEW.UPPER_NAME := UPPER(:NEW.ENAME). This extra column would have been indexed. Now with function-based indexes, we remove the need for the extra column. We begin by creating a copy of the demo EMP table in the SCOTT schema and adding some data to it: ops$tkyte@ORA11GR2> create table emp 2 as 3 select * 4 from scott.emp 5 where 1=0; Table created. ops$tkyte@ORA11GR2> insert into emp 2 (empno,ename,job,mgr,hiredate,sal,comm,deptno) 3 select rownum empno, 4 initcap(substr(object_name,1,10)) ename, 5 substr(object_type,1,9) JOB, 6 rownum MGR, 7 created hiredate, 8 rownum SAL, 9 rownum COMM, 10 (mod(rownum,4)+1)*10 DEPTNO 11 from all_objects 12 where rownum < 10000; 9999 rows created.
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CHAPTER 11 INDEXES
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Next, we will create an index on the UPPER value of the ENAME column, effectively creating a caseinsensitive index: ops$tkyte@ORA11GR2> create index emp_upper_idx on emp(upper(ename)); Index created. Finally, we ll analyze the table since, as noted previously, we need to make use of the CBO to use function-based indexes. In Oracle 10g, this step is technically unnecessary, as the CBO is used by default and dynamic sampling would gather the needed information, but gathering statistics is a more correct approach. ops$tkyte@ORA11GR2> begin 2 dbms_stats.gather_table_stats 3 (user,'EMP',cascade=>true); 4 end; 5 / PL/SQL procedure successfully completed. We now have an index on the UPPER value of a column. Any application that already issues caseinsensitive queries like this ops$tkyte%ORA11GR2> select * 2 from emp 3 where upper(ename) = 'KING'; Execution Plan ---------------------------------------------------------Plan hash value: 1576737566 ---------------------------------------------------------------------------------| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| ---------------------------------------------------------------------------------| 0 | SELECT STATEMENT | | 2 | 110 | 2 (0)| | 1 | TABLE ACCESS BY INDEX ROWID| EMP | 2 | 110 | 2 (0)| |* 2 | INDEX RANGE SCAN | EMP_UPPER_IDX | 2 | | 1 (0)| ---------------------------------------------------------------------------------Predicate Information (identified by operation id): -------------------------------------------------2 - access(UPPER("ENAME")='KING') will make use of this index, gaining the performance boost an index can deliver. Before this feature was available, every row in the EMP table would have been scanned, uppercased, and compared. In contrast, with the index on UPPER(ENAME), the query takes the constant KING to the index, range scans a little data, and accesses the table by rowid to get the data. This is very fast. This performance boost is most visible when indexing user-written functions on columns. Oracle 7.1 added the ability to use user-written functions in SQL, so we could do something like this: SQL> 2 3 4 select my_function(ename) from emp where some_other_function(empno) > 10 /
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