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CHAPTER 11 INDEXES
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Bitmap join indexes do have a prerequisite. The join condition must join to a primary or unique key in the other table. In the preceding example, DEPT.DEPTNO is the primary key of the DEPT table, and the primary key must be in place, otherwise an error will occur: ops$tkyte@ORA10G> create bitmap index emp_bm_idx 2 on emp( d.dname ) 3 from emp e, dept d 4 where e.deptno = d.deptno 5 / from emp e, dept d * ERROR at line 3: ORA-25954: missing primary key or unique constraint on dimension
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When in doubt, try it out. It is trivial to add a bitmap index to a table (or a bunch of them) and see what it does for you. Also, you can usually create bitmap indexes much faster than B*Tree indexes. Experimentation is the best way to see if they are suited for your environment. I am frequently asked, What defines low cardinality There is no cut-and-dried answer for this. Sometimes it is 3 values out of 100,000. Sometimes it is 10,000 values out of 1,000,000. Low cardinality doesn t imply single-digit counts of distinct values. Experimentation is the way to discover if a bitmap is a good idea for your application. In general, if you have a large, mostly read-only environment with lots of ad hoc queries, a set of bitmap indexes may be exactly what you need.
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Function-based indexes were added to Oracle 8.1.5. They are now a feature of Standard Edition, whereas in releases prior to Oracle9i Release 2 they were a feature of Enterprise Edition. Function-based indexes give us the ability to index computed columns and use these indexes in a query. In a nutshell, this capability allows you to have case-insensitive searches or sorts, search on complex equations, and extend the SQL language efficiently by implementing your own functions and operators and then searching on them. There are many reasons why you would want to use a function-based index, with the following chief among them: They are easy to implement and provide immediate value. They can be used to speed up existing applications without changing any of their logic or queries.
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Unlike B*Tree and bitmap indexes, in Oracle9i Release 1 function-based indexes require some initial setup before we can create and then use them.
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CHAPTER 11 INDEXES
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Note The following information applies only to Oracle9i Release 1 and before. In Oracle9i Release 2 and above, function-based indexes are usable without any setup. The Oracle SQL Reference manual for Oracle9i Release 2 is not correct in this regard it says you need these privileges, when in fact you do not.
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There are some system parameter or session settings you must use and, to be able to create them, a privilege you must have: You must have the system privilege QUERY REWRITE to create function-based indexes on tables in your own schema. You must have the system privilege GLOBAL QUERY REWRITE to create function-based indexes on tables in other schemas. For the optimizer to use function-based indexes, the following session or system variables must be set: QUERY_REWRITE_ENABLED=TRUE and QUERY_REWRITE_INTEGRITY=TRUSTED. You may enable these either at the session level with ALTER SESSION, or at the system level via ALTER SYSTEM, or by setting them in the init.ora parameter file. QUERY_REWRITE_ENABLED allows the optimizer to rewrite the query to use the functionbased index. QUERY_REWRITE_INTEGRITY tells the optimizer to trust that the code marked deterministic by the programmer is in 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 in fact 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 leadingedge 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. 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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