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CHAPTER 11 INDEXES
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The error message that accompanies this isn t too illuminating either: ops$tkyte@ORA11GR2> !oerr ora 1743 01743, 00000, "only pure functions can be indexed" // *Cause: The indexed function uses SYSDATE or the user environment. // *Action: PL/SQL functions must be pure (RNDS, RNPS, WNDS, WNPS). SQL // expressions must not use SYSDATE, USER, USERENV(), or anything // else dependent on the session state. NLS-dependent functions // are OK. We are not using SYSDATE. We are not using the user environment (or are we ). No PL/SQL functions are used, and nothing about the session state is involved. The trick lies in the format we used: YYYY. That format, given the same exact inputs, will return different answers depending on what month you call it in. For example, anytime in the month of May ops$tkyte@ORA11GR2> select to_char( to_date('2005','YYYY'), 2 'DD-Mon-YYYY HH24:MI:SS' ) 3 from dual; TO_CHAR(TO_DATE('200 -------------------01-May-2005 00:00:00 the YYYY format will return May 1, in June it will return June 1, and so on. It turns out that TO_DATE, when used with YYYY, is not deterministic! That is why the index cannot be created: it would only work correctly in the month you created it in (or insert/updated a row in). So, it is due to the user environment, which includes the current date itself. To use TO_DATE in a function-based index, you must use a date format that is unambiguous and deterministic regardless of what day it is currently.
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Function-Based Indexes Wrap-up
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Function-based indexes are easy to use and implement, and they provide immediate value. They can be used to speed up existing applications without changing any of their logic or queries. Many orders of magnitude improvement may be observed. You can use them to precompute complex values without using a trigger. Additionally, the optimizer can estimate selectivity more accurately if the expressions are materialized in a function-based index. You can use function-based indexes to selectively index only rows of interest as demonstrated earlier with the PROCESSED_FLAG example. You can, in effect, index a WHERE clause using that technique. Lastly, you can use function-based indexes to implement a certain kind of integrity constraint: selective uniqueness (e.g., The fields X, Y, and Z must be unique when some condition is true ). Function-based indexes will affect the performance of inserts and updates. Whether or not that warning is relevant to you is something you must decide. If you insert and very infrequently query the data, this might not be an appropriate feature for you. On the other hand, keep in mind that you typically insert a row once and you query it thousands of times. The performance hit on the insert (which your individual end user will probably never notice) may be offset many thousands of times by speeding up the queries. In general, the pros heavily outweigh any of the cons in this case.
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CHAPTER 11 INDEXES
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Application domain indexes are what Oracle calls extensible indexing. They allow you to create your own index structures that work just like indexes supplied by Oracle. When someone issues a CREATE INDEX statement using your index type, Oracle will run your code to generate the index. If someone analyzes the index to compute statistics on it, Oracle will execute your code to generate statistics in whatever format you care to store them in. When Oracle parses a query and develops a query plan that may make use of your index, Oracle will ask you how costly this function is to perform as it is evaluating the different plans. In short, application domain indexes give you the ability to implement a new index type that does not exist in the database as of yet. For example, if you develop software that analyzes images stored in the database, and you produce information about the images, such as the colors found in them, you could create your own image index. As images are added to the database, your code is invoked to extract the colors from the images and store them somewhere (wherever you want to store them). At query time, when the user asks for all blue images, Oracle will ask you to provide the answer from your index when appropriate. The best example of an application domain index is Oracle s own text index. This index is used to provide keyword searching on large text items. You may create a simple text index like this ops$tkyte@ORA11GR2> create index myindex on mytable(docs) 2 indextype is ctxsys.context 3 / Index created. and then use the text operators the creators of that index type introduced into the SQL language select * from mytable where contains( docs, 'some words' ) > 0; It will even respond to commands such as the following: ops$tkyte@ORA11GR2> begin 2 dbms_stats.gather_index_stats( user, 'MYINDEX' ); 3 end; 4 / PL/SQL procedure successfully completed. It will participate with the optimizer at runtime to determine the relative cost of using a text index over some other index or a full scan. The interesting thing about all of this is that you or I could have developed this index. The implementation of the text index was done without inside kernel knowledge. It was done using the dedicated, documented, and exposed API. The Oracle database kernel is not aware of how the text index is stored (the APIs store it in many physical database tables per index created). Oracle is not aware of the processing that takes place when a new row is inserted. Oracle text is really an application built on top of the database, but in a wholly integrated fashion. To you and me, it looks just like any other Oracle database kernel function, but it is not. I personally have not found the need to go and build a new exotic type of index structure. I see this particular feature as being of use mostly to third-party solution providers that have innovative indexing techniques. I think the most interesting thing about application domain indexes is that they allow others to supply new indexing technology I can use in my applications. Most people will never make use of this particular API to build a new index type, but most of us will use the end results. Virtually every application I work on seems to have some text associated with it, XML to be dealt with, or images to be stored and categorized. The interMedia set of functionality, implemented using the Application Domain Indexing feature, provides these capabilities. As time passes, the set of available index types grows. We ll take a more in-depth look at the text index in a subsequent chapter.
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