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Indexing Alternatives to B-Tree Indexes

Oracle Tips by Burleson Consulting

As you may know, Oracle offers several alternative indexing methods to enhance the standard B-tree indexes. These include bitmap indexes, function-based indexes, and reverse-key indexes.

Bitmap Indexes

It was a common misconception that bitmap indexes were only appropriate for columns with a very small number of distinct values—say, fewer than 50. Current research in Oracle8i has shown that bitmap indexes can substantially improve the speed of queries using columns with up to 1000 distinct values, because retrieval from a bitmap index is done in RAM and is almost always faster than using a traditional B-tree index. Most experienced Remote DBAs will look for table that contain columns with fewer than 1000 distinct values, build a bitmap index on these columns, and then see if the query is faster.

Function-Based Indexes

To use the alternative indexing structures, you must first identify SQL statements that are using the BIF. In the next example, we can search the v$sqlarea view to find all SQL statements that are using the to_char BIF.

   v$sqlarea  -- or stats$sql_summary
   sql_text like ‘%to_char%’;

Once identified, function-based indexes can be created to remove the full-table scans and replace them with index-range scans. For details on tuning with function-based indexes, see Chapter 23.

Reverse-Key Indexes and SQL Performance

There is, however, a major scalability danger with automatically generated synthetic keys. Every insertion to a table requires a corresponding insertion to its primary key index. If the primary key values are being generated in ascending order, then all inserts will need to change the high-order leaf block in the B-tree. There is an obvious danger here of contention for that block of the index, if several users attempt concurrent inserts (whereas the inserts to the table itself can easily be distributed to a variety of blocks by using multiple process freelists).

Prior to Oracle8, the standard strategy to avoid this problem was to ensure that the synthetic key values were not generated in order. This was done by permuting the values generated by the sequence number generator before using them. Various permutation schemes such as adding a leading check digit, or reversing the order of the digits, have been used. These schemes have the effect of distributing inserts evenly over the range of values in the index, thus preventing leaf block contention. In Oracle8, the same effect may be obtained by using a reverse-key index.

The major disadvantage of distributing inserts in this way is that the data density of index leaf blocks will be typically only 75 percent of capacity rather than almost 100 percent, making fast full-index scans on the primary key index less efficient. However, this access path is not typical and is seldom performance critical when used. So, reverse-key indexes should be used in general for synthetic primary key indexes.

This is an excerpt from "Oracle High-Performance SQL Tuning" by Donald K. Burleson, published by Oracle Press.

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