Oracle indexing tips
Oracle includes numerous data structures to improve the speed of Oracle SQL queries. Taking advantage of the low cost of disk storage, Oracle includes many new indexing algorithms that dramatically increase the speed with which Oracle queries are serviced. This article explores the internals of Oracle indexing; reviews the standard b-tree index, bitmap indexes, function-based indexes, and index-only tables (IOTs); and demonstrates how these indexes may dramatically increase the speed of Oracle SQL queries.
Indexes and blocksize
Indexes that experience lots of index range scans of index fast full scans (as evidence by multiblock reads) will greatly benefit from residing in a 32k blocksize.
Today, most Oracle tuning experts utilize the multiple blocksize feature of Oracle because it provides buffer segregation and the ability to place objects with the most appropriate blocksize to reduce buffer waste. Some of the world record Oracle benchmarks use very large data buffers and multiple blocksizes.
According to an article by Christopher Foot, author of the OCP Instructors Guide for Oracle DBA Certification, larger block sizes can help in certain situations:
"A bigger block size means more space for key storage in the branch nodes of B-tree indexes, which reduces index height and improves the performance of indexed queries."
In any case, there appears to be evidence that block size affects the tree structure, which supports the argument that data blocks affect the structure of the tree.
Indexes and clustering
The CBO's decision to perform a full-table vs. an index range scan is influenced by the clustering_factor (located inside the dba_indexes view), db_block_size, and avg_row_len. It is important to understand how the CBO uses these statistics to determine the fastest way to deliver the desired rows.
Conversely, a high clustering_factor, where the value approaches the number of rows in the table (num_rows), indicates that the rows are not in the same sequence as the index, and additional I/O will be required for index range scans. As the clustering_factor approaches the number of rows in the table, the rows are out of sync with the index.
For complete details on Oracle indexing for high performance, see my book "Oracle Tuning: The Definitive Reference".
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