Clustered Indexes   Oracle Tips by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Features for the Data Warehouse

Clustered Indexes

If we take a look at the I/O path to data we need to consider two factors; the logical I/O (CALLS) versus the physical I/O’s that are required to service the request. As Oracle scans an index, we know that each node of the index tree is read before we access the data row. To illustrate, let’s take the following simple example in table 8.1

Table 8.1 Table sizes and index sizes

Object             Customer table     last_name_idx
ROWS              1,000,000                    100,000
ROW LENGTH             160
BLOCK SIZE               16K                         16K
DATA BLOCKS        10,000                      4,000

Note: this example has been oversimplified to illustrate the principle of clustered tables, and does not consider other data block overhead such as blocks headers and space management pages.

Now, we can run an index report from the Oracle data dictionary and we see the following characteristics of the last_name_idx:

SQL> @idx_list

Index               Level Clust Ftr       # rows     reorg  dist. Keys
------------------- ----- ---------- ----------- --------- -----------
LAST_NAME_IDX 3 788,934 1,105,458 0.713 1,105,458

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This is an excerpt from "High Performance Data Warehousing", copyright 1997.

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