Order Index Columns for Performance

The order in which columns are named in the CREATE INDEX command need not correspond to the order in which they appear in the table. However, the order of columns in the CREATE INDEX statement is significant because query performance can be affected by the order chosen. In general, you should put the column expected to be used most often first in the index.

For example, assume the columns of the VENDOR_PARTS table are as shown in Figure 4 - 1.

 Figure 4 - 1. The VENDOR_PARTS Table

Assume that there are five vendors, and each vendor has about 1000 parts.

Suppose that the VENDOR_PARTS table is commonly queried by SQL statements such as the following:

SELECT * FROM vendor_parts
   WHERE part_no = 457 AND vendor_id = 1012;

To increase the performance of such queries, you might create a composite index putting the most selective column first; that is, the column with the most values

Oracle Tuning Manual 7 & 8

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