Database Reference
In-Depth Information
can help determine how much data the SELECT statement retrieves
physically from disk.
In general terms, indexes are physically shortened, presorted versions
of table rows. A WHERE clause can allow the use of specific indexes
both for reading data and for applying inherent index sorts (pre-
sorted). Therefore, WHERE clause expression orders can be
extremely important to performance, particularly when matching
indexes or, more specifically, attempting to utilize and take advantage
of physical data ordering already available in an index. In Oracle
Database 9
i,
Oracle Database 10
g
,
and beyond, the Optimizer is
intelligent to the point that WHERE clause expression sequences can
even ignore case sensitivity to a certain extent. However, strict adher-
ence to SQL coding standards is still advisable. Simply put, structur-
ing WHERE clauses in the right manner can make SQL code
enormously less complex and potentially much more efficient.
That is enough about the WHERE clause for now. The next chapter
deals with sorting query results and the ORDER BY clause.
5.5
Endnotes
1.
Oracle Performance Tuning for 9
i
and 10
g
(ISBN: 1-55558-305-9)
