If the union performed index lookups, but the table example performed a full table scan it might. Of course with iterative index usage the point is deprecated. However before they were introduced the first query would perform an FTS even if emp_type was the primary key. Even with iterative index usage the union statement is often faster.
Ian MacGregor Stanford Linear Accelerator Center [EMAIL PROTECTED] -----Original Message----- Sent: Thursday, July 31, 2003 5:54 PM To: Multiple recipients of list ORACLE-L I'm getting back to work on my union article, and I have yet another union question. Are there ever cases where a UNION might be used for performance reasons? For example, I could write: SELECT * FROM emp WHERE emp_type='HOURLY' OR emp_type='CONTRACT'; or I could write: SELECT * FROM emp WHERE emp_type='HOURLY' UNION SELECT * FROM emp WHERE emp_type='CONTRACT'; This is probably too simple of an example, but are there ever cases where using a UNION like this makes sense from a performance point-of-view? Best regards, Jonathan Gennick --- Brighten the corner where you are http://Gennick.com * 906.387.1698 * mailto:[EMAIL PROTECTED] Join the Oracle-article list and receive one article on Oracle technologies per month by email. To join, visit http://four.pairlist.net/mailman/listinfo/oracle-article, or send email to [EMAIL PROTECTED] and include the word "subscribe" in either the subject or body. -- Please see the official ORACLE-L FAQ: http://www.orafaq.net -- Author: Jonathan Gennick INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing). -- Please see the official ORACLE-L FAQ: http://www.orafaq.net -- Author: MacGregor, Ian A. INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
