Explain THIS plan.

From: Kirsch, Walter J (Northrop Grumman) <walter.kirsch-eds_at_eds.com>
Date: Tue, 14 May 2002 11:08:31 -0800
Message-ID: <F001.0046055B.20020514110831@fatcity.com>

2-cpu, 220mhz, 32-bit HPUX 11.0. Oracle 8.1.7.0.0

Could someone explain what's going on here?

This SQL takes no time at all :

	select
	  substr(username , 1, 12)  "User"


	, substr(lock_type, 1, 18)  "Lock Type"

	, substr(mode_held, 1, 18)  "Mode Held"

	  from sys.dba_lock a
	     , v$session    b
	 where /*lock_type not in ('Media Recovery','Redo Thread')
	   and*/ a.session_id = b.sid;

with explain plan:

	SELECT STATEMENT Optimizer=CHOOSE (Cost=70 Card=1 Bytes=148)
	  NESTED LOOPS (Cost=70 Card=1 Bytes=148)
	    NESTED LOOPS (Cost=60 Card=1 Bytes=141)
	      NESTED LOOPS (Cost=20 Card=1 Bytes=107)
	        FIXED TABLE (FULL) OF X$KSUSE (Cost=10 Card=1 Bytes=30)
	        FIXED TABLE (FIXED INDEX #1) OF X$KSUSE (Cost=10 Card=1
Bytes=77)
	      VIEW OF GV$_LOCK
	        UNION-ALL
	          VIEW OF GV$_LOCK1 (Cost=20 Card=2 Bytes=162)
	            UNION-ALL
	              FIXED TABLE (FULL) OF X$KDNSSF (Cost=10 Card=1
Bytes=94)
	              FIXED TABLE (FULL) OF X$KSQEQ (Cost=10 Card=1
Bytes=94)
	          FIXED TABLE (FULL) OF X$KTADM (Cost=10 Card=1 Bytes=94)
	          FIXED TABLE (FULL) OF X$KTCXB (Cost=10 Card=1 Bytes=94)
	    FIXED TABLE (FIXED INDEX #1) OF X$KSQRS (Cost=10 Card=100

Bytes=700)

whereas an uncommented predicate consumes 40 minutes of CPU (sez TOP) with this explain plan:

	SELECT STATEMENT Optimizer=CHOOSE (Cost=70 Card=1 Bytes=148)
	  NESTED LOOPS (Cost=70 Card=1 Bytes=148)
	    NESTED LOOPS (Cost=30 Card=1 Bytes=114)
	      MERGE JOIN (CARTESIAN) (Cost=20 Card=1 Bytes=37)
	        FIXED TABLE (FULL) OF X$KSUSE (Cost=10 Card=1 Bytes=30)
	        SORT (JOIN) (Cost=10 Card=1 Bytes=7)
	          FIXED TABLE (FULL) OF X$KSQRS (Cost=10 Card=1 Bytes=7)
	      FIXED TABLE (FIXED INDEX #1) OF X$KSUSE (Cost=10 Card=1
Bytes=77)
	    VIEW OF GV$_LOCK
	      UNION-ALL
	        VIEW OF GV$_LOCK1 (Cost=20 Card=2 Bytes=162)
	          UNION-ALL
	            FIXED TABLE (FULL) OF X$KDNSSF (Cost=10 Card=1 Bytes=94)
	            FIXED TABLE (FULL) OF X$KSQEQ (Cost=10 Card=1 Bytes=94)
	        FIXED TABLE (FULL) OF X$KTADM (Cost=10 Card=1 Bytes=94)
	        FIXED TABLE (FULL) OF X$KTCXB (Cost=10 Card=1 Bytes=94)

If it jams, force it. If it breaks, it needed replacing anyway. --John F Duval

-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.com
-- 
Author: Kirsch, Walter J (Northrop Grumman)
  INET: walter.kirsch-eds_at_eds.com

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Received on Tue May 14 2002 - 14:08:31 CDT