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An explain plan is a representation of the access path that is taken when a query is executed within Oracle.
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At the physical level Oracle reads blocks of data. The smallest amount of data read is a single Oracle block, the largest is constrained by operating system limits (and multiblock i/o). Logically Oracle finds the data to read by using the following methods:
Full Table Scan (FTS)    --È«±íɨÃè
Index Lookup (unique & non-unique)    --Ë÷ÒýɨÃ裨ΨһºÍ·ÇΨһ£©
Rowid    --ÎïÀíÐÐid
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When looking at a plan, the rightmost (ie most inndented) uppermost operation is the first thing that is executed. --²ÉÓÃ×îÓÒ×îÉÏ×îÏÈÖ´ÐеÄÔ­Ôò¿´²ã´Î¹ØÏµ£¬ÔÚͬһ¼¶Èç¹ûij¸ö¶¯×÷ûÓÐ×ÓID¾Í×îÏÈÖ´ÐÐ
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SQL> select  /*+parallel (e 4)*/  *  from  emp  e;
Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=82 Bytes=7134)
   1    0   TABLE ACCESS* (FULL) OF 'EMP' (Cost=1 Card=82 Bytes=7134):Q5000
--[:Q5000]±íʾÊDz¢Ðз½Ê½
   1 PARALLEL_TO_SERIAL            SELECT /*+ NO_EXPAND ROWID(A1) */ A1."EMPNO"
                                   ,A1."ENAME",A1."JOB",A1."MGR",A1."HI
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SELECT STATEMENT [CHOOSE] Cost=1234 --CostÓÐÖµ£¬²ÉÓÃCBO
SELECT STATEMENT [CHOOSE]           --CostΪ¿Õ£¬²ÉÓÃRBO(9IÊÇÈç´ËÏÔʾµÄ)
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PARENT1
**FIRST CHILD
****FIRST GRANDCHILD
**SECOND CHILD
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SQL*Plus: Release 9.2.0.1.0 - Production on ÐÇÆÚËÄ 11ÔÂ 5 09:08:04 2009
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select sess.sid,sess.serial#, lo.oracle_username,lo.os_user_name,ao.object_name,lo.locked_mode
from v$locked_object lo,dba_objects ao,v$session sess
where ao.object_id = lo.object_id
and lo.session_id = sess.sid;
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SELECT A.OWNER,
A.OBJECT_NAME,
B.XIDUSN,
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B.XIDSQN,
B.SESSION_ID,
B.ORACLE_USERNAME,
B.OS_USER_NAME,
B.PROCESS,
B.LOCKED_MODE,
C.MACHINE,
C.STATUS,
C.SERVER,
C.SID,
C.SERIAL#,
C.PROGRAM
from ALL_OBJECTS A,
V$LOCKED_OBJECT B,
SYS.GV_$SESSION C
WHERE ( A.OBJE ......
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