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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´Î¹Øϵ£¬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
ÓÅ»¯Ä£Ê½ÊÇCHOOSEµÄÇé¿öÏ£¬¿´Cost²ÎÊýÊÇ·ñÓÐÖµÀ´¾ö¶¨²ÉÓÃCBO»¹ÊÇRBO£º
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
Here the same principles apply