📄 oracle常见问题常用技巧-4.htm
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第一句：
<p>SQL> select * from mytables where myid>=1; </p>
<p>已选择325917行。</p>
<p>Execution Plan</p>
<p>----------------------------------------------------------</p>
<p>0 SELECT STATEMENT Optimizer=CHOOSE (Cost=3132 Card=318474 Byt es=141402456)</p>
<p>1 0 TABLE ACCESS (FULL) OF 'MYTABLES' (Cost=3132 Card=318474 Byt es=141402456)</p>
<p>Statistics</p>
<p>----------------------------------------------------------</p>
<p>7 recursive calls</p>
<p>89 db block gets</p>
<p>41473 consistent gets</p>
<p>19828 physical reads</p>
<p>0 redo size</p>
<p>131489563 bytes sent via SQL*Net to client</p>
<p>1760245 bytes received via SQL*Net from client</p>
<p>21729 SQL*Net roundtrips to/from client</p>
<p>1 sorts (memory)</p>
<p>0 sorts (disk)</p>
<p>325917 rows processed</p>
<p>第二句：</p>
<p>Execution Plan</p>
<p>----------------------------------------------------------</p>
<p>0 SELECT STATEMENT Optimizer=CHOOSE (Cost=346 Card=663 Bytes=2 94372)</p>
<p>1 0 TABLE ACCESS (BY INDEX ROWID) OF 'MYTABLES' (Cost=346 Card=663</p>
<p>Bytes=294372)</p>
<p>2 1 INDEX (RANGE SCAN) OF 'PK_MYTABLES' (UNIQUE) (Cost=5 Card=663)</p>
<p>Statistics</p>
<p>----------------------------------------------------------</p>
<p>1278 recursive calls</p>
<p>0 db block gets</p>
<p>6647 consistent gets</p>
<p>292 physical reads</p>
<p>0 redo size</p>
<p>3544898 bytes sent via SQL*Net to client</p>
<p>42640 bytes received via SQL*Net from client</p>
<p>524 SQL*Net roundtrips to/from client</p>
<p>1 sorts (memory)</p>
<p>0 sorts (disk)</p>
<p>7838 rows processed </p>
<p> 显而易见，第1句没有使用索引，第2句使用了主键索引pk_mytables. FF的巨大影响由此可见一斑。由此想到，我们在写sql 语句时，如果预先估计一下
FF, 你就几乎可以预见到 Oracle 会否使用索引。 </p>
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