dual 表的性能优化（Performance tuning of dual tables）.docVIP

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dual 表的性能优化（Performance tuning of dual tables）.doc

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dual 表的性能优化（Performance tuning of dual tables）

dual 表的性能优化（Performance tuning of dual tables） The performance optimization of Dual table Dual said a special system, it is only a field, a record. In our code, often use it to produce some special values, such as the system time: Select sysdate from dual; Under normal circumstances, the above statement will do a full table scan of the dual table, 3 consistent gets. This is a very small number. But if you put in a big circle, will have a very large consistent gets (in fact, our system will have such problem). How to optimize it, consistent gets instant only decreased by 1, will be in a large loop is a very large value. First look at the SQL trace: SQL select sysdate from dual; Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE 10 TABLE ACCESS (FULL) OFDUAL Statistics ---------------------------------------------------------- 0 recursive calls 0 dB block gets 3 consistent gets 0 physical reads ...... 1 rows processed Found on the full table scan. Whether can be considered by an index to avoid? SQL connect sys/sys as SYSDBA Connected. SQL SQL create table T_DUAL (DUMMY VARCHAR2 (1)); Table created. SQL SQL insert into t_dual values (X); 1 row created. SQL commit; Commit complete. SQL SQL create index t_dual_idx on t_dual (dummy); Index created. SQL analyze table t_dual compute statistics for table for all indexes for all indexed columns; Table analyzed. SQL create view x_$dual as select t_dual where dummy=X* from; View created. SQL grant select on x_$dual to public; Grant succeeded. SQL connect demo/demo Connected. SQL create synonym DUAL for sys.x_$dual; Synonym created. Look at the optimization results: SQL set autot trace SQL select sysdate from dual; Execution Plan ---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=1 Bytes=1) 10 INDEX (RANGE SCAN) OFT_DDUAL_IDX(NON-UNIQUE) (Cost=1 Card=1 Bytes=1) Statistics -------------------------------------------