《一个SQL Tuning例子》的评论

作者：bunny harness

bunny harness — Sat, 17 Mar 2012 03:17:32 +0000

Hi…

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作者：Stuart Arehart

Stuart Arehart — Fri, 16 Mar 2012 23:50:27 +0000

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作者：roger

roger — Tue, 28 Feb 2012 14:58:30 +0000

to jump2009:

这个是应用里面的sql，写法是固定的，程序产生的语句！

作者：jump2009

jump2009 — Tue, 28 Feb 2012 09:21:15 +0000

为什么写成CELL_PH_NUM like ‘13912345678’,一般直接写成CELL_PH_NUM = ‘13912345678’ 要like后面一般都加’%’

作者：roger

roger — Tue, 28 Feb 2012 01:42:14 +0000

to francis: 没事，都是同事嘛！

作者：francis

francis — Mon, 27 Feb 2012 15:48:40 +0000

真旭，这个问题还真是麻烦你了啊，我向你表示歉意啊

作者：roger

roger — Mon, 27 Feb 2012 02:17:39 +0000

to 路人:

看下这个就明白了：

Parameter: _LIKE_WITH_BIND_AS_EQUALITY
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Introduced:      8.1.6
Range of values: TRUE | FALSE
Related:

Description:
~~~~~~~~~~~~
 Treat LIKE predicate with bind as an equality predicate for costing 
 purposes.

 This parameter forces the optimizer to treat expressions of the form
 [indexed-column like :b1] similar to [index-column = :b1].

 Normally CBO assigns internal default selectivity estimates for the
 LIKE operator but this is nowhere near the true selectivity and can
 cause an index access to be rejected.

 NOTE:  Default selectivity has changed from earlier releases

     Release           Selectivity
     < 9.2.x               25%
    >= 9.2.x               5%
       
 As of 8.1.7.2 and Oracle9i this parameter also enabled equality 
 costing for expressions of the form:
     function(column) LIKE function(:bind)

Parameter: _LIKE_WITH_BIND_AS_EQUALITY

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Introduced: 8.1.6

Range of values: TRUE | FALSE

Description:

~~~~~~~~~~~~

Treat LIKE predicate with bind as an equality predicate for costing

purposes.

This parameter forces the optimizer to treat expressions of the form

[indexed-column like :b1] similar to [index-column = :b1].

Normally CBO assigns internal default selectivity estimates for the

LIKE operator but this is nowhere near the true selectivity and can

cause an index access to be rejected.

NOTE: Default selectivity has changed from earlier releases

Release Selectivity

< 9.2.x 25%

>= 9.2.x 5%

As of 8.1.7.2 and Oracle9i this parameter also enabled equality

costing for expressions of the form:

function(column) LIKE function(:bind)

作者：路人

路人 — Sun, 26 Feb 2012 14:33:45 +0000

没弄明白，能不能解释一下，以前没见过这个案例，谢谢。