http://log.medcl.net 记录生活 Tue, 29 Nov 2011 15:39:56 +0000 en hourly 1 http://wordpress.org/?v=3.1.2 http://log.medcl.net/item/2010/03/change-sql-optimization-experience/ http://log.medcl.net/item/2010/03/change-sql-optimization-experience/#comments Sun, 07 Mar 2010 03:56:51 +0000 medcl http://log.medcl.net/?p=240 24000/12 sal > 2000 sal*12 > 24000 如果SQL语句包括第一种情况，优化器会简单地把它转变成第二种。 优化器不会简化跨越比较符的表达式，例如第三条语句，鉴于此，应尽量写用常量跟字段比较检索的表达式，而不要将字段置于表达式当中。否则没有办法优化，比如如果sal上有索引，第一和第二就可以使用，第三就难以使用。 2 操作符优化： 优化器把使用LIKE操作符和一个没有通配符的表达式组成的检索表达式转换为一个“=”操作符表达式。 例如：优化器会把表达式ename LIKE 'SMITH'转换为ename = 'SMITH' 优化器只能转换涉及到可变长数据类型的表达式，前一个例子中，如果ENAME字段的类型是CHAR(10)， 那么优化器将不做任何转换。 一般来讲LIKE比较难以优化。 其中： ~~ IN 操作符优化： 优化器把使用IN比较符的检索表达式替换为等价的使用“=”和“OR”操作符的检索表达式。 例如，优化器会把表达式ename IN ('SMITH','KING','JONES')替换为 ename = 'SMITH' OR [...]]]> http://log.medcl.net/item/2010/03/change-sql-optimization-experience/feed/ 0 http://log.medcl.net/item/2010/03/sort-multiple-fields-sql/ http://log.medcl.net/item/2010/03/sort-multiple-fields-sql/#comments Fri, 05 Mar 2010 11:59:05 +0000 medcl http://log.medcl.net/?p=236 http://log.medcl.net/item/2010/03/sort-multiple-fields-sql/feed/ 0 http://log.medcl.net/item/2010/01/case-statement-in-the-sql-processing-conditions-used-in-the-data/ http://log.medcl.net/item/2010/01/case-statement-in-the-sql-processing-conditions-used-in-the-data/#comments Fri, 22 Jan 2010 11:35:04 +0000 medcl http://log.medcl.net/?p=198 http://log.medcl.net/item/2010/01/case-statement-in-the-sql-processing-conditions-used-in-the-data/feed/ 0 http://log.medcl.net/item/2010/01/set-parseonly-with-the-set-noexec-transact-sql/ http://log.medcl.net/item/2010/01/set-parseonly-with-the-set-noexec-transact-sql/#comments Tue, 19 Jan 2010 06:37:13 +0000 medcl http://log.medcl.net/?p=196 http://log.medcl.net/item/2010/01/set-parseonly-with-the-set-noexec-transact-sql/feed/ 1 http://log.medcl.net/item/2010/01/random-read-data-from-the-database/ http://log.medcl.net/item/2010/01/random-read-data-from-the-database/#comments Tue, 19 Jan 2010 06:11:43 +0000 medcl http://log.medcl.net/?p=194 http://log.medcl.net/item/2010/01/random-read-data-from-the-database/feed/ 0 http://log.medcl.net/item/2010/01/t-sql-as-the-same-as-dealing-with-an-array-of-strings-separated-string/ http://log.medcl.net/item/2010/01/t-sql-as-the-same-as-dealing-with-an-array-of-strings-separated-string/#comments Tue, 19 Jan 2010 03:08:47 +0000 medcl http://log.medcl.net/?p=191 http://log.medcl.net/item/2010/01/t-sql-as-the-same-as-dealing-with-an-array-of-strings-separated-string/feed/ 0 http://log.medcl.net/item/2010/01/common-sql-string-functions-faqs/ http://log.medcl.net/item/2010/01/common-sql-string-functions-faqs/#comments Tue, 19 Jan 2010 02:58:17 +0000 medcl http://log.medcl.net/?p=190 http://log.medcl.net/item/2010/01/common-sql-string-functions-faqs/feed/ 0 http://log.medcl.net/item/2009/10/data-comparison-of-different-versions-of-the-parameters/ http://log.medcl.net/item/2009/10/data-comparison-of-different-versions-of-the-parameters/#comments Wed, 28 Oct 2009 06:08:13 +0000 medcl http://log.medcl.net/?p=180 http://log.medcl.net/item/2009/10/data-comparison-of-different-versions-of-the-parameters/feed/ 0 http://log.medcl.net/item/2009/09/set-quoted_identifier/ http://log.medcl.net/item/2009/09/set-quoted_identifier/#comments Wed, 30 Sep 2009 01:56:25 +0000 medcl http://log.medcl.net/?p=172 http://log.medcl.net/item/2009/09/set-quoted_identifier/feed/ 0 http://log.medcl.net/item/2009/09/sql%e5%87%bd%e6%95%b0%e5%a4%a7%e5%85%a8%e5%8f%8a%e7%94%a8%e6%b3%95/ http://log.medcl.net/item/2009/09/sql%e5%87%bd%e6%95%b0%e5%a4%a7%e5%85%a8%e5%8f%8a%e7%94%a8%e6%b3%95/#comments Sat, 12 Sep 2009 11:43:49 +0000 medcl http://log.medcl.net/?p=164 http://log.medcl.net/item/2009/09/sql%e5%87%bd%e6%95%b0%e5%a4%a7%e5%85%a8%e5%8f%8a%e7%94%a8%e6%b3%95/feed/ 5