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充分利用B*Tree所以不存储全为NULL的列值，可以对关心的枚举值，而且枚举值行数很少，查询要走索引，建立函数索引，减少索引的管理与存储开销，并且提高索引的使用效率，是很有效的。<br><br>看了sqlserver,PostgreSQL支持这个，这个叫部分索引/过滤索引<br>语法是<br>create index ... on tab(column)<br>where ...<br><br>Partial index<br>From Wikipedia, the free encyclopedia<br>In databases, a partial index, also known as filtered index is an index which has some condition applied to it so that it includes a subset of rows in the table.<br>This allows the index to remain small, even though the table may be rather large, and have extreme selectivity.<br>Suppose you have a transaction table where entries start out with STATUS = 'A' (active), and then may pass through other statuses ('P' for pending, 'W' for "being worked on") before reaching a final status, 'F', at which point it is no longer likely to be processed again.<br>In PostgreSQL, a useful partial index might be defined as:<br> create index partial_status on txn_table (status) where status in ('A', 'P', 'W');<br>This index would not bother storing any of the millions of rows that have reached "final" status, 'F', and would allow queries looking for transactions that still "need work" to efficiently search via this index.<br>Similarly, a partial index can be used to index only those rows where a column is not null, which will be of benefit when the column usually is null.<br> create index partial_object_update on object_table (updated_on) where updated_on is not null;<br>This index would allow the following query to read only the updated tuples:<br> select * from object_table where updated_on is not null order by updated_on;<br>It is not necessary that the condition be the same as the index criterion; Stonebraker's paper below presents a number of examples with indexes similar to the following:<br> create index partial_salary on employee(age) where salary > 2100;<br>[edit]Support<br><br>In SQL Server, this type of index is called a filtered index.[1] Partial indexes have been supported in PostgreSQL since version 7.2, released in February 2002.[2]<br>MySQL as of version 5.4 does not support partial indexes.[3] In MySQL, the term "partial index" is sometimes used to refer to prefix indexes, where only a truncated prefix of each value is stored in the index. This is another technique for reducing index size.[4]<br>[edit]References<br><br>^ SQL Server 2008 Documentation: Filtered Index Design Guidelines. Microsoft TechNet.<br>^ "PostgreSQL Documentation: Release Notes: Release 7.2". PostgreSQL. Retrieved 2009-10-09. "Enable partial indexes (Martijn van Oosterhout)"<br>^ Razzoli, Federico (2008-10-05). "Bug #4990 Functional Indexes". bugs.mysql.com. Retrieved 2009-07-14.<br>^ Ronald Bradford (2009-07-22). "Understanding Different MySQL Index Implementations".<br>[edit]External links
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您好，知我药妆肌肤顾问很高兴帮助您。<br>推荐你尝试以下方法:<br>1.生姜祛痘印面膜<br>用法：把生姜切片后敷在痘印处，先紧紧按一分钟，然后让生姜片自己贴在脸上，大概5—10分钟后觉得局部发热，再过几分钟觉得热热的取下。每周一次亦可。<br>功效：因为生姜里的成分可以刺激血液循环，加速皮肤恢复。<br>注意：这款生姜去痘印面膜只针对痘印，可不要往痘痘上贴哦。另外，如果皮肤敏感的mm也不要用。