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"TABLESAMPLE" in DB2

 
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rajesh_1183

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Joined: 24 Nov 2005
Posts: 121
Location: Tadepalligudem

PostPosted: Wed Jan 04, 2006 12:38 pm    Post subject: "TABLESAMPLE" in DB2
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Hi all,

can any body explain what the following query performs

SELECT * FROM staff TABLESAMPLE BERNOULLI(10);

Thanks in adv,
Rajesh.
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nikyojin

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Joined: 05 Oct 2005
Posts: 94

PostPosted: Wed Jan 04, 2006 1:51 pm    Post subject: Re: "TABLESAMPLE" in DB2
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Hi Rajesh,
DB2 UDB V8.1.2 adds the ability to do efficient sampling of data for SQL queries in the engine. Sampling is supported in DB2 UDB V8.1.2 with the new TABLESAMPLE clause, which is added to the FROM clause of the SQL statement. The TABLESAMPLE clause includes the approximate percentage of that table to sample, called the sampling percentage or sampling rate. For example, if the sampling rate is set to 0.1, then only 1/10 of a one percent of data will be sampled. This would mean that in a 10,000-row table, only 10 rows would be accessed ? this may or may not be appropriate for your business.

The TABLESAMPLE clause includes either the BERNOULLI or SYSTEM specification. BERNOULLI sampling examines every row to determine whether or not it is included in the answer set, while SYSTEM sampling does the sampling on a page-by-page basis. While SYSTEM sampling will be generally faster, it may result in skewed results if your calculations are based on a column that is also used for clustering the table, or for multi-dimensional organization of the data. In this case, BERNOULLI sampling may be more appropriate.

Hope this answers ur Query....
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rajesh_1183

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PostPosted: Wed Jan 04, 2006 2:59 pm    Post subject:
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Thanks nikyojin
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