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Interpreting Explain output in Plan_table

 
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vijikesavan

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Joined: 04 Oct 2006
Posts: 118
Location: NJ, USA

PostPosted: Thu Oct 12, 2006 3:33 am    Post subject: Interpreting Explain output in Plan_table
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Hi,
I have run a explain to optimize a query. Found the value of a column prefetch as 'D' . Not sure wht this means?
Any pointers would help.

Thanks,
Viji
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arka_nandi

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Joined: 15 Dec 2005
Posts: 2

PostPosted: Mon Oct 16, 2006 10:15 am    Post subject: Plan table - Prefetch
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Below are posible values for the prefetch column in the plan table and their explanation -

S = pure sequential prefetch;
L = prefetch through a page list;
D = optimizer expects dynamic prefetch;
blank = unknown at bind time or no prefetch.

Hope this helps.

- Arka.
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mkarmegam

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Joined: 28 May 2006
Posts: 13
Location: USA

PostPosted: Thu Oct 19, 2006 3:25 am    Post subject: Re: Interpreting Explain output in Plan_table
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More details...
| 5.11.5.2 Dynamic prefetch (PREFETCH=D)


| Dynamic prefetch can reduce paging and improve performance over sequential
| prefetch for some data access that involves data that is not on
| consecutive pages. When DB2 expects that dynamic prefetch will be used,
| DB2 sets PREFETCH=D. At runtime, dynamic prefetch might or might not
| actually be used. However, DB2 expects dynamic prefetch and optimizes for
| that behavior.


| When dynamic prefetch is used: Dynamic prefetch is used in prefetch
| situations when the pages that DB2 will access are distributed in a
| nonconsecutive manner. If the pages are distributed in a sufficiently
| consecutive manner, sequential prefetch is used instead.
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