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Re: When does Oracle use 'Index Fast Scan'

From: David Hau <>
Date: Mon, 26 Jan 2004 19:29:26 -0800
Message-ID: <>

I assume you're talking about the Fast Full Index Scan. This is used when the index contains all the columns necessary to answer the query.

It's faster than a Full Table Scan because indexes are smaller than entire rows, so a Fast Full Index Scan will scan fewer blocks than a Full Table Scan.

It's faster than an Index Range Scan firstly because Fast Full Index Scan scans the blocks in sequential order, whereas the Index Range Scan traverses the B-tree index structure in scanning the blocks, resulting in a random access I/O pattern which is slower. This is also why the Oracle documentation says that with a Fast Full Index Scan, the result is not sorted by the index key (because the result is not obtained by traversing the index structure.) Secondly, the better performance is also because the Fast Full Index Scan uses multiblock reads and is capable of parallel operation, whereas the Index Range Scan is capable of neither.

Dave. wrote:

> I have found that the vast majority of time that Oracle chooses this
> method, my statistics are stale and the query is sub-optimal. One
> time, Oracle changed from a 'range scan' to this type of scan with a
> FIRST_ROWS hint and this reduced performance.
> This is just a full scan of the index, one block at a time right? When
> would this ever be superior to a Fast Full Scan or a Range Scan?

Please see the official ORACLE-L FAQ:
Author: David Hau

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Received on Mon Jan 26 2004 - 21:29:26 CST

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