Re: When does Oracle use 'Index Fast Scan'

If you look up "Logical I/O" in the Master Glossary of the Oracle docs, it's defined as:

"A block read which may or may not be satisfied from the buffer cache."

So a logical I/O is always a *block* read. It does not take into acount whether you're doing a multiblock read or not. One reason this is so is because the CBO does not know whether the block is already in the buffer cache or not. If it's already in the cache, then multiblock read is not an issue anymore.

The init parameter db_file_multiblock_read_count is what makes the CBO favor fast full index scan over a non-fast scan like full index scan or index range scan. The LIO itself does not take into consideration the aspect of multiblock read.

Regards,
Dave

ryan.gaffuri_at_cox.net wrote:

>i thought an index_fs only read 1 block per i/o? same with an index range
>scan because they are using random access?
>----- Original Message -----
>To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com>
>Sent: Tuesday, January 27, 2004 5:29 PM
>
>
>
>
>>An index fast full scan and an index full scan both need to access all
>>the blocks of an index. The only difference between them is that the
>>index_ffs accesses the blocks in the order of the blocks (and uses
>>multiblock read), whereas the index_fs accesses the blocks in the order
>>of the b tree index. In terms of # logical I/Os, they are exactly the
>>
>>
>same.
>
>
>>OTOH, an index range scan by definition is a _range_ scan, and need to
>>access only a subset of the blocks of an index. Because of this, it'll
>>have a lower # logical I/Os than an index_ffs.
>>
>>Regards,
>>Dave
>>
>>
>>ryan.gaffuri_at_cox.net wrote:
>>
>>
>>
>>>ive found that index_ffs typically incur higher logical I/Os that index
>>>
>>>
>range scans. so its not just access speeds.
>
>
>>>
>>>
>>>>From: David Hau <davehau123_at_netscape.net>
>>>>Date: 2004/01/27 Tue AM 11:54:26 EST
>>>>To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
>>>>Subject: Re: When does Oracle use 'Index Fast Scan'
>>>>
>>>>This is where the access time of your disks (or SAN) makes a difference.
>>>> If your disks have really fast access time, then a random-access
>>>>pattern would not cause much performance degradation and so a range scan
>>>>would not be slow at all, even though it's traversing the b-tree index
>>>>structure. If you're only striping together disks with relatively slow
>>>>access time (e.g. using a striped IDE disk array), then you have high
>>>>throughput but not that fast an access time. In this case, fast full
>>>>index scan would be much faster than an index range scan because the
>>>>fast full scan reads the blocks sequentially and a sequential disk I/O
>>>>requires only positioning the head once (assuming the disk is not
>>>>fragmented). The rest of the time depends on the throughput. If you
>>>>stripe together a large enough number of IDE disks, then your throughput
>>>>is great but your access time is still the access time of a single IDE
>>>>drive which is not that fast.
>>>>
>>>>This is assuming you need to do a physical I/O to obtain the blocks. Of
>>>>course, if the blocks already reside in the buffer cache, then it's a
>>>>different story.
>>>>
>>>>Regards,
>>>>Dave
>>>>
>>>>
>>>>ryan.gaffuri_at_cox.net wrote:
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>>btw, in many cases range scan is faster than a fast full scan. Range
>>>>>
>>>>>
>scan recursively hits the nodes that are needed and skips the ones that are
>not. So it reads less blocks.
>
>
>>>>>So if you are looking for a 'range' or a specific value, range scan
>>>>>
>>>>>
>beats fast full scan most of the time. Less Logical and Physical I/Os.
>
>
>>>>>test it and hint your queries
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>From: David Hau <davehau123_at_netscape.net>
>>>>>>Date: 2004/01/26 Mon PM 10:34:25 EST
>>>>>>To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
>>>>>>Subject: Re: When does Oracle use 'Index Fast Scan'
>>>>>>
>>>>>>Correction: the Index Range Scan can be parallelized when it involves
>>>>>>multiple partitions.
>>>>>>
>>>>>>- Dave
>>>>>>
>>>>>>
>>>>>>David Hau wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>>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.
>>>>>>>
>>>>>>>Regards,
>>>>>>>Dave.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>ryan.gaffuri_at_cox.net 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: http://www.orafaq.net
>>>>>>--
>>>>>>Author: David Hau
>>>>>>INET: davehau123_at_netscape.net
>>>>>>
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>>>>>>
>>>>>>
>>>>>
>>>>>
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>>>>Please see the official ORACLE-L FAQ: http://www.orafaq.net
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>>>>Author: David Hau
>>>> INET: davehau123_at_netscape.net
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>>>
>>--
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>>Author: David Hau
>> INET: davehau123_at_netscape.net
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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: David Hau
  INET: davehau123_at_netscape.net

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