Re: Sort and index

Thanks for the quick response
Andrei

Dave Held wrote:

>>-----Original Message-----
>>From: Andrei Gaspar [mailto:andi@softnrg.dnttm.ro]
>>Sent: Monday, April 18, 2005 10:36 AM
>>To: pgsql-performance@postgresql.org
>>Subject: [PERFORM] Sort and index
>>
>>I thought that an index can be used for sorting.
>>I'm a little confused about the following result:
>>
>>create index OperationsName on Operations(cOperationName);
>>explain SELECT * FROM Operations ORDER BY cOperationName;
>>                              QUERY PLAN
>>--------------------------------------------------------------
>>---------
>> Sort  (cost=185.37..189.20 rows=1532 width=498)
>>   Sort Key: coperationname
>>   ->  Seq Scan on operations  (cost=0.00..104.32 rows=1532 width=498)
>>(3 rows)
>>
>>Is this supposed to be so?
>>
>>
>
>Since you are fetching the entire table, you are touching all the rows.
>If the query were to fetch the rows in index order, it would be seeking
>all over the table's tracks.  By fetching in sequence order, it has a
>much better chance of fetching rows in a way that minimizes head seeks.
>Since disk I/O is generally 10-100x slower than RAM, the in-memory sort
>can be surprisingly slow and still beat indexed disk access.  Of course,
>this is only true if the table can fit and be sorted entirely in memory
>(which, with 1500 rows, probably can).
>
>__
>David B. Held
>Software Engineer/Array Services Group
>200 14th Ave. East,  Sartell, MN 56377
>320.534.3637 320.253.7800 800.752.8129
>
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>
>
>
>


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