From: Andrei Gaspar <andi@softnrg.dnttm.ro>
To: Dave Held <dave.held@arrayservicesgrp.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Sort and index
Date: Mon, 18 Apr 2005 19:11:38 +0300
Message-ID: <4263DC3A.5070509@softnrg.dnttm.ro> (raw)
In-Reply-To: <49E94D0CFCD4DB43AFBA928DDD20C8F9026184B6@asg002.asg.local>
References: <49E94D0CFCD4DB43AFBA928DDD20C8F9026184B6@asg002.asg.local>
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
>
>---------------------------(end of broadcast)---------------------------
>TIP 4: Don't 'kill -9' the postmaster
>
>
>
>
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To: pgsql-performance@postgresql.org
Cc: andi@softnrg.dnttm.ro, dave.held@arrayservicesgrp.com
Subject: Re: Sort and index
In-Reply-To: <4263DC3A.5070509@softnrg.dnttm.ro>
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