From: Peter Brawley
Date: January 14 2011 1:02pm
Subject: Re: Rewrite SQL to stop table scan
List-Archive: http://lists.mysql.com/mysql/224058
Message-Id: <4D304980.3030203@earthlink.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

On 1/14/2011 4:52 AM, Bruce Ferrell wrote:
> How would you rewrite the following SQL so that is doesn't do a full
> table scan.   It does in fact do a full scan in spite of the time
> clause.  It's been
> making me nuts for months.
>
> select count(*) as count
> from alerts where (unix_timestamp(stamp)>  (unix_timestamp(now()) -
> '300' ) )
> and devid = '244';
How about indexing on stamp, devid, then writing ... where stamp > 
current_timestamp ...?

PB

------

> Thanks in advance,
>
> Bruce
>
> P.S.
>
> I've tried it this way:
>
> select count(*) as count  from alerts where ((unix_timestamp(stamp)>
> (unix_timestamp(now()) - '300' ) )) and devid = '244';
>
> and explain always says this:
>
> +--+---------------+------+---------+------+---------+-------------+
> | id | select_type | table  | type | possible_keys | key  | key_len |
> ref  | rows    | Extra       |
> +----+-------------+--------+------+---------------+------+---------+------+---------+-------------+
> |  1 | SIMPLE      | alerts | ALL  | NULL          | NULL | NULL
> |NULL | 2041284 | Using where |
> +----+-------------+--------+------+---------------+------+---------+------+---------+-------------+
>
> And it's structured this way:
>
> | Field                    | Type    | Null  |  Key | Default
>               | Extra
> | id                        | varchar(60)    | NO   | MUL |
>              |
> | stamp                  | timestamp     | NO   | MUL |
> CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
>
>
>