RE: serial and sequential scan

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04-20-2008, 07:40 AM

Willem Roos

Posts: n/a

RE: serial and sequential scan

How many records in your table? If there are only a few a sequential
scan is better. You can easily calculate it by getting the table and
index row sizes & calculating how many records fit on a page. Now figure
out how many pages a sequential scan will touch & how many an index
lookup.

> -----Original Message-----
> From: owner-informix-list@iiug.org
> [mailto

wner-informix-list@iiug.org] On Behalf Of hajek@nspuh.cz
> Sent: 15 February 2005 12:24
> To: informix-list@iiug.org
> Subject: serial and sequential scan
>
> Hi,
> is this normal behaviour (IDS 7.31, hp-ux 10.20)?
> from dbschema:
> create table ano_k_out_hem
> (
> id_pozad serial not null ,
> num_vysl decimal(14,6),
> text_vysl char(30),
> .......
> primary key (id_pozad) constraint u3332_3326
> );
>
> from sqexplain.log:
> QUERY:
> ------
> update ano_k_out_hem set status_apl = "Z" where id_pozad = 2610405
>
> Estimated Cost: 3
> Estimated # of Rows Returned: 1
>
> 1) ano_k_out_hem: SEQUENTIAL SCAN
>
> Filters: amis.ano_k_out_hem.id_pozad = 2610405
>
> from dbaccess table info:
> Index name Owner Type Cluster Columns
> 1006_3439 amis unique No id_pozad
>
>
> So - why the sequential scan? (Update statistics (medium) is being
> executed regularly)
>
> Thanks, Michal
>
>

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RE: serial and sequential scan