RE: A question about performance

Tiffany,

A good rule of thumb is that if a query returns >5-20% of the rows in a table, a full table scan will be faster.

If a query returns between 5 and 20%, test each execution plan to see which is faster.

You will have to determine how many of your rows have subscr_id > 0 to know which plan may be faster.

Try this:

SELECT
        (taba.row2 / tab.COUNT(1)) x 100 "PERCENT_SELECTED" FROM

	SUBSCRIBER tab,
	(SELECT COUNT(1) row2 FROM subscriber WHERE subscr_id > 0) taba;

The definitive test would be to run each statement and trace it using TKPROF to see which is more economical.

A quick and dirty way would be to run each in SQL*Plus with SET TIMING ON first. Whichever is faster wins.

Jack
ocp x 2
Senior @ xol.com

-----Original Message-----
From: Yosi Greenfield [mailto:yosi_at_comhill.com] Sent: Wednesday, June 21, 2000 10:59 AM
To: Multiple recipients of list ORACLE-L Subject: Re: A question about performance

Hi,

The optimizer will probably ignore the index and do a full table scan anyway.
If not, it would have to do at least two reads (index and data) and still return every row of the table. In any case, leave out the where clause - it could only hurt you.

hth

yosi

TiffanyDu_at_pcdc.com.tw wrote:

> Hi All:
> There is a question about performance.
> Table name : subscriber , primary key : subscr_id , with
> over 5000000 records .
>
> CASE 1 : select *
> from subscriber ;
> (FULL TABLE SCAN)
>
> CASE 2 : select *
> from subscriber
> where subscr_id > 0 ;
> (USE INDEX)
>
> Could anyone tell me which ones performance is better , or
> they are the same ??
>
> Thank you in advance.
>
>
> Tiffany
> E-mail tiffanydu_at_pcdc.com.tw
>
>
> --
> Author: =?big5?B?VGlmZmFueSBEdSAoIKf5p0qq4iAp?=
> INET: TiffanyDu_at_pcdc.com.tw
>
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-- 
Author: Yosi Greenfield
  INET: yosi_at_comhill.com

Fat City Network Services    -- (858) 538-5051  FAX: (858) 538-5051
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