Thread: quickest query to check a table for existance of a value in a field

Hi

I have a reasonable sized table (~2m rows), with an indexed field for which 
there are ~200 possible values.

I want a quick way to find out if there are any rows matching a specified 
value for this indexed field.  I dont care how many there are or what their 
properties are, just if there are any or not.

Here's a quick example:

CREATE TABLE mytable(c1 INT4 PRIMARY KEY, c2 INT2);
CREATE INDEX myindex ON mytable(c2);

SELECT COUNT(c2) AS num FROM mytable WHERE c2=1234;

The time it takes to execute the above query appears to be of the order O(n) 
- is there an O(1) query to do it - given that I only want to know if num>0 
or if num=0?  (the function COUNT() doesn't have to be used).

Any help appreciated,

Thanks

--
Shane

> I want a quick way to find out if there are any rows matching a
> specified value for this indexed field.  I dont care how many there
> are or what their properties are, just if there are any or not.
> 
> SELECT COUNT(c2) AS num FROM mytable WHERE c2=1234;
>
> Thanks
> Shane


Try: select 1 as yes from mytable where c2=1234 limit 1;or: select 1 as yes where exists( select 1 from mytable where
c2=1234
);

would be interesting to see differences in performance on your data


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Hi

Thanks, just tried this on my dev box (650,000 rows, ~50,000 per ID), the 
original way (COUNT()) takes ~1/2 second (guestimating), while your way is as 
instant as I can tel.. 

Beauty :)

--
Shane

On Monday 25 Mar 2002 9:41 pm, chester c young wrote:
> > I want a quick way to find out if there are any rows matching a
> > specified value for this indexed field.  I dont care how many there
> > are or what their properties are, just if there are any or not.
> >
> > SELECT COUNT(c2) AS num FROM mytable WHERE c2=1234;
> >
> > Thanks
> > Shane
>
> Try: select 1 as yes from mytable where c2=1234 limit 1;
>  or: select 1 as yes where exists( select 1 from mytable where c2=1234
> );
>
> would be interesting to see differences in performance on your data
>
>
> __________________________________________________
> Do You Yahoo!?
> Yahoo! Movies - coverage of the 74th Academy Awards®
> http://movies.yahoo.com/
>
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