Thread: what is "cost"?

what is "cost"?

From
Gregory Holston
Date:
Hi All,

I just did an explain and was wondering what is meant by cost.  I just 
got a 72.983 cost from a query I found not working.  Any info or pointers 
would be appreciated.

Thanks,

-Greg

Running:Apache/1.3.1 (Unix) PHP/3.0.3 Postgres 6.2


Re: [SQL] what is "cost"?

From
Bruce Momjian
Date:
> 
> Hi All,
> 
> I just did an explain and was wondering what is meant by cost.  I just 
> got a 72.983 cost from a query I found not working.  Any info or pointers 
> would be appreciated.

Just added to explain manual page:
The cost value is only meaningful to the optimizer in comparing variousquery plans.

--  Bruce Momjian                        |  http://www.op.net/~candle maillist@candle.pha.pa.us            |  (610)
853-3000+  If your life is a hard drive,     |  830 Blythe Avenue +  Christ can be your backup.        |  Drexel Hill,
Pennsylvania19026
 


Re: [SQL] what is "cost"?

From
Gregory Holston
Date:

On Fri, 23 Apr 1999, Bruce Momjian wrote:

> > 
> > Hi All,
> > 
> > I just did an explain and was wondering what is meant by cost.  I just 
> > got a 72.983 cost from a query I found not working.  Any info or pointers 
> > would be appreciated.
> 
> Just added to explain manual page:
> 
>     The cost value is only meaningful to the optimizer in comparing various
>     query plans.
> 
I got the 72.98 on this table while doing a simple "select * from 
publication" meanwhile I got nothing higher than 4 on any other table. Is this 
indicative of a problem?

-Greg


Re: [SQL] what is "cost"?

From
Bruce Momjian
Date:
> >     The cost value is only meaningful to the optimizer in comparing various
> >     query plans.
> > 
> I got the 72.98 on this table while doing a simple "select * from 
> publication" meanwhile I got nothing higher than 4 on any other table. Is this 
> indicative of a problem?

No.  It means the optimizer thinks it is going to have to do more work
to complete the query.  All other methods it used to complete the query
had a higher cost.  It is related to the number of rows in the table,
the indexes available, sorts needed, joins, etc.

--  Bruce Momjian                        |  http://www.op.net/~candle maillist@candle.pha.pa.us            |  (610)
853-3000+  If your life is a hard drive,     |  830 Blythe Avenue +  Christ can be your backup.        |  Drexel Hill,
Pennsylvania19026