Обсуждение: what is "cost"?
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
> > 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
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
> > 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