covering index query

hi all,
can anybody help me out with a covering index query and
say how exactly it helps out in data retreival?
thanks,
james bondFrom the SQL 2000 Books Online:
<Excerpt href="http://links.10026.com/?link=createdb.chm::/cm_8_des_05_2ri0.htm">
Covered queries can improve performance. Covered queries are queries where
all the columns specified in the query are contained within the same index.
For example, a query retrieving columns a and b from a table that has a
composite index created on columns a, b, and c is considered covered.
Creating indexes that cover a query can improve performance because all the
data for the query is contained within the index itself; only the index
pages, not the data pages, of the table must be referenced to retrieve the
data, thereby reducing overall I/O. Although adding columns to an index to
cover queries can improve performance, maintaining the extra columns in the
index incurs update and storage costs.
</Excerpt>
--
Hope this helps.
Dan Guzman
SQL Server MVP
"jamesbond" <anonymous@.discussions.microsoft.com> wrote in message
news:063201c3b57d$0b2e0290$a301280a@.phx.gbl...
> hi all,
> can anybody help me out with a covering index query and
> say how exactly it helps out in data retreival?
> thanks,
> james bond