Re: help! cannot understand "leading subset of columns" in index usage

strepxe_at_yahoo.co.uk wrote:
> hi,
> i am preparing for my first ocp exam and am having a little difficulty
> understanding the concept of indexes. the book says:
>
> "b-tree indexes can be used if any combination of leading columns of
> the index is used in the SQL statement"
>
> it then gives an example of table t1 with index inv_pk on columns c1
> and c2.
>
> table t1 has columns a,b,c1,c2.
>
> it then says the statement: "select count(a) from t1 where c1 = 1;"
> will used the index as it uses a leading subset of columns as will
> "select count(c1) from t1 where c2 = 2;".
>
> however "select count(a) from t1 where c2 = 1;" will not work as it
> does not use a leading subset.
>
> am i correct in saying from this that the use of the b-tree index will
> depend purely on the order in which the columns were specified when the
> index was created? if i am correct on this can anyone explain why this
> is?

You would be correct but I think the premise of the question is not. At least not with any currently supported version of Oracle.

Perhaps I am misreading this but I think the "offical" answer to the question is incorrect: No surprise.

-- 
Daniel A. Morgan
http://www.psoug.org
damorgan_at_x.washington.edu
(replace x with u to respond)