Index Key Column Order

Question

I've several queries running on a table (for fetching data), to improve the performance of these queries, we've an Index in place, however, the query optimizer suggests upto 94% cost reduction possibility because of the order of the Index key columns in
40% of the queries.
I know that if I create an Index that matches the order of columns, the result will be better. However, I would like to know is there a possibility of changing order of columns in query itself so that it can use the existing Index? (I tried it but didn't
make any difference).
Also, I want to know how does SQL Server decides a key column order for any query?

Answer 1

Reordering the predicates in your query will not help.
If your query has exact matches on all columns in the index, then the order of the columns in the index key doesn't matter.
For example:
CREATE TABLE t
(col1 char(5)
,col2 char(5)
,description varchar(200)
CREATE INDEX ix ON t (col1, col2)
SELECT description
FROM t
WHERE col1 = 'abc'
AND col2 = 'def'
However, if not all index key columns are filtered, or they are not filtered with equality, then the order does matter.
For example:
SELECT description
FROM t
WHERE col1 LIKE 'a%'
AND col2 = 'def'
In the query above, an index on (col2, col1) is more effective than an index on (col1, col2). If col1 is the leading column, the first match (and every next match) of "col2 = 'def'" has to be scanned within the results of "col1 LIKE 'a%'".
If  col2 is the leading column, then no col2 keys with the unmatched values have to be skipped. In other words, fewer keys have to be scanned.
Maybe your query selects a range of values of the index's leading column...
Gert-Jan