Yes, it looks like..

CREATE INDEX name ON table (Col1,Col2);

You could also do something like:

CREATE INDEX name ON table (Col1||Col2);
(same as last_name+first_name sort of)

>1) is it more efficient to use a composite index or a separate index on each field

Depends on the SQL statement you are using.

But seperate a key concept(s) in indexes are
- to keep them as compact as possible (less rows need to be scanned off disk and less memory needs to be used.)
- if the index does not have many different values i.e. only 0 or 1 use a bitmap index.

If you do statements like..
update table set col1=2167,col2=212 where col1=1 and col2=2;
Then use seperate indexes..

If you do something like...
update table set col1=2167,col2=212 where col1||col2=1||2;
Use the composite method.

Let me just say the the || is highly ineffiecient I can't see why you would ever need to use and many DBA's would crap in their pants if they saw this coding style. Maybe a view would be appropriate. That being said I only index on key fields or fields which are more or less unique and are used extensively in where clauses.



>2) if the composite index is more efficient, can you create a composite index on numeric fields w/o converting them to strings first?

I don't think so you need to convert to strings.

As I said depends on what you are trying to do.

HTH