index with multiple columns - ok when doing query on only one column?

Question

If I have an table

create table sv ( id integer, data text )

and an index:

create index myindex_idx on sv (id,text)

would this still be usefull if I did a query

select * from sv where id = 10

My reason for asking is that i'm looking through a set of tables with out any indexes, and seeing different combinations of select queries. Some uses just one column other has more than one. Do I need to have indexes for both sets or is an all-inclusive-index ok? I am adding the indexes for faster lookups than full table scans.

Example (based on the answer by Matt Huggins):

select * from table where col1 = 10
select * from table where col1 = 10 and col2=12
select * from table where col1 = 10 and col2=12 and col3 = 16

could all be covered by index table (co1l1,col2,col3) but

select * from table where col2=12

would need another index?

Answer 1

It should be useful since an index on (id, text) first indexes by id, then text respectively.

• If you query by id, this index will be used.
• If you query by id & text, this index will be used.
• If you query by text, this index will NOT be used.

Edit: when I say it's "useful", I mean it's useful in terms of query speed/optimization. As Sune Rievers pointed out, it will not mean you will get a unique record given just ID (unless you specify ID as unique in your table definition).