Palle,
> Is it always bad to create index xx on yy (field1, field2, field3);
No, it seldom bad, in fact. I have some indexes that run up to seven
columns, becuase they are required for unique keys.
Indexes of 3-4 columns are often *required* for many-to-many join tables.
I'm afraid that you've been given some misleading advice.
> I guess the problem is that the index might often grow bigger than the
> table, or at least big enough not to speed up the queries?
Well, yes ... a 4-column index on a 5-column table could be bigger than the
table if allowed to bloat and not re-indexed. But that's just a reason for
better maintainence.
--
-Josh Berkus
Aglio Database Solutions
San Francisco