> 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

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