Question

Is there any reason to add indices to table with few (less than 10) elements?
It is frequently used in joins, but I’m not sure if index on such small table increase its performance or decrease.

Edit: rows in table are quite small: 7 columns, with total size smaller than 500 bytes, so I think that it could be stored in server memory (it is answer to Paul Sanwald note)

Answer 1

It is unlikely that a table that size will ever use the index. If the table will be stable in size and always be small, I would not add indexes.

From Books Online:

Indexing small tables may not be
optimal because it can take the query
optimizer longer to traverse the index
searching for data than to perform a
simple table scan. Therefore, indexes
on small tables might never be used,
but must still be maintained as data
in the table changes.

So it likely to be a poor idea to index small tables.