When should I use index on table? From how many rows index makes sense? If I have table with constant rows, just edited come columns (not in 'where' clause), makes index sense even if table has just about 15 rows? EDIT: Can be in such case non-index selecting/reading more effective than index read? EDIT: Now I am working with firebird 2.5, but most of the time I am using SQL Server 2005/2008.

How to decide when use index on table column

When should I use index on table?

From how many rows index makes sense?
If I have table with constant rows, just edited come columns (not in 'where' clause), makes index sense even if table has just about 15 rows? EDIT: Can be in such case non-index selecting/reading more effective than index read?

EDIT: Now I am working with firebird 2.5, but most of the time I am using SQL Server 2005/2008.