need index?

[fengguangjin]

The fields under ORDER BY clause need a index to speed up query? Also how about GROUP BY clause?
Next question is, if a field, say 'tankno', is used in a composite index, but in second SQL's WHERE clause only this field tankno is used, a seperate index should be built to speed up the second query?

Thanks?
Victor

 

[Marco Kalter]

For order by / group by operations on a large table an index will help. If a column is included in a composite index, and not on the first position, an extra index can be useful.