We should collect statistics on following cases..
- When statistics are stale, you can help stats on table to find last stats collection date.
- On all Non-Unique indices·
- On Non-index join columns·
- On Primary Index of small tables·
- On Primary Index of a Join Index·
- On Secondary Indices defined on any join index·
- When Join index columns that frequently appear on any additional join index columns that frequently appear in WHERE search conditions·
- On Columns that frequently appear in WHERE search conditions or in the ON clause of joins