フリー問題

問題文

A shop rebuilds every index once a month in the belief that this keeps the extra fetches down. The count of extra fetches on the tables keeps rising anyway. Why does the practice not help?

選択肢

1. The extra fetches come from the blocks that the recycle pool discards early so the rows have to be read again on the next access.
2. The extra fetches come from the rows that outgrew their block inside the table so any amount of work on the indexes leaves the real cause of those fetches untouched.
3. The extra fetches come from the leaf blocks that a rebuild leaves half empty so a rebuild that runs every month makes the situation slightly worse each time.
4. The extra fetches come from the statistics that grew stale between the rebuilds so the optimizer keeps choosing an access path that reads more blocks than it needs.