Chirag (10/31/2008)
Say there is a big data modification (update statement) going on and not all pages affected by the update stmt are in the buffer/memory. Will the pages not in the memory be first fetched into the memory and then changed ? Or will the change be in the log and then written to the disk without the pages being brought to the memory?
The query processor can only operate on pages in memory. If a query (read or change) requires a page that isn't in memory then the query processor requests that page and puts the query that needs it into a waiting state. The storage engine will go and fetch that page (issuing the appropriate IO requests to the OS), put it into memory and then signal the query processor that the page is there. The query that needed it will then be made runnable and can continue processing.
Bit of an over-simplification, but serves to explain.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability