Curt Sampson wrote: > > On Thu, 18 Apr 2002, Michael Loftis wrote: > > > mlw wrote: > > > > >The supposed advantage of a sequential read over an random read, in > > >an active multitasking system, is a myth. If you are executing one > > >query and the system is doing only that query, you may be right. > > > > > >Execute a number of queries at the same time, the expected benefit > > >of a sequential scan goes out the window. The OS will be fetching > > >blocks, more or less, at random. > > On a system that has neither read-ahead nor sorting of I/O requests, > yes. Which systems are you using that provide neither of these > facilities?
This only happens if the OS can organize the I/O requests in such a manner. It is a non-trivial function. > > > In a multi-tasking system it's always cheaper to fetch less blocks, no > > matter where they are. Because, as you said, it will end up more or > > less random onf a system experiencing a larger number of queries. > > Invariably a process or thread will lose its quantum when it submits > an I/O request. (There's nothing left for it to do, since it's waiting > for its data to be read, so there's nothing for it to execute.) This statement is verifiably false. What a program does after it submits an I/O requests is VERY OS and state specific. If an I/O request is made for a disk block, which is in read-ahead cache, a number of operating systems my return right away. ---------------------------(end of broadcast)--------------------------- TIP 6: Have you searched our list archives? http://archives.postgresql.org
