Re: Releasing memory during External sorting?
От | Mark Lewis |
---|---|
Тема | Re: Releasing memory during External sorting? |
Дата | |
Msg-id | 1127497383.23567.218.camel@archimedes обсуждение исходный текст |
Ответ на | Re: Releasing memory during External sorting? (Tom Lane <tgl@sss.pgh.pa.us>) |
Ответы |
Re: Releasing memory during External sorting?
|
Список | pgsql-performance |
operations != passes. If you were clever, you could probably write a modified bubble-sort algorithm that only made 2 passes. A pass is a disk scan, operations are then performed (hopefully in memory) on what you read from the disk. So there's no theoretical log N lower-bound on the number of disk passes. Not that I have anything else useful to add to this discussion, just a tidbit I remembered from my CS classes back in college :) -- Mark On Fri, 2005-09-23 at 13:17 -0400, Tom Lane wrote: > Ron Peacetree <rjpeace@earthlink.net> writes: > > 2= No optimal external sorting algorithm should use more than 2 passes. > > 3= Optimal external sorting algorithms should use 1 pass if at all possible. > > A comparison-based sort must use at least N log N operations, so it > would appear to me that if you haven't got approximately log N passes > then your algorithm doesn't work. > > regards, tom lane > > ---------------------------(end of broadcast)--------------------------- > TIP 9: In versions below 8.0, the planner will ignore your desire to > choose an index scan if your joining column's datatypes do not > match
В списке pgsql-performance по дате отправления: