Re: [SQL] Questions about vacuum analyze

> I didn't know much about it either until the comments in psort.c led me
> to read the relevant parts of Knuth.  Tape rewind and disk seek times

Yes, that's what I did too.

> are not remotely comparable, especially when you're talking about N tape
> units versus one disk unit --- using more files (tapes) to minimize
> rewind time can be a big win on tapes, but put those same files on a
> disk and you're just spreading the seek time around differently.  More
> to the point, though, tape-oriented algorithms tend to assume that tape
> space is free.  Polyphase merge doesn't care in the least that it has
> several copies of any given record laying about on different tapes, only
> one of which is of interest anymore.
> 
> I know how to get the sort space requirement down to 2x the actual data
> volume (just use a balanced merge instead of the "smarter" polyphase)
> but I am thinking about ways to get it down to data volume + a few
> percent with an extra layer of bookkeeping.  The trick is to release
> and recycle space as soon as we have read in the tuples stored in it...

The thing I liked about the existing algorithm is that it did the
sorting without the page faulting/thrashing caused by many sort
algorithms.  I can see better space reuse as being a big win because we
would get even less page faulting because the replaced page would
already be in memory.

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