Re: [HACKERS] A Better External Sort?

Поиск
Список
Период
Сортировка
От Zeugswetter Andreas DAZ SD
Тема Re: [HACKERS] A Better External Sort?
Дата
Msg-id E1539E0ED7043848906A8FF995BDA5797EFA40@m0143.s-mxs.net
обсуждение исходный текст
Ответ на [HACKERS] A Better External Sort?  (Ron Peacetree <rjpeace@earthlink.net>)
Ответы Re: [HACKERS] A Better External Sort?  ("Jim C. Nasby" <jnasby@pervasive.com>)
Список pgsql-performance
> In my original example, a sequential scan of the 1TB of 2KB
> or 4KB records, => 250M or 500M records of data, being sorted
> on a binary value key will take ~1000x more time than reading
> in the ~1GB Btree I described that used a Key+RID (plus node
> pointers) representation of the data.

Imho you seem to ignore the final step your algorithm needs of
collecting the
data rows. After you sorted the keys the collect step will effectively
access the
tuples in random order (given a sufficiently large key range).

This random access is bad. It effectively allows a competing algorithm
to read the
whole data at least 40 times sequentially, or write the set 20 times
sequentially.
(Those are the random/sequential ratios of modern discs)

Andreas

В списке pgsql-performance по дате отправления:

Предыдущее
От: "R, Rajesh (STSD)"
Дата:
Сообщение: Query in SQL statement
Следующее
От: Richard Huxton
Дата:
Сообщение: Re: Which one FreeBSD or Linux