Re: GIN improvements part 1: additional information

On 01/13/2014 07:07 PM, Alexander Korotkov wrote:

I've fixed this bug and many other bug. Now patch passes test suite that
I've used earlier. The results are so:

Operations time:
          event         |     period
-----------------------+-----------------
  index_build           | 00:01:47.53915
  index_build_recovery  | 00:00:04
  index_update          | 00:05:24.388163
  index_update_recovery | 00:00:53
  search_new            | 00:24:02.289384
  search_updated        | 00:27:09.193343
(6 rows)

Index sizes:
      label     |   size
---------------+-----------
  new           | 384761856
  after_updates | 667942912
(2 rows)

Also, I made following changes in algorithms:

    - Now, there is a limit to number of uncompressed TIDs in the page.

    After reaching this limit, they are encoded independent on if they can fit
    page. That seems to me more desirable behaviour and somehow it accelerates
    search speed. Before this change times were following:

          event         |     period
-----------------------+-----------------
  index_build           | 00:01:51.467888
  index_build_recovery  | 00:00:04
  index_update          | 00:05:03.315155
  index_update_recovery | 00:00:51
  search_new            | 00:24:43.194882
  search_updated        | 00:28:36.316784
(6 rows)

Hmm, that's strange. Any idea why that's happening? One theory is that when you re-encode the pages more aggressively, there are fewer pages with a mix of packed and unpacked items. Mixed pages are somewhat slower to scan than fully packed or fully unpacked pages, because GinDataLeafPageGetItems() has to merge the packed and unpacked items into a single list. But I wouldn't expect that effect to be large enough to explain the results you got.

    - Page are not fully re-encoded if it's enough to re-encode just last
    segment.

Great! We should also take advantage of that in the WAL record that's written; no point in WAL-logging all the segments, if we know that only last one was modified.