On 01/05/2016 10:38 AM, Tomas Vondra wrote:


There shouldn't be a difference between the two approaches (although I
guess there could be if one left a larger pending list than the other,
as pending lists is very space inefficient), but since you included
9.5 in your test I thought it would be interesting to see how either
patched version under 9.6 compared to 9.5.

Well, turns out there's a quite significant difference, actually. The
index sizes I get (quite stable after multiple runs):

    9.5 : 2428 MB
    9.6 + alone cleanup : 730 MB
    9.6 + pending lock : 488 MB

So that's quite a significant difference, I guess. The load duration for
each version look like this:

    9.5                 : 1415 seconds
    9.6 + alone cleanup : 1310 seconds
    9.6 + pending lock  : 1380 seconds

I'd say I'm happy with sacrificing ~5% of time in exchange for ~35%
reduction of index size.

The size of the index on 9.5 after VACUUM FULL (so pretty much the
smallest index possible) is 440MB, which suggests the "pending lock"
patch does a quite good job.

The gin_metapage_info at the end of one of the runs (pretty much all the
runs look exactly the same) looks like this:

                   pending lock   alone cleanup      9.5
  pending_head                2               2   310460
  pending_tail              338             345   310806
  tail_free_size            812             812      812
  n_pending_pages           330             339      347
  n_pending_tuples         1003            1037     1059
  n_total_pages               2               2        2
  n_entry_pages               1               1        1
  n_data_pages                0               0        0
  n_entries                   0               0        0
  version                     2               2        2

So almost no difference, except for the pending_* attributes, and even
in that case the values are only different for 9.5 branch. Not sure what
conclusion to draw from this - maybe it's necessary to collect the
function input while the load is running (but that'd be tricky to
process, I guess).

Are we going to anything about this? While the bug is present in 9.5 (and possibly other versions), fixing it before 9.6 gets out seems important because reproducing it there is rather trivial (while I've been unable to reproduce it on 9.5).


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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