On 03/26/2016 10:18 AM, Tatsuo Ishii wrote:
Fair point. Attached is v18 of the patch, after pgindent cleanup.

Here are some feedbacks to v18 patch.

1) regarding examples in create_statistics manual

Here are numbers I got. "with statistics" referrers to the case where
multivariate statistics are used.  "without statistics" referrers to the
case where multivariate statistics are not used. The numbers denote
estimated_rows/actual_rows. Thus closer to 1.0 is better. Some numbers
are shown as a fraction to avoid 0 division. In my understanding case
1, 3, 4 showed that multivariate statistics superior.

        with statistics without statistics
case1   0.98            0.01
case2   98/0            1/0

The case2 shows that functional dependencies assume that the conditions used in queries won't be incompatible - that's something this type of statistics can't fix.

case3   1.05            0.01
case4   1/0             103/0
case5   18.50           18.33
case6   111123/0        1111123/0

The last two lines (case5 + case6) seem a bit suspicious. I believe those are for the histogram data, and I do get these numbers:

case5    0.93 (5517 / 5949)         42.0 (249943 / 5949)
case6    100/0                      100/0

Perhaps you've been using the version before the bugfix, with ANALYZE on the wrong table?

2) following comments by me are not addressed in the v18 patch.

- There's no docs for pg_mv_statistic (should be added to "49. System

- The word "multivariate statistics" or something like that should
  appear in the index.

- There are some explanation how to deal with multivariate statistics
  in "14.1 Using Explain" and "14.2 Statistics used by the Planner"

Yes, those are valid omissions. I plan to address them, and I'd also considering adding a section to 65.1 (How the Planner Uses Statistics), explaining more thoroughly how the planner uses multivariate stats.


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

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