Hi,
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
Catalogs")
- 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"
section.
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.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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