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  1. MariaDB Server
  2. MDEV-643

LP:668491 - Wrong result for SELECT COUNT(*) FROM t in pbxt.preload

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          Description

          CURRENT_TEST: pbxt.preload
--- /home/knielsen/devel/maria/my/work-5.1-mysqltest/mysql-test/suite/pbxt/r/preload.result	2010-05-26 09:39:54.000000000 +0200
+++ /home/knielsen/devel/maria/my/work-5.1-mysqltest/mysql-test/suite/pbxt/r/preload.reject	2010-10-29 21:12:28.000000000 +0200
@@ -40,10 +40,10 @@
 insert into t1(b) select b from t2;
 select count(*) from t1;
 count(*)
-33448
+33440
 select count(*) from t2;
 count(*)
-20672
+20668
 flush tables;
 flush status;
 show status like "key_read%";

          This failure is not deterministically reproducable, only happens rarely. But
          we've seen it occasionally in our Buildbot over time:

          http://buildbot.askmonty.org/buildbot/reports/cross_reference#branch=&revision=&platform=&dt=&bbnum=&typ=&info=&fail_name=pbxt.preload&fail_variant=&fail_info_short=&fail_info_full=&limit=100

          I was able to repeat as follows in latest MariaDB 5.1:

          (cd mysql-test && for i in `seq 1 1000` ; do ./mtr --parallel=3 pbxt.preload pbxt.preload pbxt.preload || exit 1 ; done)

          However, the failure seems to be quite elusive, so YMMV...

          The failure looks quite strange, the test just inserts a bunch of rows and
          then SELECT COUNT( * ) from the tables return a few rows too few.

          I am speculating if this could be caused by the PBXT engine returning some
          statistics about table size to mysqld, which is then taken by the optimiser as the actual count, so it optimises away a table scan? Either by mistake (so the fix would be to have PBXT tell mysqld that the count returned is an estimate only), or deliberately (so something would be wrong with the accuracy of the optimised row count)?

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                ratzpo Rasmus Johansson added a comment -

                Launchpad bug id: 668491

                Show
                ratzpo Rasmus Johansson added a comment - Launchpad bug id: 668491
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                elenst Elena Stepanova added a comment -

                I couldn't reproduce it either on current maria/5.3 or on maria/5.1 of the time of the bug report; however, it doesn't mean much as the description explains that the problem was difficult to reproduce.
                Anyway, I'm setting it to Minor and removing the upcoming 5.3 release from the list because PBXT is not a priority. Probably, we will want just to close it.

                Show
                elenst Elena Stepanova added a comment - I couldn't reproduce it either on current maria/5.3 or on maria/5.1 of the time of the bug report; however, it doesn't mean much as the description explains that the problem was difficult to reproduce. Anyway, I'm setting it to Minor and removing the upcoming 5.3 release from the list because PBXT is not a priority. Probably, we will want just to close it.
                Hide
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                psergey Sergei Petrunia added a comment -

                PBXT is no longer developed

                Show
                psergey Sergei Petrunia added a comment - PBXT is no longer developed

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                  • Assignee:
                    Unassigned
                    Reporter:
                    knielsen Kristian Nielsen
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                    • Created:
                      Updated:
                      Resolved: