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  1. MariaDB Server
  2. MDEV-4369

column_stats.histogram contents doesnt make sense

        Details

        • Type: Bug
        • Status: Closed
        • Priority: Major
        • Resolution: Fixed
        • Affects Version/s: None
        • Fix Version/s: None
        • Component/s: None
        • Labels:

          Description

          I used DBT-3 data, scale=10, InnoDB engine. I create statistical tables like
          specified in scripts/mysql_system_tables.sql.

          Then I use the following to generate the histogram:

          MariaDB [dbt3sf10]> set histogram_size=200;
Query OK, 0 rows affected (0.00 sec)

MariaDB [dbt3sf10]> set histogram_type='single_prec_hb';
Query OK, 0 rows affected (0.00 sec)
MariaDB [dbt3sf10]> analyze table customer persistent for all;

          Then I want to take a look at the histogram:

          MariaDB [dbt3sf10]> select *,hex(histogram),length(histogram) from mysql.column_stats where column_name='c_acctbal' and table_name='customer'\G
*************************** 1. row ***************************
          db_name: dbt3sf10
       table_name: customer
      column_name: c_acctbal
        min_value: -999.99
        max_value: 9999.99
      nulls_ratio: 0.0000
       avg_length: 8.0000
    avg_frequency: 1.8319
        hist_size: 200
        hist_type: 
        histogram: 	
 "%'),.0357:<>ACEHJLOQSVXZ\_acfhjmoqtvx{}������������������������������������������������������                     
                                                                                                                                 P                   �?                �w��             mmen
   hex(histogram): 020406090B0D10121417191B1E20222527292C2E303335373A3C3E414345484A4C4F515356585A5C5F616366686A6D6F717476787B7D7F828486898B8D90929497999B9EA0A2A5A7A9ACAEB0B2B5B7B9BCBEC0C3C5C7CACCCED1D3D5D8DADCDFE1E3E6E8EAEDEFF1F4F6F8FBFD00000001000000000000000300000000000000000000000B00000003500000000000000100000000000000000000000000F03F00000000000000000000000000000000F0770298C67F00000000000000000000010000006D6D656E

          Look at the hex(histogram) data. The first half of it has growing values:
          02,04,06,09,0B,0D, ..., F1,F4,F6,F8,FB,FD.. Then it goes back to zero, and
          continues at zero, with a few spikes.

          As far as I understand the meaning of values in a histogram, they should always
          increase. That is, histogram[i+1] >= histogram[i]. This is not the case for
          this histogram.

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                relates to

                Task - A task that needs to be done. MDEV-4145 Take into account the selectivity of single-table range predicates on non-indexed columns when searching for the best execution plan

                • Major - Major loss of function.
                • Closed

                  Activity

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                  psergey Sergei Petrunia added a comment -

                  Graphic plot of values in the histogram

                  Show
                  psergey Sergei Petrunia added a comment - Graphic plot of values in the histogram
                  Hide
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                  igor Igor Babaev added a comment -

                  A fix for the bug has been pushed into maria-10.0-mwl253.

                  Show
                  igor Igor Babaev added a comment - A fix for the bug has been pushed into maria-10.0-mwl253.

                    People

                    • Assignee:
                      igor Igor Babaev
                      Reporter:
                      psergey Sergei Petrunia
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                      Dates

                      • Created:
                        Updated:
                        Resolved: