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  1. MariaDB Server
  2. MDEV-5962

EITS: value "position" calculated incorrectly for CHAR(n) columns

        Details

        • Type: Bug
        • Status: Closed
        • Priority: Major
        • Resolution: Fixed
        • Affects Version/s: 10.0.9
        • Fix Version/s: 10.0.10
        • Component/s: None
        • Labels:

          Description

          Let's see how histograms work for CHAR columns

          create table ten(a int);
insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

create table t1(
  c_mktsegment char(10)
);

insert into t1 select 'AUTOMOBILE' from ten;
insert into t1 select 'FURNITURE' from ten;
insert into t1 select 'HOUSEHOLD' from ten;
insert into t1 select 'MACHINERY' from ten;

set histogram_size=20;
set use_stat_tables='preferably';
set optimizer_use_condition_selectivity=4;

analyze table t1 persistent for all;

          Now, put a breakpoint in Field::pos_in_interval_val_str and run:

          explain extended select count(*) from t1 where c_mktsegment ='ABCD';

          Let's see 

          (gdb) print mp
  $334 = 4702394920804032544
(gdb) print minp
  $335 = 4707761685061911113
(gdb) print maxp
  $336 = 5566804592153216338
(gdb) print mp_prefix
  $337 = "    DCBA"
(gdb) print minp_prefix
  $338 = "IBOMOTUA"
(gdb) print maxp_prefix
  $339 = "RENIHCAM"
(gdb) p maxp > minp
  $340 = true
(gdb) p mp < minp
  $342 = true

          So, mp < minp < maxp . This is expected. Now , run this line

            n= mp - minp;

          and

          (gdb) print n
  $343 = 1.8441377309451674e+19
(gdb) p ((double) mp) - ((double) minp)
  $344 = -5366764257878016

          We use unsigned substraction, and end up with the wrong value_position number. The return value of Field::pos_in_interval_val_str() is 1 although it should have been 0.

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                  • Assignee:
                    psergey Sergei Petrunia
                    Reporter:
                    psergey Sergei Petrunia
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                    • Created:
                      Updated:
                      Resolved: