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  1. MariaDB Server
  2. MDEV-27576

Use DESC indexes for MIN/MAX optimization

Details

    Description

      As of c10e10c6, opt_sum.cc is still stubbed for DESC indexes

            if (part->key_part_flag & HA_REVERSE_SORT)
      		 
              break; // TODO MDEV-13756

      Given that we are about to close MDEV-13756, it looks strange: either it was forgotten and needs to be enabled, or maybe it requires a better reference.

      Given the above, the optimization expectedly doesn't work.

      --source include/have_sequence.inc
      		 
       
      		 
      create or replace table t1 (id int, key(id));
      		 
      insert into t1 select seq from seq_1_to_100 order by rand(1);
      		 
      explain extended select max(id) from t1 where id > 50;
      		 
       
      		 
      create or replace table t1 (id int, key(id desc));
      		 
      insert into t1 select seq from seq_1_to_100 order by rand(1);
      		 
      explain extended select max(id) from t1 where id > 50;
      		 
       
      		 
      # Cleanup
      		 
      drop table t1;

      With the ASC index, it is

      preview-10.8-MDEV-13756-desc-indexes c10e10c6

      		 
      +------+-------------+-------+------+---------------+------+---------+------+------+----------+------------------------------+
      		 
      | id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                        |
      		 
      +------+-------------+-------+------+---------------+------+---------+------+------+----------+------------------------------+
      		 
      |    1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | Select tables optimized away |
      		 
      +------+-------------+-------+------+---------------+------+---------+------+------+----------+------------------------------+

      With the DESC index, it is

      +------+-------------+-------+-------+---------------+------+---------+------+------+----------+--------------------------+
      		 
      | id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra                    |
      		 
      +------+-------------+-------+-------+---------------+------+---------+------+------+----------+--------------------------+
      		 
      |    1 | SIMPLE      | t1    | range | id            | id   | 5       | NULL | 50   |   100.00 | Using where; Using index |
      		 
      +------+-------------+-------+-------+---------------+------+---------+------+------+----------+--------------------------+

      Attachments

        Issue Links

          is caused by

          Task - A task that needs to be done. MDEV-13756 Implement descending index: KEY (a DESC, b ASC)

          • Blocker - Blocks development and/or testing work, production could not run.
          • Closed
          relates to

          New Feature - A new feature of the product, which has yet to be developed. MDEV-32732 Support DESC indexes in loose scan optimization

          • Critical - Crashes, loss of data, severe memory leak.
          • In Testing

          Activity

            Transition Time In Source Status Execution Times
            Yuchen Pei made transition -
            Open In Progress
            		495d 16h 55m 1
            Yuchen Pei made transition -
            In Progress In Review
            		152d 17h 25m 1
            Yuchen Pei made transition -
            Stalled In Review
            		13h 1m 1
            Sergei Petrunia made transition -
            In Review Stalled
            		10d 18h 17m 2
            Yuchen Pei made transition -
            Stalled In Testing
            		18h 13m 1
            Lena Startseva made transition -
            In Testing Stalled
            		27d 13h 25m 1
            Yuchen Pei made transition -
            Stalled Closed
            		3d 7h 43m 1

            People

              ycp Yuchen Pei
              elenst Elena Stepanova
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              Dates

                Created:
                Updated:
                Resolved:

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