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  1. MariaDB Server
  2. MDEV-27576

Use DESC indexes for MIN/MAX optimization

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      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 |
      		 
      +------+-------------+-------+-------+---------------+------+---------+------+------+----------+--------------------------+

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          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

          Task - A task that needs to be done. MDEV-32732 Support DESC indexes in loose scan optimization

          • Major - Major loss of function.
          • Stalled

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              ycp Yuchen Pei
              elenst Elena Stepanova
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              Dates

                Created:
                Updated:
                Resolved:

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