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  1. MariaDB Server
  2. MDEV-379

optimizer, join buffer, and LIMIT

        Details

        • Type: Bug
        • Status: Open
        • Priority: Minor
        • Resolution: Unresolved
        • Affects Version/s: 5.5.25, 5.3.7
        • Fix Version/s: 5.5
        • Component/s: None
        • Labels:
          None

          Description

          In some cases optimizer chooses an execution plan that is usually faster, but is much slower when only a few result rows are

          Take DBT-3 data with MyISAM tables:

          create view v_orders as
  select * from
     orders join
     customer on (o_custkey = c_custkey) join
     nation on (c_nationkey = n_nationkey) join
     region on (n_regionkey = r_regionkey)

          A simple

          select * from v_orders limit 3

          uses the following plan

          +------+-------------+----------+------+-----------------------+---------------+---------+--------------------------+--------+---------------------------------------------------------------------------+
| id   | select_type | table    | type | possible_keys         | key           | key_len | ref                      | rows   | Extra                                                                     |
+------+-------------+----------+------+-----------------------+---------------+---------+--------------------------+--------+---------------------------------------------------------------------------+
|    1 | SIMPLE      | region   | ALL  | PRIMARY               | NULL          | NULL    | NULL                     |      5 |                                                                           |
|    1 | SIMPLE      | nation   | ref  | PRIMARY,i_n_regionkey | i_n_regionkey | 5       | dbt3m.region.r_regionkey |      5 | Using join buffer (flat, BKA join); Key-ordered Rowid-ordered scan        |
|    1 | SIMPLE      | customer | ref  | PRIMARY,i_c_nationkey | i_c_nationkey | 5       | dbt3m.nation.n_nationkey | 180000 | Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan |
|    1 | SIMPLE      | orders   | ref  | i_o_custkey           | i_o_custkey   | 5       | dbt3m.customer.c_custkey |     15 | Using join buffer (incremental, BKA join); Key-ordered Rowid-ordered scan |
+------+-------------+----------+------+-----------------------+---------------+---------+--------------------------+--------+---------------------------------------------------------------------------+

          Same plan is used without LIMIT clause.

          It requires join buffers to be filled before any result rows can be produced,
          and the query takes a minute to complete (with LIMIT 3).

          With mrr=off the same query returns in a fraction of a second.

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                  • Assignee:
                    Unassigned
                    Reporter:
                    serg Sergei Golubchik
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                    Dates

                    • Created:
                      Updated: