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                              WORKLOG TASK
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TASK...........: Make EXPLAIN always show materialization separately
CREATION DATE..: Mon, 29 Mar 2010, 06:45
SUPERVISOR.....: Igor
IMPLEMENTOR....: 
COPIES TO......: 
CATEGORY.......: Client-BackLog
TASK ID........: 110 (http://askmonty.org/worklog/?tid=110)
VERSION........: Benchmarks-3.0
STATUS.........: Un-Assigned
PRIORITY.......: 60
WORKED HOURS...: 0
ESTIMATE.......: 0 (hours remain)
ORIG. ESTIMATE.: 0

PROGRESS NOTES:



DESCRIPTION:

At the moment, SJM-Materialization is shown in EXPLAIN output in this way:

MariaDB [j45]> explain select * from ot where a in (select b from it1);
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows
| Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | PRIMARY     | ot    | ALL  | NULL          | NULL | NULL    | NULL |   10
|             |
|  1 | PRIMARY     | it1   | ALL  | NULL          | NULL | NULL    | NULL |   10
| Materialize |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+

MariaDB [j45]> explain select * from ot where a in (select it1.b from it1, it2);
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows
| Extra                              |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------------+
|  1 | PRIMARY     | ot    | ALL  | NULL          | NULL | NULL    | NULL |   10
|                                    |
|  1 | PRIMARY     | it1   | ALL  | NULL          | NULL | NULL    | NULL |   10
| Start materialize                  |
|  1 | PRIMARY     | it2   | ALL  | NULL          | NULL | NULL    | NULL |   10
| End materialize; Using join buffer |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------------+

This WL task is to change the output format so it will look as follows:
- Tables inside the SJM-nest are displayed as a separate select
- within the master select, there is a line that denotes SJM-lookup or
  SJM-Scan.

The above examples will look as follows:

MariaDB [j45]> explain select * from ot where a in (select b from it1);
+----+-------------+------------+--------+---------------+--------------+---------+----------+------+-------+
| id | select_type | table      | type   | possible_keys | key          |
key_len | ref      | rows | Extra |
+----+-------------+------------+--------+---------------+--------------+---------+----------+------+-------+
|  1 | PRIMARY     | ot         | ALL    | NULL          | NULL         | NULL 
  | NULL     |   10 |       |
|  1 | PRIMARY     | SUBQUERY#2 | eq_ref | distinct_key  | distinct_key | 5    
  | j45.ot.a |    1 |       |
|  2 | SUBQUERY    | it1        | ALL    | NULL          | NULL         | NULL 
  | NULL     |   10 |       |
+----+-------------+------------+--------+---------------+--------------+---------+----------+------+-------+

MariaDB [j45]> explain select * from ot where a in (select it1.b from it1, it2);
+----+-------------+------------+--------+---------------+--------------+---------+----------+------+-------------------+
| id | select_type | table      | type   | possible_keys | key          |
key_len | ref      | rows | Extra             |
+----+-------------+------------+--------+---------------+--------------+---------+----------+------+-------------------+
|  1 | PRIMARY     | ot         | ALL    | NULL          | NULL         | NULL 
  | NULL     |   10 |                   |
|  1 | PRIMARY     | SUBQUERY#2 | eq_ref | distinct_key  | distinct_key | 5    
  | j45.ot.a |    1 |                   |
|  2 | SUBQUERY    | it1        | ALL    | NULL          | NULL         | NULL 
  | NULL     |   10 |                   |
|  2 | SUBQUERY    | it2        | ALL    | NULL          | NULL         | NULL 
  | NULL     |   10 | Using join buffer |
+----+-------------+------------+--------+---------------+--------------+---------+----------+------+-------------------+

The rationale behind the change is:
- Unification of EXPLAIN output with MWL#90
- The new format is more natural representation of what is going on,
  conceptually-wise (and may be soon be code-wise)
- The new format allows to display E(#records-in-temp-table) for the 
  SJM-Scan case (and for SJM-lookup that number doesn't matter that much)
- The new format doesn't put anything into "Extra" column and that's good 
  because that column is already overloaded and horizontal screen space is
  precious (while vertical is not so much).


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