Bug #44191 EXPLAIN PARTITIONS reports all rows instead of rows in examined partitions
Submitted: 9 Apr 2009 17:33
Reporter: Kyle Joiner Email Updates:
Status: Verified Impact on me:
None 
Category:MySQL Server: Optimizer Severity:S4 (Feature request)
Version:5.1 OS:Any
Assigned to: CPU Architecture:Any
Tags: explain, partitioning, rows estimate
Triage: Needs Triage: D5 (Feature request)

[9 Apr 2009 17:33] Kyle Joiner
Description:
When you perform and EXPLAIN PARTITIONS SELECT the explain plan reports all a count of all rows if no indexes are used.  It would be logical that it only report the row count for the partitions examined.

How to repeat:
From the 5.1 manual:

http://dev.mysql.com/doc/refman/5.1/en/partitioning-info.html

 Suppose that you have a table trb1 defined and populated as follows:

CREATE TABLE trb1 (id INT, name VARCHAR(50), purchased DATE)
    PARTITION BY RANGE(id)
    (
        PARTITION p0 VALUES LESS THAN (3),
        PARTITION p1 VALUES LESS THAN (7),
        PARTITION p2 VALUES LESS THAN (9),
        PARTITION p3 VALUES LESS THAN (11)
    );

INSERT INTO trb1 VALUES
    (1, 'desk organiser', '2003-10-15'),
    (2, 'CD player', '1993-11-05'),
    (3, 'TV set', '1996-03-10'),
    (4, 'bookcase', '1982-01-10'),
    (5, 'exercise bike', '2004-05-09'),
    (6, 'sofa', '1987-06-05'),
    (7, 'popcorn maker', '2001-11-22'),
    (8, 'aquarium', '1992-08-04'),
    (9, 'study desk', '1984-09-16'),
    (10, 'lava lamp', '1998-12-25');

You can see which partitions are used in a query such as SELECT * FROM trb1;, as shown here:

mysql> EXPLAIN PARTITIONS SELECT * FROM trb1\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: trb1
   partitions: p0,p1,p2,p3
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 10
        Extra: Using filesort

In this case, all four partitions are searched. However, when a limiting condition making use of the partitioning key is added to the query, you can see that only those partitions containing matching values are scanned, as shown here:

mysql> EXPLAIN PARTITIONS SELECT * FROM trb1 WHERE id < 5\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: trb1
   partitions: p0,p1
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 10
        Extra: Using where

The rows do not total 10 from partitions p0 and p1.

Suggested fix:
Have Explain results only count the rows from the selected partitions.