My understanding of what is happening here is this:
The 'rows' column of EXPLAIN output is an estimate of how many rows
MySQL thinks it will likely have to examine in a table to get your
answer. When there's an index, it will hopefully be able to use that
to exmaine a small subset of the rows in the table.
Problem here is, MySQL thinks it will have to examine 1463 of 1950
rows. At that point (or any point higher than about 30%) MySQL will
decide that a table scan may be faster. Hence the decision to not use
the d_id index.
When you drop the index, MySQL can no longer plan to eliminate any
rows using an index, so it knows up front it will have to do a table
scan, giving you the 1950 answer for the table with no d_id index.
What's probably happening is that you have a large grouping of the
5098 number in your data, based on a quick read of your query.
Make sense?
Dan
On 10/25/06, David Hillman <[EMAIL PROTECTED]> wrote:
All;
Am I crazy, or doesn't this have to be an optimizer/explain bug?
SQL interspersed with comments follow...
mysql> CREATE TEMPORARY TABLE `table_a` ( `s_id` int(11) NOT
NULL
default '0', `r_id` int(11) NOT NULL default '0', `d_id` int
(11) NOT NULL default '0', `status` enum('open','close') NOT NULL
default 'open', key `s_id` ( `s_id` ), key `d_id` ( `d_id` ) )
SELECT MAX(fs.s_id) as s_id, fs.r_id, fs.d_id,
fs.status FROM
table_c AS fs WHERE fs.d_id=5098 AND fs.status='close' GROUP BY
fs.r_id;
Query OK, 1950 rows affected (0.03 sec)
Records: 1950 Duplicates: 0 Warnings: 0
We created a temp table, and stuck 1950 rows in it.
mysql> explain SELECT fs.s_id, fs.r_id, fst.* FROM
table_a
AS fs LEFT OUTER JOIN table_b AS fst ON
fs.s_id=fst.s_id LEFT OUTER JOIN table_d AS ff ON
ff.f_id=fst.f_id WHERE fs.d_id='5098' AND ff.status='active';
+----+-------------+-------+--------+----------------
+---------------+---------+-------------------------+------
+-------------+
| id | select_type | table | type | possible_keys |
key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+----------------
+---------------+---------+-------------------------+------
+-------------+
| 1 | SIMPLE | fs | ALL | d_id | NULL |
NULL | NULL | 1463 | Using where |
| 1 | SIMPLE | fst | ref | s_id | s_id | 4 |
database.fs.s_id | 1 | |
| 1 | SIMPLE | ff | eq_ref | PRIMARY,status |
PRIMARY | 4 | database.fst.f_id | 1 | Using where |
+----+-------------+-------+--------+----------------
+---------------+---------+-------------------------+------
+-------------+
3 rows in set (0.00 sec)
Now when we join on that temporary table_a ( aka "fs" ), there's
two interesting things happening. One, there's a possible_key called
d_id, but it's not using it. Two, it's allegedly an "ALL" join, but
only showing 1463 of the 1950 rows.
mysql> alter table table_a drop index d_id;
Query OK, 1950 rows affected (0.01 sec)
Records: 1950 Duplicates: 0 Warnings: 0
Now we drop the "possible_key" that it wasn't using anyway,
and... run the same explain...
mysql> explain SELECT fs.s_id, fs.r_id, fst.* FROM
table_a
AS fs LEFT OUTER JOIN table_b AS fst ON
fs.s_id=fst.s_id LEFT OUTER JOIN table_d AS ff ON
ff.f_id=fst.f_id WHERE fs.d_id='5098' AND ff.status='active';
+----+-------------+-------+--------+----------------
+---------------+---------+-------------------------+------
+-------------+
| id | select_type | table | type | possible_keys |
key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+----------------
+---------------+---------+-------------------------+------
+-------------+
| 1 | SIMPLE | fs | ALL | NULL |
NULL | NULL | NULL | 1950 | Using where |
| 1 | SIMPLE | fst | ref | s_id | s_id | 4 |
database.fs.s_id | 1 | |
| 1 | SIMPLE | ff | eq_ref | PRIMARY,status |
PRIMARY | 4 | database.fst.f_id | 1 | Using where |
+----+-------------+-------+--------+----------------
+---------------+---------+-------------------------+------
+-------------+
3 rows in set (0.00 sec)
Now that join is still an "ALL", but it's looking at more rows?
How can it look at more than "ALL" rows? Why does dropping an index
that it wasn't using change, well, anything? Can someone make some
sense of this for me?
This is on version 4.1.18. Thanks.
--
David Hillman
LiveText, Inc
1.866.LiveText x235
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