Am I correct in the statment second query is faster? This query gets
executed several million times.
mysql> explain select A.* from files as A where A.pathref=129286 and
version=(select max(version) from files as B where A.pathref=B.pathref);
+----+--------------------+-------+------+-------------------+-----------+---------+---------------------+------+-------------+
| id | select_type | table | type | possible_keys | key |
key_len | ref | rows | Extra |
+----+--------------------+-------+------+-------------------+-----------+---------+---------------------+------+-------------+
| 1 | PRIMARY | A | ref | pathref_2,pathref | pathref_2 |
4 | const | 1 | Using where |
| 2 | DEPENDENT SUBQUERY | B | ref | pathref_2,pathref | pathref_2 |
4 | crisfield.A.pathref | 1 | Using index |
+----+--------------------+-------+------+-------------------+-----------+---------+---------------------+------+-------------+
2 rows in set (0.01 sec)
mysql> explain select A.* from files as A where A.pathref=129286 and
version=(select max(version) from files as B where 129286=B.pathref);
+----+-------------+-------+-------+---------------------------+-----------+---------+-------------+------+------------------------------+
| id | select_type | table | type | possible_keys | key |
key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------------------+-----------+---------+-------------+------+------------------------------+
| 1 | PRIMARY | A | const | pathref_2,pathref,version | pathref_2 |
8 | const,const | 1 | |
| 2 | SUBQUERY | NULL | NULL | NULL | NULL |
NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+-------+---------------------------+-----------+---------+-------------+------+------------------------------+
2 rows in set (0.00 sec)
On Wed, 7 Sep 2005, Gleb Paharenko wrote:
Hello.
so why does 'explain select * from files where
(pathref,version)=(129286,0);' scan the whole table?
It is documented that this syntax can't be optimized yet. See:
http://dev.mysql.com/doc/mysql/en/row-subqueries.html
Jason Pyeron <[EMAIL PROTECTED]> wrote:
On Tue, 6 Sep 2005, Dan Nelson wrote:
In the last episode (Sep 06), Jason Pyeron said:
there is an unique key index 'pathref_2 (pathref,version)' on this
table.
so why does 'explain select * from files where
(pathref,version)=(129286,0);' scan the whole table?
I have to admit I have never seen this syntax used in a where clause
before.
never thought not to use it, SQL servers are just big set processing
engines.
Does "where pathref=129286 and version=0" optimize any better?
yes, it scans only one row.
A quick test of mysql-5.0.11 indicates that it doesn't use indexes at
all with the (field1,field2,..)=(value1,value2,..) syntax. Feel free
to file a bug :)
http://bugs.mysql.com/13024
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