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« Grid control therapy | Main | D.I.Y. MySQL 5.1 monitoring »
Thursday
Jan182007

Using EXPLAIN EXTENDED to see view query rewrites

At the MySQL Mini Conference in Sydney this week we discussed how to use EXPLAIN EXTENDED to view the rewrites undertaken by the MySQL optimizer.  IN particular, to see if MySQL performs a merge of the query into the view definition, or if it creates a temporary table.

It can be tricky to optimize queries using views, since it's often hard to know exactly how the query will be resovled - will MySQL push merge the text of the query and the view, or will it use a temporary table containing the views result set and then apply the query clauses to that?

In general, MySQL merges query text except when the view definition includes a GROUP BY or UNION.  But to be sure we can use EXPLAIN EXTENDED.  This also helps when we get confusing output in the EXPLAIN output.

For instance if we have a view definition like this:

 

CREATE VIEW user_table_v AS
     SELECT *
       FROM information_schema.tables ist
        WHERE table_type='BASE TABLE';

and try and explain a query like this:

 

explain select * from user_table_v WHERE table_schema='mysql'\G

We get output like this, which might be difficult to interpret unless we know the view defition:

 

*************************** 1. row ***************************
          id: 1
  select_type: SIMPLE
        table: ist
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 2
     filtered: 100.00
        Extra: Using where

Note the table "ist",  only by looking at the view definition can we interepret this.  But if we do an EXPLAIN EXTENDED followed by a SHOW WARNINGS we see the exact text:

*************************** 1. row ***************************
  Level: Note
   Code: 1003
Message: select `ist`.`TABLE_NAME` AS `TABLE_NAME` from `information_schema`.`tables` `ist` where ((
`ist`.`TABLE_SCHEMA` = _utf8'mysql') and (`ist`.`TABLE_TYPE` = _utf8'BASE TABLE'))
1 row in set (0.00 sec)

And from this we can see that MySQL did indeed merge the WHERE clauses of both the query and the view definition.

If we look at the output for a view like this:

CREATE VIEW table_types_v AS
       SELECT table_type,count(*)
         FROM information_schema.tables ist
        GROUP BY table_type;

The we see the following output, in which we can see that MySQL created a temporary table and then applied the WHERE clause from the query:

*************************** 1. row ***************************
          id: 1
  select_type: PRIMARY
        table: NULL
         type: NULL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: NULL
     filtered: NULL
        Extra: Impossible WHERE noticed after reading const tables
*************************** 2. row ***************************
          id: 2
  select_type: DERIVED
        table: ist
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 2
     filtered: 100.00
        Extra: Using temporary; Using filesort
2 rows in set, 1 warning (0.00 sec)

*************************** 1. row ***************************
  Level: Note
   Code: 1003
Message: select `table_types_v`.`table_type` AS `table_type`,`table_types_v`.`count(*)` AS `count(*)
` from `mysql`.`table_types_v` where (`table_types_v`.`table_type` = _utf8'BASE TABLE')
1 row in set (0.00 sec)

EXPLAIN EXTENDED is an invaluable tool for tuning SQL statements, and even more so when working with views.

Reader Comments (2)

Still far from a proper explain plan like the ones of Oracle or PostgreSQL ...
January 19, 2007 | Unregistered Commenterp
very interesting, but I don't agree with youIdetrorce
December 16, 2007 | Unregistered CommenterIdetrorce

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