= Query Analyzer Plugin for MySQL on Rails

MODIFIED by John Eberly originally take from http://svn.nfectio.us

The Query Analyzer plugin will expand the usability of your log files
by providing query analysis using the MySQL query execution plan. Each SQL
select query will be 'EXPLAIN'ed and added to the log files right below
the original query.

Using this plugin and a good understanding of the results, you will be 
able to analyze and optimize the queries your application is making.

Refer to http://www.mysql.org/doc/refman/5.0/en/explain.html for more
information on understanding the results. 

= Installation

script/plugin install git://github.com/jeberly/query-analyzer.git

= Example Use

Here is a real life usage of the plugin that detected the omission of indexes on
a table. In this case, it was a join table and the keys didn't have indexes (silly me!).
Names have been changed to protect the innocent (and make it fit 80 columns)

# development.log

P Load (0.008669)
 => SELECT p.* FROM p INNER JOIN d ON p.id = d.p_id WHERE (d.p_id = 2 AND ((d.type = 'P'))) 

Analyzing P Load
   
select_type | key_len | type   | Extra       | id | possible_keys | rows | table  | ref    | key    
----------------------------------------------------------------------------------------------------
SIMPLE      |         | ALL    | Using where | 1  |               | 74   |    d   |        |        
SIMPLE      | 4       | eq_ref | Using where | 1  | PRIMARY       | 1    |    p   | d.p_id | PRIMARY


= Analyze the results

Looking at the results of the execution plan, we can see that the lookup in
the d table is missing an index (possible_keys=null) and performed a full 
table scan (type=ALL) to satisfy the WHERE condition. In this case, there was only one 
row that matched the condition in the table, but MySQL still had to search all 74 rows 
in the table to find it, a key indicator of a missing
or malformed index(es). Once it has pulled all the records to satisfy the WHERE, it then
starts the p table join. This time, it was able to match d.p_id to p.id using
the PRIMARY key on the p table. The type=eq_ref indicates a 1 to 1 match against a primary 
or unique column.

Lets add some indexes to the join table and see if we can cut that full table scan down in the
number of rows it needs to search.

#> script/generate Migration AddIndexesToD

# file: 005_add_indexes_to_d
class AddIndexesToD < ActiveRecord::Migration
  def self.up
    add_index :d, [ :p_id, :type ]
    add_index :d, :type
  end

  def self.remove
    remove_index :d, [ :p_id, :type ]
    remove_index :d, :type  
  end
end


Now that we have an index on the foreign_key column and type, lets re-run the query and
see if we got rid of that full table scan.

# development.log

P Load (0.009011)   
 => SELECT p.* FROM p INNER JOIN d ON p.id = d.p_id WHERE (d.p_id = 2 AND ((d.type = 'P'))) 

Analyzing P Load
   
select_type | key_len | type   | Extra       | id | possible_keys                  | rows | table | ref    | key                             
------------------------------------------------------------------------------------------------------------------------------
SIMPLE      | 255     | ref    | Using where | 1  | d_p_id_type_index,d_type_index | 1    |   d   | const  | d_p_id_type_index
SIMPLE      | 4       | eq_ref | Using where | 1  | PRIMARY                        | 1    |   p   | d.p_id | PRIMARY                         

Okay. Now MySQL is using an index satisfy the WHERE condition. Using the index, it was able to
find the single row that matched, preventing the full table scan.


Credits:
The extension of the Array class for printing the columnized records was originally
written by Peter Cooper who adapted it from Courtenay from #caboose.

http://www.rubyinside.com/columnized-text-datasets-in-rails-71.html
http://habtm.com/articles/2006/06/10/pretty-tables-for-ruby-objects

Released under the MIT license (download your own if you need it)