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Query Analyzer

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 http://svn.nfectio.us/plugins/query_analyzer

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)

Vitals

Repository http://svn.nfectio.us/plugins/query_analyzer
License Rails' (MIT)
Rating (19 votes)
Owner Bob Silva
Created 22 September 2006

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