how to optimize mysql query : counting category and subcategory with single Query from two tables

Question

I need help to optimize this MySQL query for better and fast performance.

Here is the SQL FIDDLE with Query and table structure.

basically I have two tables

tbl_category

CREATE TABLE IF NOT EXISTS `tbl_category` (
  `category_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `category_name` varchar(20) NOT NULL,
  `parent_category_id` int(10) unsigned DEFAULT NULL,      
  PRIMARY KEY (`category_id`),
  UNIQUE KEY `category_name` (`category_name`,`parent_category_id`),
  KEY `category_parent_id` (`parent_category_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1;

--
-- data for table `tbl_auction`
-- 
+-------------+---------------+--------------------+-----------------+
| category_id | category_name | parent_category_id | category_status | 
+-------------+---------------+--------------------+-----------------+
|           1 | Boats         |               NULL | a               | 
|           2 | Books         |               NULL | a               | 
|           3 | Building      |               NULL | a               | 
|           4 | Cars          |               NULL | a               | 
|           5 | Electrical    |                  3 | a               | 
|           6 | Hardware      |                  3 | a               | 
|           7 | Heating       |                  3 | a               | 
|           8 | Miscellaneous |                  3 | a               | 
|           9 | Plumbing      |                  3 | a               | 
|          10 | Tools         |                  4 | a               | 
|          11 | Lights        |                  4 | a               | 
|          12 | Miscellaneous |                  4 | a               | 
+-------------+---------------+--------------------+-----------------+

tbl_auction

CREATE TABLE IF NOT EXISTS `tbl_auction` (
  `auction_id` int(10) unsigned NOT NULL AUTO_INCREMENT, 
  `auction_category` int(10) unsigned NOT NULL 
   COMMENT 'either store subcategory OR main category if no subcategory',
  `auction_title` varchar(100) NOT NULL,     
  PRIMARY KEY (`auction_id`),
  KEY `auction_category` (`auction_category`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1;

--
-- data for table `tbl_auction`
--

+------------+------------------+---------------+
| auction_id | auction_category | auction_title | 
+------------+------------------+---------------+-
|          1 |                1 | one           | 
|          2 |                2 | two           | 
|          3 |                5 | five          | 
|          4 |                6 | six           | 
|          5 |                5 | five2         | 
|          6 |                8 | eight         | 
|          7 |               11 | eleven        | 
|          8 |               11 | eleven2       | 
|          9 |               10 | ten           | 
|         10 |                2 | two2          | 
|         11 |               12 | twelve        | 
+------------+------------------+---------------+

Now I want to count all auctions category wise as well as if some category have subcategory then sum that auction to main category

I made below query

SELECT auction_category AS categoryID, COUNT(*) AS total
      FROM `tbl_auction`
      GROUP BY auction_category
UNION ALL
SELECT parent_category_id  AS categoryID, COUNT( * ) AS total
      FROM  `tbl_auction` ta
      INNER JOIN tbl_category tc ON tc.category_id = ta.auction_category
      WHERE parent_category_id IS NOT NULL
      GROUP BY parent_category_id

although above query returns expected results But I think there may be some optimized way to solve this. kindly guide me and suggest me optimized Query.

Thanks

Answer 1

What's killing performance is the filesort triggered by one of the GROUP BYs and/or the UNION. You can rewrite the query without UNION as:

SELECT pcat.category_id, COUNT(*)
  FROM  tbl_category pcat
  LEFT JOIN tbl_category subcat 
    ON subcat.parent_category_id = pcat.category_id 
  INNER JOIN tbl_auction a 
    ON a.auction_category = IFNULL(subcat.category_id,pcat.category_id)
GROUP BY pcat.category_id;

This has the same effect (double-counting each auction, once under its subcat, once under the parent cat), and the EXPLAIN shows no filesort. If it does when the data get larger, you may try adding the ORDER BY NULL trick.

Also if real-time accuracy is not super important but the query is executed frequently consider caching the result.