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JOIN Performance & Charsets

 | October 21, 2008 | 


We have written before about the importance of using numeric types as keys, but maybe you’ve inherited a schema that you can’t change or have chosen string types as keys for a specific reason. Either way, the character sets used on joined columns can have a significant impact on the performance of your queries.

Take the following example, using the InnoDB storage engine:

t1 has been populated with 100,000 records, while t2 has 400,000. The data set fit easily in memory.

Notice the differences in key_len and the explicit call to CONVERT() in the WHERE clause. This is a result of the joined columns being of different character sets.

The above query took an average of 4.33 seconds to execute with t1 as utf8 and t2 as latin1. Converting both tables to utf8 resulted in an average execution time of 3.12 seconds and had the following EXPLAIN:

Notice here how there is no CONVERT() required and the key_len on both tables match. Just this simple change resulted in more than a 25% improvement in average execution time, from 4.33 to 3.12 seconds.

This test was performed with MySQL 5.0.67, FreeBSD 7, on a box with 2GB RAM.

Ryan Lowe

Ryan was a principal consultant and team manager at Percona until July 2014. He has experience with many database technologies in industries such as health care, telecommunications, and social networking.


  • I have one problem with join.

    I need to reduce query time to it optimized level. Details are as under

    SELECT SQL_CALC_FOUND_ROWS p.*, FLOOR(p.prodratingtotal/p.prodnumratings) AS prodavgrating, 0 AS prodgroupdiscount, pi.* , (IF(p.prodname=’gold’, 10000, 0) + IF(p.prodcode=’gold’, 10000, 0) + ((MATCH (ps.prodname) AGAINST (‘gold’)) * 10) + MATCH (ps.prodname,ps.prodcode,ps.proddesc,ps.prodsearchkeywords) AGAINST (‘gold’)) AS score FROM products p LEFT JOIN product_images pi ON (p.productid = pi.imageprodid AND pi.imageisthumb = 1) INNER JOIN product_search ps ON p.productid = ps.productid WHERE p.prodvisible = 1 AND (ps.prodcode = ‘gold’ OR TRUE) AND (MATCH (ps.prodname,ps.prodcode,ps.proddesc,ps.prodsearchkeywords) AGAINST (‘gold’)) ORDER BY score DESC LIMIT 20
    EXECUTION TIME: 2.5000+ seconds
    products: 31,000 records
    product_images: 92,000 records
    product_search: 57,000 records

    1 SIMPLE ps fulltext prodname prodname 0 1 Using where; Using temporary; Using filesort
    1 SIMPLE p eq_ref PRIMARY,i_products_rating_vis,i_products_added_vis,i_products_sortorder_vis PRIMARY 4 shoppingcart_5521.ps.productid 1 Using where
    1 SIMPLE pi ref i_product_images_imageprodid i_product_images_imageprodid 5 shoppingcart_5521.p.productid,const 1

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