… is MRR Key-ordered Scan. Let’s see what this optimization actually is. Key-ordered Scan for BKA With this optimization the idea of…’ and l_shipdate > ’1995-03-09′ group by l_orderkey, o_orderdate, o_shippriority order by revenue desc, o_orderdate LIMIT 10; In-memory workload Now let’s see how effective are the join optimizations when…
Post: Multi Range Read (MRR) in MySQL 5.6 and MariaDB 5.5
March 21, 2012 By 10 Comments
… key_column, pk_column FROM tbl WHERE key_column=x ORDER BY key_column (Note that secondary keys in InnoDB contain primary… were made in the MySQL config: optimizer_switch=’index_condition_pushdown=off’ optimizer_switch=’mrr=on’ optimizer_switch=’mrr_sort_keys=on’ (only… = n_nationkey group by c_custkey, c_name, c_acctbal, c_phone, n_name, c_address, c_comment order by revenue desc LIMIT…
Comment: What does Using filesort mean in MySQL?
March 15, 2012 By 27 Comments
I looked at the “ORDER BY OPTIMIZATION” section in MySQL manual but could not find a reason … * FROM table1 ORDER BY a LIMIT 1698, 1 This one uses filesort (Extra: Using filesort): EXPLAIN SELECT * FROM table1 ORDER BY a LIMIT 1699… does not. I just see this note: With EXPLAIN SELECT … ORDER BY, you can check whether MySQL can use indexes to resolve…
Post: How FLUSH TABLES WITH READ LOCK works with Innodb Tables
March 23, 2012 By 20 Comments
… FLUSH TABLES WITH READ LOCK does not work as optimally as you could think it works. Even though with… to wait for any SELECT statement to complete in order to complete locking tables. This means if you have… to be blocked too, which among other tools used by mysql client, so you might get a feel you…
Post: ORDER BY ... LIMIT Performance Optimization
September 1, 2006 By 93 Comments
… need to know about ORDER BY … LIMIT optimization to avoid these problems ORDER BY with LIMIT is most common use of ORDER BY in interactive applications with… indexes could be used on each of the tables to optimize order by, or at least local sort could be used for each… one direction. If you have ORDER BY col1, col2 it can be optimized using index, if you have ORDER BY col1 DESC, col2 DESC same…
Post: Possible optimization for sort_merge and UNION ORDER BY LIMIT
September 18, 2007 By 10 Comments
… index for the order by. Ranges as well as IN lists make this optimization impossible, not even speaking about index merge optimization. Lets look… get resulted rows in sorted order. Once this is implemented similar approach could be used for optimizing ORDER BY with IN. This original query…
Post: MySQL Performance - eliminating ORDER BY function
October 17, 2007 By 6 Comments
… you would learn about MySQL Performance Optimization is to avoid using functions when comparing constants or order by. Ie use indexed_col=N… you look only at ORDER BY clause you would see first query which sorts by function is able to avoid order by while second which… this case MySQL Optimizer is rather smart and is able to do this even if we have function in ORDER BY and exactly…
Post: Using index for ORDER BY vs restricting number of rows.
February 16, 2007 By 7 Comments
… with MySQL Optimizer I frequently run into is making poor decision when it comes to choosing between using index for ORDER BY or… * from goods where cat_id=5 and seller_id=1 order by price desc limit 10 \G *************************** 1. row *************************** id: 1 select…(cat_id) where cat_id=5 and seller_id=1 order by price desc limit 10 \G *************************** 1. row *************************** id: 1 select…
Post: Using GROUP BY WITH ROLLUP for Reporting Performance Optimization
September 17, 2007 By 12 Comments
… rows which are already traversed and counted for group by operation. The obvious optimization is to get rid of LIMIT 10 and… “a%” group by grp with rollup order by cnt desc limit 11; ERROR 1221 (HY000): Incorrect usage of CUBE/ROLLUP and ORDER BY Oops. Bad luck – for some reason you can’t use order by together with ROLLUP. This does…
Post: Is it query which needs to be optimized ?
March 6, 2007 By 9 Comments
… are couple of examples: GROUP BY Consider SELECT COUNT(*) cnt, page FROM log GROUP BY page ORDER BY cnt DESC limit 10, the query… as possible. ORDER BY I already wrote about ORDER BY LIMIT Optimization so I will not repeat it here. In the nutshell ORDER BY LIMIT can be… ORDER BY, these also can cause a lot of troubles. So designing schema make sure for queries you’re going to run ORDER BY…
