… is MRR Key-ordered Scan. Let’s see what this optimization actually is. Key-ordered Scan for BKA With this optimization the idea of…_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 the…
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…_nationkey group by c_custkey, c_name, c_acctbal, c_phone, n_name, c_address, c_comment order by revenue desc LIMIT 20; In…
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, 1 Can… does not. I just see this note: With EXPLAIN SELECT … ORDER BY, you can check whether MySQL can use indexes to resolve…
Post: ORDER BY ... LIMIT Performance Optimization
September 1, 2006 By 93 Comments
… ORDER BY implementation, especially together with LIMIT is often the cause of MySQL Performance problems. Here is what you 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 large…
Post: Possible optimization for sort_merge and UNION ORDER BY LIMIT
September 18, 2007 By 10 Comments
… get resulted rows in sorted order. Once this is implemented similar approach could be used for optimizing ORDER BY with IN. This original query… where c1=5 order by ord desc limit 10) union (select * from utest where c2=5 order by ord desc limit 10) order by ord desc limit 10 \G… fact MySQL is unable to handle even basic UNION with limit (without order by) optimally – in creates result set for the union fully and…
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 to… much 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 very… ORDER BY, these also can cause a lot of troubles. So designing schema make sure for queries you’re going to run ORDER BY .. LIMIT…
Post: Four ways to optimize paginated displays
September 24, 2008 By 26 Comments
…? I typically see queries like this: select …. from … order by …. limit X, 20 If the ORDER BY can’t use an index (commonly the case…. There are ways to optimize so you don’t have to do quite so much offsetting and limiting. I wrote about this… list. With that in mind, here are four suggestions for optimizing paginated displays that can give significantly better performance. On the…
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… goods where cat_id=5 and seller_id=1 order by price desc limit 10 \G *************************** 1. row *************************** id: 1 select_type: SIMPLE…
Post: Using GROUP BY WITH ROLLUP for Reporting Performance Optimization
September 17, 2007 By 12 Comments
… already traversed and counted for group by operation. The obvious optimization is to get rid of LIMIT 10 and just fetch all groups… “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…
Post: MySQL: Followup on UNION for query optimization, Query profiling
August 14, 2006 By 12 Comments
… exactly happens during query execution – very helpful for MySQL Performance optimization: mysql> flush status; Query OK, 0 rows affected (0.00…=19 order by last_online desc limit 10) UNION ALL (select * from people where age=20 order by last_online desc limit 10) ORDER BY last_online desc limit…
