…of 20.08k queries were captured which are actually invocations of 167 different queries. Following that there are summaries of… UNION…of number of times this query was executed. Now let’s take a look at the values in the 95% column…MySQL. The end result might be that you end up limiting the number of…
Post: Identifying the load with the help of pt-query-digest and Percona Server
December 29, 2011 By 3 Comments
Post: Multi Column indexes vs Index Merge
September 19, 2009 By 31 Comments
…20 work similar way – MySQL just stops at different value. The enumerated ranges such…index on another column. This is because MySQL is able to estimate number of rows it…columns. In this case the combined index is useless and MySQL has an option of doing full table scan or doing the Union (instead of…
Post: UNION vs UNION ALL Performance
October 5, 2007 By 15 Comments
…Numbers would be different but it should not change general point of having optimization of moving LIMIT inside of union…columns with cardinality of 100, having about 40.000.000 of rows select * from test.abc where i=5 union…
Post: Using UNION to implement loose index scan in MySQL
August 10, 2006 By 15 Comments
… 1 row in set (0.00 sec) Note number of rows has decreased from 90556 to …Lets see how query times differ in these cases: mysql> SELECT sql_no_…MySQL Optimizer and change the query so it can handle it well…. use UNION: mysql… or birth instead of age, and put zip as first column in the …
Post: Database access Optimization in Web Applications.
September 12, 2006 By 10 Comments
… links WHERE domain = ‘mysql.com’; will return only one row while may require to scan hundreds of thousands of rows (or index entries… in single UNION (with padding to accommodate different types and number of columns) – this is not a good practice. However if you can reduce number of queries…
Post: ORDER BY ... LIMIT Performance Optimization
September 1, 2006 By 100 Comments
…together with LIMIT is often the cause of MySQL Performance problems. Here is what you …UNION workaround I already wrote about. So what if you have application which can perform search on many different columns…need to rethink your indexing. Also note – number of records scanned to provide result set …
