… of performance, or of accuracy of randomness. The ORDER BY RAND() solution is known to have poor performance,…columns in a compound index, then the order would matter, and it would have to match the order of columns…to determine the last episode of each TV show. Couldn’t you have used a SUBQUERY to…
Post: The small improvements of MySQL 5.6: Duplicate Index Detection
May 31, 2013 By Leave a Comment
…: 0 Duplicates: 0 Warnings: 1 mysql> SHOW WARNINGS\G *************************** 1. row *************************** Level: Note Code:… it has the exact column definition in the same order. In other words, …set of MySQL utilities were introduced by Oracle recently, which includes mysqlindexcheck, …
Post: ORDER BY ... LIMIT Performance Optimization
September 1, 2006 By 100 Comments
…order by by last column, in fact index can be used for ORDER BY if sorting is done by leading column(s). Note however columns following column used for order by… 1 row in set (0.03 sec) See – “rows” is showing us there are estimated 1.6 million …
Post: How much overhead is caused by on disk temporary tables
August 16, 2007 By 26 Comments
…”c” column. mysql> explain select count(*) cnt,c from gt group by c order by null limit 10; … how it happens in practice. mysql> show global status like “key%”; +————————+———+ | Variable_name | Value | …
Post: Feature Idea: Finding columns which query needs to access
November 17, 2006 By 16 Comments
… some data by denormalizing tables. So far it has to be done manually – look at SELECT clause, WHERE clause, ORDER BY GROUP BY and… tool would show mapping of WHERE clauses to the tables as well as other clauses such as GROUP BY and ORDER BY. For complex queries especially if they do not refer to the columns with table prefix it…
Post: Implications of Metadata Locking Changes in MySQL 5.5
February 1, 2013 By 9 Comments
…COLUMN c; session7 > select * from test_2 order by id; session8 > select * from test_2 order by id; session9 > select * from test_2 order by…
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 key columns) Buffer each pk_column… reporting high numbers for Innodb_buffer_pool_read_ahead which shows that the access pattern was sequential and hence InnoDB decided…
Post: A case for MariaDB's Hash Joins
May 31, 2012 By Leave a Comment
…8G. Let me show the breakdown of dataset size by the tables that …> ’1996-05-01′ GROUP BY o_orderkey ORDER BY num_items DESC LIMIT 10; …by the column l_orderkey which is the left-most PK column, so reading the table orders in PK order…
Post: Eventual Consistency in MySQL
November 18, 2011 By 8 Comments
…_COLUMN_NAME: B 2 rows in set (0.00 sec) This shows that test.Bar has columns X,Y which reference test.Foo columns….CONSTRAINT_NAME, ‘.’, K.COLUMN_NAME) ORDER BY K.ORDINAL_POSITION), ‘) = (‘, GROUP_CONCAT(CONCAT(K.REFERENCED_TABLE_NAME, ‘.’, K.REFERENCED_COLUMN_NAME) ORDER BY K.ORDINAL_POSITION…
Post: MySQL Indexing Best Practices: Webinar Questions Followup
August 16, 2012 By 4 Comments
… column and using order by on another column. do i need to add the index on column using order by clause. A: If index is used for ORDER BY… filesort for group by by this column. This works because by scanning data in index order MySQL gets data in already sorted order and looks at… the leaf nodes? your slide showed single but the fact that you can use and index for “order by desc” indicates a double…
