… compromise, either of performance, or of accuracy of randomness. The ORDER BY RAND() solution is known to have poor performance, but it… the order of columns in the index. Example: WHERE (last_name, first_name) = (‘Karwin’, ‘Bill’); Q: On the Dynamic Pivot, the straight join… did not use the STRAIGHT_JOIN, the query optimizer reordered the tables. It seemed to prefer an index-scan of 7 rows in…
Post: ORDER BY ... LIMIT Performance Optimization
September 1, 2006 By 100 Comments
… – expressions or functions will block index usage for order by. Sort by column in leading table if you have JOIN with ORDER BY … LIMIT you should try… is possible to use index to do ORDER BY with JOIN MySQL still will not be able to use it as Optimizer is not smart… case there is index (k,j) on the table so indexes could be used on each of the tables to optimize order by, or at…
Post: Join Optimizations in MySQL 5.6 and MariaDB 5.5
April 4, 2012 By 4 Comments
…: optimizer_switch=’index_condition_pushdown=off’ optimizer_switch=’mrr=on’ optimizer_switch=’mrr_sort_keys=on’ optimizer_switch=’mrr_cost_based=off’ optimizer_switch=’join_… order by revenue desc, o_orderdate LIMIT 10; In-memory workload Now let’s see how effective are the join optimizations when …
Post: A case for MariaDB's Hash Joins
May 31, 2012 By Leave a Comment
… query_cache_type=0 optimizer_switch=’index_condition_pushdown=on’ optimizer_switch=’mrr=on’ optimizer_switch=’mrr_sort_keys=on’ optimizer_switch=’mrr_cost_based… orders AS o INNER JOIN lineitem AS l ON o_orderkey=l_orderkey WHERE o_orderdate > ’1996-05-01′ GROUP BY o_orderkey ORDER BY…
Post: MySQL Indexing Best Practices: Webinar Questions Followup
August 16, 2012 By 4 Comments
… need an index on a field which is used to join tables in table2 Q: In regards to extending an index being better… ORDER BY B will use index (A,B) for sorting optimization, for more complicated conditions you will need to use something like Trick “Unionizing Order by… both previous and next pages in index order. Note however it is not really requirement for ORDER BY DESC optimization – you can still traverse BTREE…
Post: Joining on range? Wrong!
May 17, 2010 By 10 Comments
… `tags`, there is index on (`itm_prd_id`, `itm_order_timestamp`) in `items_ordered` and indexes on other columns used in joins. Let’s verify… to optimize those simple conditions to match both product id and date range by index and read only the relevant rows. This affects joins…, ADD INDEX itm_prd_id__and__itm_order_date (itm_prd_id, itm_order_date); UPDATE items_ordered SET itm_order_date = DATE(itm_order_timestamp…
Post: Is it query which needs to be optimized ?
March 6, 2007 By 9 Comments
… are using indexes, try to have queries index covered and avoid joins as 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…
Post: Database access Optimization in Web Applications.
September 12, 2006 By 10 Comments
… GROUP BY Queries and Sort Queries – SELECT name,descr FROM titles ORDER BY rank DESC LIMIT 10 – If there is no proper index and… just because schema is not optimally indexed – this is easy. For example our ORDER BY rank query is such – adding index on rank column maxes… be obsessed with this. I’ve seen people trying to join all queries in single UNION (with padding to accommodate different…
Post: The MySQL optimizer, the OS cache, and sequential versus random I/O
April 28, 2008 By 17 Comments
… with one join order takes an hour with another, and the optimizer chose the poorer of the two join orders. Why is one join order so much slower than the other, and why did the optimizer… the indexes are fully in memory. When you divide this by 100 I/O operations per second, and then divide that by 86400…
Post: Multi Range Read (MRR) in MySQL 5.6 and MariaDB 5.5
March 21, 2012 By 10 Comments
…optimizer_switch=’index_condition_pushdown=off’ optimizer_switch=’mrr=on’ optimizer_switch=’mrr_sort_keys=on’ (only on MariaDB 5.5) optimizer…by c_custkey, c_name, c_acctbal, c_phone, n_name, c_address, c_comment order by… works only with joins and specifically with Block Access Join Algorithms. So…
