… Access Traditionally, MySQL always uses Nested Loop Join to join two or more tables. What this means is that, select rows from first… as probe step. This join algorithm works best for highly selective queries, and obviously when the first operand used in the build…. You can read more about these variables here. The query used is: select l_orderkey, sum(l_extendedprice * (1 – l_discount)) as…
Post: High-Performance Click Analysis with MySQL
December 22, 2008 By 13 Comments
… denormalization is important is that nested-loop joins on large data sets are very expensive. If MySQL supported sort-merge or…. If you do it on the master with INSERT..SELECT queries, it will propagate to the slaves and it’ll be… out-of-the-box than MySQL is, especially for more complex queries. Percona is not tied to MySQL, although we’re most famous…
Post: MySQL: Followup on UNION for query optimization, Query profiling
August 14, 2006 By 14 Comments
…for using full key. Be careful however with these nested IN clauses. MySQL has to internally build all possible combinations for row…exactly happens during query execution – very helpful for MySQL Performance optimization: mysql> flush status; Query OK, 0 rows affected (0.00 sec) mysql> SELECT sql_no…
Post: MySQL 5.5 and MySQL 5.6 default variable values differences
February 21, 2013 By 8 Comments
… values from MySQL 5.5.30 and MySQL 5.6.10 to the different tables and ran the query: mysql [localhost] {msandbox} (test) > select var55… my opinion. MySQL 5.6 adds a lot more optimizer switches which you can play with: mysql [localhost] {msandbox} (test) > select * from var55…_condition_pushdown=on,mrr=on,mrr_cost_based=on,block_nested_loop=on,batched_key_access=off,materialization=on,semijoin=on…
Post: Why MySQL could be slow with large tables ?
June 9, 2006 By 141 Comments
…mysql> select count(pad) from large; +————+ | count(pad) | +————+ | 31457280 | +————+ 1 row in set (4 min 58.63 sec) mysql> select…MySQL single query runs as single thread (with exeption of MySQL Cluster) and MySQL issues IO requests one by one for query… using nested loops is …
Post: Moving Subtrees in Closure Table Hierarchies
February 14, 2011 By 18 Comments
… subtrees can be tricky in both Closure Table and the Nested Sets model. It can be easier with the Adjacency… IN (SELECT descendant FROM TreePaths WHERE ancestor = ‘D’) AND ancestor NOT IN (SELECT descendant FROM TreePaths WHERE ancestor = ‘D’); But MySQL complains: “You… can’t DELETE and SELECT from the same table in a single query in MySQL. But we can use MySQL‘s multi-table DELETE…
Post: How adding another table to JOIN can improve performance ?
August 1, 2008 By 12 Comments
… us now replace the range with IN list in this query: mysql> explain select sum(events) from info where d IN(’2007-01… is estimated same in original query) and we get the range clause on “info” table instead nested loops join – exactly what we…: mysql> select * from g; +—–+ | gr | +—–+ | 10 | | 20 | | 30 | | 40 | | 50 | | 60 | | 70 | | 80 | | 90 | | 100 | +—–+ 10 rows in set (0.00 sec) mysql…
Post: How is join_buffer_size allocated?
July 5, 2010 By 9 Comments
When examining MySQL configuration, we quite often want to know how various buffer … use an index. This is because we know that the nested loop is effectively going to do a table scan on… = b.col2 You can see these un-indexed queries in SHOW STATUS as Select_full_join, and you want zero of them…
Post: 3 ways MySQL uses indexes
September 12, 2009 By 28 Comments
… which is not very selective. A lot of this mistakes come from missing one very important MySQL limitation – once MySQL runs into the… improved query performance. Beware however of large nested enumerated ranges they are very hard on the optimizer. This just describes how MySQL uses…
Post: A case for MariaDB's Hash Joins
May 31, 2012 By Leave a Comment
…Nested Loop (BNL) Join of MySQL 5.5.24, Batched Key Access (BKA) Join of MySQL 5.6.5 and Block Nested…innodb_flush_method=O_DIRECT query_cache_size=0 query_cache_type=0 MySQL 5.6.5 Configuration innodb… its EXPLAIN output as returned by MySQL 5.5 is as follows: SELECT s_nationkey, l_shipmode, count(*)…
