tgabi, MySQL Will only use only first column of multiple column index in case you have Range on the first column like col1 BETWEEN 5 and 10 – it is exactly skill of creating multi column indexes to ensure they… makes hell a lot of difference) Saying MySQL can’t use multiple column indexes is simular to saying MySQL always does full table scan – if…
Comment: How to find wrong indexing with glance view
August 22, 2008 By 27 Comments
…. Speaking about index merge – indeed there are cases when index merge works when multiple column index does not however if they both work multiple column index are most likely to be the winner. There are always exceptions. Even running with MySQL defaults can be…
Post: 3 ways MySQL uses indexes
September 12, 2009 By 28 Comments
… see how many index parts are actually used for row lookup. Very common problem I see is multi column indexes which are used… MySQL uses single index – there are more complex rules of how indexes will be used if you look at multiple indexes usage with “index merge” Using Index… to couple of columns index can be simply much smaller than the data which is one of the reason covering indexes help to…
Post: Edge-case behavior of INSERT...ODKU
October 27, 2012 By 6 Comments
… the columns from the INSERT part of the statement corresponding to the columns in the secondary index, and …to be considered as being defined first. So, MySQL checks our INSERT, sees that the next …our INSERT … ODKU on a table with multiple unique indexes such as this one would be equivalent to …
Post: Multi-Column IN clause - Unexpected MySQL Issue
April 4, 2008 By 21 Comments
… massive amount of urls. To save on indexes instead of using URL we index CRC32 of the URL which allows to find… using multiple column IN makes MySQL to pick doing full table scan in this case, even though the cardinality on the first column is… used MySQL feature out there. For given application case we could simply rewrite query using more standard single column IN clause: mysql> explain…
Post: Wow. My 6 year old MySQL Bug is finally fixed in MySQL 5.6
August 30, 2012 By 2 Comments
… still being with MySQL) and running MySQL 4.1 I…index | +—-+————-+——-+——+—————+——+———+——-+——-+————————–+ 1 row in set, 1 warning (0.01 sec) 4bil is out of range for unsigned column… which would use multiple table and the column type was …
Comment: Multiple column index vs multiple indexes
August 26, 2008 By 13 Comments
What about the impact of multiple column index on ORDER BY speed ? e.g. : SELECT * FROM a WHERE b=1 AND c=2 ORDER BY d Here, an index on (b,c,d) will avoid MySQL using filesorting. If three indexes are put on b, c and d, will MySQL be able to use index merge on d to avoid filesorting ?
Comment: Database problems in MySQL/PHP Applications
August 11, 2006 By 13 Comments
…the database to do row lookups by multiple columns of varchars is likely. Don’t do …traditional database background. > Things are different with MySQL. Good design will help dictate how …re here to help. Partial agreement: “Use Indexes smartly.” Indexing columns that have very low cardinality will …
Post: Join Optimizations in MySQL 5.6 and MariaDB 5.5
April 4, 2012 By 4 Comments
…, then worst case you could be reading the same page multiple times into the buffer pool. So considering this drawback, Batched… point index lookups to one or more index range lookups. This means MySQL can employ many other optimizations like for example if columns other then the secondary key columns are…
Comment: How to find wrong indexing with glance view
August 23, 2008 By 27 Comments
…’, mysql can use either indexes built on multiple columns or intersection of multiple accesses by indexes. When we talk about index_merge_intersection, we should understand that mysql finds one set using an index…
