… rows are read from table1, the values of indexed columns that will be used to perform a lookup…i.e. in index order and then the lookup will be performed. So key-ordered scan is basically an… note that the following changes were made on MySQL 5.6 config: optimizer_switch=’index_condition_pushdown=off’…
Post: Multi Range Read (MRR) in MySQL 5.6 and MariaDB 5.5
March 21, 2012 By 10 Comments
…, then steps above are changed to the following: SELECT key_column, pk_column FROM tbl WHERE key_column=x ORDER BY key_column (Note that secondary… that traditional index lookup (for non-index-only columns) involves, reading an index record, and then using the PK column value in the index record to…
Post: Multi Column indexes vs Index Merge
September 19, 2009 By 30 Comments
… values in sorted order. For example when you query AGE=18 with single column BTREE index MySQL will dive into the index to find first matching row and when will continue scanning index in order until… 660ms to execute. now if we change it: If we disable index on the second column we get a full table scan: mysql…
Post: A rule of thumb for choosing column order in indexes
June 5, 2009 By 15 Comments
… of thumb I sometimes use to decide which columns should come first in an index. This is not specific to MySQL, it’s generally applicable to any database server with b-tree indexes. And there are a bunch of subtleties, but I will…
Comment: How to find wrong indexing with glance view
March 8, 2010 By 27 Comments
…(operator_id) ); create index order_date_idx on orders(order_date); Okay so we know that we’ll never delete or change item_type ids… I need an index on order_date to help with finding orders for a given date. 8 columns 6 indexes on single columns. This is silly. You can imagine a table with more columns and…
Post: ANALYZE: MyISAM vs Innodb
September 3, 2008 By 9 Comments
… accuracy at the order of magnitude is enough…Changed: 245760 Warnings: 0 mysql> show index from antest; +——–+————+———-+————–+————-+———–+————-+———-+——–+——+————+———+ | Table | Non_unique | Key_name | Seq_in_index | Column…
Post: 3 ways MySQL uses indexes
September 12, 2009 By 27 Comments
… see how many index parts are actually used for row lookup. Very common problem I see is multi column indexes which are used…ranges MySQL will use both key parts. Hence if you change this predicate to A IN (5,6,7,8,9…=5 ORDER BY B ; A=5 ORDER BY B DESC; A>5 ORDER BY A ; A>5 ORDER BY A,B ; A>5 ORDER BY…
Comment: Database problems in MySQL/PHP Applications
August 11, 2006 By 12 Comments
… completely because InnoDB uses a clustered index based on (in order) the primary key you specify … that all the changes are being made on the master and all those changes are making…’re here to help. Partial agreement: “Use Indexes smartly.” Indexing columns that have very low cardinality will often …
Post: Extending Index for Innodb tables can hurt performance in a surprising way
May 22, 2010 By 9 Comments
… (0.00 sec) The obvious optimization is to extend index from column (a) to column (a,b) right which will make it faster… index changes from mk-duplicate-key-checker key checker when it comes to redundant indexes. If you have query plans depending on Innodb ordering of data by primary key inside indexes they can become…
Post: Shard-Query EC2 images available
May 11, 2011 By 2 Comments
…Note dates may not be in date_id order‘; /*!40101 SET character_set_client = @saved_cs_client …241 Data_length: 1616904192 Max_data_length: 0 Index_length: 539279360 Data_free: 4194304 Auto_increment: …selected in the list of columns to show (click show/hide to change the columns) That the “public dns…
