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If you execute frequently those kind of queries: SELECT C1 FROM tbl WHERE source = 'x' and state = 'y' Then it might be advisable to build an additional index on both source and state. Maybe this question Should I create an index for non key columns? might help you to understand this concept.


Cardinality Even with high cardinality, the tipping point used by the MySQL Query Optimizer is either the key distribution or the storage engine. Back on November 13, 2012, I discussed how lopsided keys can make the Query Optimizer choose different indexes (sometime not choose and index at all) : Must an index cover all selected columns for it to be used ...


The valid syntax is close to your second try, but you need to escape the column names with backticks not with single quotes: ALTER TABLE `blog` CHANGE COLUMN `read-more` `read_more` VARCHAR(255) NOT NULL;

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