This post was initially going to be two sets of polls: “What is the maximum number of columns in MySQL?” and “What is the minimum maximum number of columns in MySQL?”. Before you read on, ponder those questions and come up with your own answers… and see if you’re right or can prove me wrong!

Back in 2009, I finished what seemed an epic task in the Drizzle code base: banishing the FRM file. Why? We felt it was not a good idea to keep arbitrary and obscure limitations from the 1980s alive in the 21st century and instead wanted a modular system where the storage engines themselves owned their own metadata. This was a radical departure from the MySQL philosophy, and one that has truly paid off in the Drizzle code base. However… for those using MySQL, Percona Server, MariaDB or any other of the MySQL branches/forks, you …

The purpose of this post is to describe what covering indexes are and how they can be used to improve the performance of queries. People mostly use indexes to filter or sort the results but not much thought is given to actually reduce the disk reads by using proper indexes. So I will show you how to reduce disk reads and hence improve the performance of queries by utilizing indexes properly.

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Certain complex MySQL SELECT and subquery statements will not allow the use of the * wildcard and you will need to fill in the entire column list of a given table. Consider the following simplified example, a SELECT statement that contains 3 columns. The asterisk here refers to all columns, which is actually the 3 columns listed in the GROUP BY clause:

SELECT
 IF(
  EXISTS(
  SELECT *
  FROM (
   SELECT *
   FROM `dbName_A`.`tableName_A`
   UNION ALL
   SELECT *
   FROM `dbName_B`.`tableName_B`
   AS `compareTables`
  GROUP BY `column_1`, `column_2`, `column_3`
  HAVING COUNT(*) = 1),
 1, 0);

Imagine if it were dozens of columns instead of just 3. You can't simply put in the * wildcard like 'GROUP BY * '. The above example will not …