DISTINCT Optimization

DISTINCT combined with ORDER BY needs a temporary table in many cases.

Because DISTINCT may use GROUP BY, learn how MariaDB works with columns in ORDER BY or HAVING clauses that are not part of the selected columns. See , "GROUP BY and HAVING with Hidden Columns".

In most cases, a DISTINCT clause can be considered as a special case of GROUP BY. For example, the following two queries are equivalent:

SELECT DISTINCT c1, c2, c3 FROM t1
WHERE c1 > const;
SELECT c1, c2, c3 FROM t1
WHERE c1 > const GROUP BY c1, c2, c3;

Due to this equivalence, the optimizations applicable to GROUP BY queries can be also applied to queries with a DISTINCT clause. Thus, for more details on the optimization possibilities for DISTINCT queries, see , "GROUP BY Optimization".

When combining LIMIT row_count with DISTINCT, MariaDB stops as soon as it finds row_count unique rows.

If you do not use columns from all tables named in a query, MariaDB stops scanning any unused tables as soon as it finds the first match. In the following case, assuming that t1 is used before t2 (which you can check with EXPLAIN), MariaDB stops reading from t2 (for any particular row in t1) when it finds the first row in t2:

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