MySQL Manual
5 MySQL Optimisation
- 5.2 Optimising
SELECTs and Other Queries- 5.2.1
EXPLAINSyntax (Get Information About aSELECT) - 5.2.2 Estimating Query Performance
- 5.2.3 Speed of
SELECTQueries - 5.2.4 How MySQL Optimises
WHEREClauses - 5.2.5 How MySQL Optimises
IS NULL - 5.2.6 How MySQL Optimises
DISTINCT - 5.2.7 How MySQL Optimises
LEFT JOINandRIGHT JOIN - 5.2.8 How MySQL Optimises
ORDER BY - 5.2.9 How MySQL Optimises
LIMIT - 5.2.10 Speed of
INSERTQueries - 5.2.11 Speed of
UPDATEQueries - 5.2.12 Speed of
DELETEQueries - 5.2.13 Other Optimisation Tips
- 5.2.1
5.2.6 How MySQL Optimises DISTINCT
DISTINCT is converted to a GROUP BY on all columns,
DISTINCT combined with ORDER BY will in many cases also
need a temporary table.
When combining LIMIT row_count with DISTINCT, MySQL will stop
as soon as it finds row_count unique rows.
If you don't use columns from all used tables, MySQL will stop the scanning of the not used tables as soon as it has found the first match.
SELECT DISTINCT t1.a FROM t1,t2 where t1.a=t2.a;
In the case, assuming t1 is used before t2 (check with
EXPLAIN), then MySQL will stop reading from t2 (for that
particular row in t1) when the first row in t2 is found.
