(This section is incomplete; MySQL does many optimizations.)
In general, when you want to make a slow
SELECT ... WHERE faster, the
first thing to check is whether or not you can add an index. All references
between different tables should usually be done with indexes. You can use the
EXPLAIN command to determine which indexes are used for a
Some of the optimizations performed by MySQL are listed below:
Removal of unnecessary parentheses:
((a AND b) AND c OR (((a AND b) AND (c AND d))))
-> (a AND b AND c) OR (a AND b AND c AND d)
(a<b AND b=c) AND a=5
-> b>5 AND b=c AND a=5
Constant condition removal (needed because of constant folding):
(B>=5 AND B=5) OR (B=6 AND 5=5) OR (B=7 AND 5=6)
-> B=5 OR B=6
Constant expressions used by indexes are evaluated only once.
COUNT(*) on a single table without a
WHERE is retrieved
directly from the table information. This is also done for any
expression when used with only one table.
Early detection of invalid constant expressions. MySQL quickly
detects that some
SELECT statements are impossible and returns no rows.
HAVING is merged with
WHERE if you don't use
BY or group functions (
For each sub join, a simpler
WHERE is constructed to get a fast
WHERE evaluation for each sub join and also to skip records as
soon as possible.
All constant tables are read first, before any other tables in the query.
A constant table is:
All the following tables are used as constant tables:
An empty table or a table with 1 row.
A table that is used with a
WHERE clause on a
UNIQUE index or a
PRIMARY KEY, where all index parts are used with constant expressions.
mysql> SELECT * FROM t WHERE primary_key=1;
mysql> SELECT * FROM t1,t2
WHERE t1.primary_key=1 AND t2.primary_key=t1.id;
The best join combination to join the tables is found by trying all
possibilities :(. If all columns in
ORDER BY and in
come from the same table, then this table is preferred first when joining.
If there is an
ORDER BY clause and a different
GROUP BY clause,
or if the
ORDER BY or
contains columns from tables other than the first table in the join
queue, a temporary table is created.
If you use
SQL_SMALL_RESULT, MySQL will use an in-memory
DISTINCT is converted to a
GROUP BY on all columns,
DISTINCT combined with
ORDER BY will in many cases also need
a temporary table.
Each table index is queried and the best index that spans fewer than 30% of
the rows is used. If no such index can be found, a quick table scan is used.
In some cases, MySQL can read rows from the index without even
consulting the data file. If all columns used from the index are numeric,
then only the index tree is used to resolve the query.
Before each record is output, those that do not match the
Some examples of queries that are very fast:
mysql> SELECT COUNT(*) FROM tbl_name;
mysql> SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name;
mysql> SELECT MAX(key_part2) FROM tbl_name
mysql> SELECT ... FROM tbl_name
ORDER BY key_part1,key_part2,... LIMIT 10;
mysql> SELECT ... FROM tbl_name
ORDER BY key_part1 DESC,key_part2 DESC,... LIMIT 10;
The following queries are resolved using only the index tree (assuming
the indexed columns are numeric):
mysql> SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val;
mysql> SELECT COUNT(*) FROM tbl_name
WHERE key_part1=val1 AND key_part2=val2;
mysql> SELECT key_part2 FROM tbl_name GROUP BY key_part1;
The following queries use indexing to retrieve the rows in sorted
order without a separate sorting pass:
mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,...
mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,...