7.2.4. WHERE Clause Optimization
This section discusses optimizations that can be made for
processing WHERE clauses. The examples use
SELECT statements, but the same optimizations
apply for WHERE clauses in
DELETE and UPDATE
statements.
Work on the MySQL optimizer is ongoing, so this section is
incomplete. MySQL performs a great many optimizations, not all
of which are documented here.
Some of the optimizations performed by MySQL follow:
-
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)
-
Constant folding:
(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 for MyISAM and
MEMORY tables. This is also done for any
NOT NULL 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 do not use GROUP
BY or aggregate functions
(COUNT(), MIN(), and
so on).
For each table in a join, a simpler WHERE
is constructed to get a fast WHERE
evaluation for the table and also to skip rows as soon as
possible.
-
All constant tables are read first before any other tables
in the query. A constant table is any of the following:
An empty table or a table with one row.
A table that is used with a WHERE
clause on a PRIMARY KEY or a
UNIQUE index, where all index parts
are compared to constant expressions and are defined as
NOT NULL.
All of the following tables are used as constant tables:
SELECT * FROM t WHERE primary_key=1;
SELECT * FROM t1,t2
WHERE t1.primary_key=1 AND t2.primary_key=t1.id;
The best join combination for joining the tables is found by
trying all possibilities. If all columns in ORDER
BY and GROUP BY clauses come
from the same table, that 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 GROUP BY
contains columns from tables other than the first table in
the join queue, a temporary table is created.
If you use the SQL_SMALL_RESULT option,
MySQL uses an in-memory temporary table.
Each table index is queried, and the best index is used
unless the optimizer believes that it is more efficient to
use a table scan. At one time, a scan was used based on
whether the best index spanned more than 30% of the table,
but a fixed percentage no longer determines the choice
between using an index or a scan. The optimizer now is more
complex and bases its estimate on additional factors such as
table size, number of rows, and I/O block size.
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, only the index tree is used to resolve
the query.
Before each row is output, those that do not match the
HAVING clause are skipped.
Some examples of queries that are very fast:
SELECT COUNT(*) FROM tbl_name;
SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name;
SELECT MAX(key_part2) FROM tbl_name
WHERE key_part1=constant;
SELECT ... FROM tbl_name
ORDER BY key_part1,key_part2,... LIMIT 10;
SELECT ... FROM tbl_name
ORDER BY key_part1 DESC, key_part2 DESC, ... LIMIT 10;
MySQL resolves the following queries using only the index tree,
assuming that the indexed columns are numeric:
SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val;
SELECT COUNT(*) FROM tbl_name
WHERE key_part1=val1 AND key_part2=val2;
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:
SELECT ... FROM tbl_name
ORDER BY key_part1,key_part2,... ;
SELECT ... FROM tbl_name
ORDER BY key_part1 DESC, key_part2 DESC, ... ;
