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MySQL - Speed of SELECT Queries ?
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 SELECT.
Some general tips:
To help MySQL optimize queries better, run myisamchk
--analyze on a table after it has been loaded with
relevant data. This updates a value for each index part
that indicates the average number of rows that have the
same value. (For unique indexes, this is always 1, of
course.). MySQL will use this to decide which index to
choose when you connect two tables with 'a non-constant
expression'. You can check the result from the analyze
run by doing SHOW INDEX FROM table_name and examining
the Cardinality column.
To sort an index and data according to an index, use
myisamchk --sort-index --sort-records=1 (if you want to
sort on index 1). If you have a unique index from which
you want to read all records in order according to that
index, this is a good way to make that faster. Note,
however, that this sorting isn't written optimally and
will take a long time for a large table!
How MySQL Optimizes WHERE Clauses ?
The WHERE optimizations are put in the SELECT part here
because they are mostly used with SELECT, but the same
optimizations apply for WHERE in DELETE and UPDATE
Also note that this section is incomplete. MySQL does
many optimizations, and we have not had time to document
Some of the optimizations performed by MySQL are listed
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
(B>=5 AND B=5) OR (B=6 AND 5=50) OR (B=7 AND 5=6)
-> B=5 OR B=6
Constant expressions used by indexes are evaluated only
COUNT(*) on a single table without a WHERE is retrieved
directly from the table information. This is also done
for any NOT NULL expression when used with only one
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 GROUP BY or
group functions (COUNT(), MIN()...).
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:
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 and the index parts are
defined as NOT NULL.
All the following tables are used as constant tables:
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 GROUP BY 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 GROUP BY 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
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 HAVING clause are skipped.
Some examples of queries that are very fast:
mysql> SELECT COUNT(*) FROM tbl_name;
mysql> SELECT MIN(key_part1),MAX(key_part1) FROM
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
mysql> SELECT COUNT(*) FROM tbl_name
WHERE key_part1=val1 AND key_part2=val2;
mysql> SELECT key_part2 FROM tbl_name GROUP BY
The following queries use indexing to retrieve the rows
in sorted order without a separate sorting pass:
mysql> SELECT ... FROM tbl_name ORDER BY
. mysql> SELECT ... FROM tbl_name ORDER BY key_part1
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