1. Optimize the where statement
Although it is the where condition of the optimized select statement; it also applies to update and delete. Also avoid optimizations that speed up arithmetic operations at the expense of readability, because mysql does such optimizations; mysql performs the following optimizations:
★
Remove unnecessary brackets
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((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)
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★
Constant folding: an optimization technique used in modern compilers
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(a<b AND b=c) AND a=5
→
b>5 AND b=c AND a=5
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★
Remove constant condition (required for constant folding)
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(B>=5 AND B=5) OR (B=6 AND 5=5) OR (B=7 AND 5=6)
→
B=5 OR B=6
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★
Constant expressions used by index are evaluated only once.
★
In a single table without where condition, COUNT(*) is directly obtained from the table information for MyISAM and MEMORY tables. The same is true for single tables with NOT NULL expressions.
★
Early detection of invalid constant expressions. MySQL quickly detects certain SELECT statements that are impossible and return no rows.
★
If you do not use GROUP BY or aggregate functions (
COUNT()
,
MIN()
, etc.), HAVING will be merged by WHERE.
★ 对于连接中的每个表，WHERE构造一个更简单 的表达式以快速 对标WHERE评估，并尽快跳过行。
★ 在查询中的所有常量表要先于其他表被读取。常数表是以下任何一项：
1）空表或只有一行的表
2）使用主键或唯一索引查询的表，所有索引部分与常量表达式比较，且被定义为NOT NULL。
|
常量表
SELECT
*
FROM
t
WHERE
primary_key
=
1
;
SELECT
*
FROM
t1
,
t2
WHERE
t1
.
primary_key
=
1
AND
t2
.
primary_key
=
t1
.
id
;
|
★ 通过尝试所有可能性来找到加入表格的最佳联合组合。如果 ORDER BY和GROUP BY子句的所有列来自同一个表，那么在连接时首先选择该表。
★ 如果有一个ORDER BY子句和一个不同的GROUP BY子句，或者如果 ORDER BY或GROUP BY 包含表的列不是连接队列表的列，则会创建一个临时表。
★ 如果你使用SQL_SMALL_RESULT 修饰符，MySQL使用内存中的临时表
★ 除非优化器认为使用索引比全表扫描更高效，查询才会使用索引进行查询。曾经，是否超过表格的30％作为是否使用索还是使用全表扫描，但是固定百分比不再决定使用索引或全表扫描。优化器现在更加复杂，并基于其他因素，如表大小，行数和I / O块大小。
★ 在某些情况下，MySQL可以从索引中读取行，而无需咨询数据文件。如果索引中使用的所有列都是数字，则仅使用索引树来解析查询。
★ 在输出每行之前，不匹配 HAVING子句的内容将被跳过。
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查询速度很快的例子：
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;
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假设索引列是数字的，MySQL仅使用索引树来解析以下查询：
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;
|
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以下查询使用索引来按照排序顺序检索行，而无需单独的排序传递：
SELECT ... FROM
tbl_name
ORDER BY
key_part1
,
key_part2
,... ;
SELECT ... FROM
tbl_name
ORDER BY
key_part1
DESC,
key_part2
DESC, ... ;
|