Removal of unnecessary parentheses:删除不必要的括号：

   ((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:连续折叠：

   (a<b AND b=c) AND a=5
-> b>5 AND b=c AND a=5

Constant condition removal:恒定条件移除：

   (b>=5 AND b=5) OR (b=6 AND 5=5) OR (b=7 AND 5=6)
-> b=5 OR b=6

In MySQL 8.0.14 and later, this takes place during preparation rather than during the optimization phase, which helps in simplification of joins. 在MySQL 8.0.14及更高版本中，这发生在准备阶段而不是优化阶段，这有助于简化连接。See Section 8.2.1.9, “Outer Join Optimization”, for further information and examples.有关更多信息和示例，请参阅第8.2.1.9节“外部连接优化”。

Constant expressions used by indexes are evaluated only once.索引使用的常量表达式只计算一次。

Beginning with MySQL 8.0.16, comparisons of columns of numeric types with constant values are checked and folded or removed for invalid or out-of-rage values:从MySQL 8.0.16开始，检查数值类型的列与常量值的比较，并折叠或删除无效或超出范围的值：

# CREATE TABLE t (c TINYINT UNSIGNED NOT NULL);
  SELECT * FROM t WHERE c ≪ 256;
-≫ SELECT * FROM t WHERE 1;

See Section 8.2.1.14, “Constant-Folding Optimization”, for more information.详见第8.2.1.14节，“恒定折叠优化”。

COUNT(*) on a single table without a WHERE is retrieved directly from the table information for MyISAM and MEMORY tables. 不带WHERE的单个表上的COUNT(*)直接从MyISAM和内存表的表信息中检索。This is also done for any NOT NULL expression when used with only one table.当只与一个表一起使用时，也会对任何NOT NULL表达式执行此操作。

Early detection of invalid constant expressions. 早期检测无效常量表达式。MySQL quickly detects that some SELECT statements are impossible and returns no rows.MySQL很快检测到一些SELECT语句是不可能的，并且不返回任何行。

HAVING is merged with WHERE if you do not use GROUP BY or aggregate functions (COUNT(), MIN(), and so on).如果不使用GROUP BY或聚焦函数（COUNT()、MIN()等），HAVING将与WHERE合并。

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.对于联接中的每个表，构造一个更简单的WHERE来快速计算表的WHERE，并尽快跳过行。

All constant tables are read first before any other tables in the query. 在查询中，首先读取所有常量表，然后再读取任何其他表。A constant table is any of the following:常数表是下列任一项：

All of the following tables are used as constant tables:以下所有表格都用作常量表格：

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. 通过尝试各种可能性，可以找到连接表的最佳连接组合。If all columns in ORDER BY and GROUP BY clauses come from the same table, that table is preferred first when joining.如果ORDER BY和GROUP BY子句中的所有列都来自同一个表，则在联接时首选该表。

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.如果存在ORDER BY子句和不同的GROUP BY子句，或者ORDER BY或GROUP BY包含联接队列中第一个表以外的表中的列，则会创建一个临时表。

If you use the SQL_SMALL_RESULT modifier, MySQL uses an in-memory temporary table.如果您使用SQL_SMALL_RESULT修饰符，MySQL将使用内存中的临时表。

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. 查询每个表索引，并使用最佳索引，除非优化器认为使用表扫描更有效。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. 有一次，扫描是基于最佳索引是否跨越表的30%以上而使用的，但固定的百分比不再决定使用索引还是扫描的选择。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.现在，优化器更加复杂，它的估计基于其他因素，如表大小、行数和I/O块大小。

In some cases, MySQL can read rows from the index without even consulting the data file. 在某些情况下，MySQL甚至不需要查阅数据文件就可以从索引中读取行。If all columns used from the index are numeric, only the index tree is used to resolve the query.如果索引中使用的所有列都是数字，则只有索引树用于解析查询。

Before each row is output, those that do not match the HAVING clause are skipped.在输出每一行之前，将跳过那些与HAVING子句不匹配的行。