# Logical transformations:
- Outer to inner joins transformation.
- Negation elimination. ##### 否认消除 not ( t1.a > 10 OR t2.b > 3) => (t1.a <=10 AND t2.b <= 3)
- Equality/constant propagation. ##### 等式和常量传播 t1.a = t2.a AND t2.a = 9 AND (t1.a <= 10 AND t2.b <=3 OR (t1.b = t2.b + 7 AND t2.b = 5)) => t1.a = 9 AND t2.a = 9 AND (9 <= 10 AND t2.b <=3 OR (t1.b = 5 + 7 AND t2.b = 5))
- Evaluate const expressions. ##### 求常量值 t1.a = 9 AND t2.a = 9 AND (9 <= 10 AND t2.b <=3 OR (t1.b = 5 + 7 AND t2.b = 5)) => t1.a = 9 AND t2.a = 9 AND (9 <= 10 AND t2.b <=3 OR (t1.b = 12 AND t2.b = 5))
- Trivial condtion removal. ##### 不重要的条件移除 t1.a = 9 AND t2.a = 9 AND (9 <= 10 AND t2.b <=3 OR (t1.b = 12 AND t2.b = 5)) => t1.a = 9 AND t2.a = 9 AND (t2.b <=3 OR (t1.b = 12 AND t2.b = 5))
- Partition pruning. ##### 分区修剪是一种非常有效的性能特性。分析修剪分析SQL中的WHERE 和FROM字句，从而在查询中消除不不必要分区
- COUNT(*), MIN(), MAX() constant substitution in case of implicit grouping.
- ORDER BY optimization.
# Perform cost-based optimization of table order and access path selection. See JOIN::make_join_plan()
- Assign cost to operations
The main cost-based optimization:
Index and access method:
Table scan (表扫)
Index scan (索引扫)
Range scan (范围扫)
Index lookup (ref access) (索引查找(通过索引扫再引用访问原表))
Join order ()
Join buffering strategy ()
Subquery strategy ()
Cost Estimate (消耗估算)
The cost for executing a query
Cost unit: 消耗单元
read a random data page from disk 读取随机页
Main cost factors: 消耗因素(CPU IO)
IO cost:
#pages read from table 从表中读页
#pages read from index 从索引中读页
CPU cost:
Evaluating query conditions 评估查询条件
Comparing keys/records 比较键和记录
Sorting keys 排序键
- Assign cost to partial or alternative plans
- Search for plan with lowest cost
