PostgreSQL在何处处理 sql查询之六十

开始

用 explain来看hash join:

postgres=# explain select dept.no_emps,emp.age from dept,emp where emp.name = dept.mgr and dept.dept_name = 'shoe';
QUERY PLAN
------------------------------------------------------------------
Hash Join  (cost=19.30..45.07 rows=23 width=8)
Hash Cond: ((emp.name)::text = (dept.mgr)::text)
->  Seq Scan on emp  (cost=0.00..21.30 rows=1130 width=42)
->  Hash  (cost=19.25..19.25 rows=4 width=42)
->  Seq Scan on dept  (cost=0.00..19.25 rows=4 width=42)
Filter: ((dept_name)::text = 'shoe'::text)
(6 rows)

postgres=#

我在代码里加点调试信息，出现的是这样的结果

---Before T_OpExpr, context is
---Before T_RelabelType, context is (
---For T_RelabelType ,Before get_rule_expr_paren, context is (
---Before T_Var, context is ((
.......In get_Variable, levelsup is: 0
.......var->varno is: 65001
---Before T_Var, context is ((
.......In get_Variable, levelsup is: 0
.......var->varno is: 2
attnum is: 1
-----------------2
-----------------3
______________IN get_variable, attname is :name
---After T_Var, context is ((emp.name
__________________________________________
---After T_Var, context is ((emp.name
__________________________________________
---For T_RelabelType ,After get_rule_expr_paren, context is ((emp.name)::text
---After T_RelabelType, context is ((emp.name)::text
__________________________________________
---Before T_RelabelType, context is ((emp.name)::text =
---For T_RelabelType ,Before get_rule_expr_paren, context is ((emp.name)::text =
---Before T_Var, context is ((emp.name)::text = (
.......In get_Variable, levelsup is: 0
.......var->varno is: 65000
---Before T_Var, context is ((emp.name)::text = (
.......In get_Variable, levelsup is: 0
.......var->varno is: 65001
---Before T_Var, context is ((emp.name)::text = (
.......In get_Variable, levelsup is: 0
.......var->varno is: 1
attnum is: 2
-----------------2
-----------------3
______________IN get_variable, attname is :mgr
---After T_Var, context is ((emp.name)::text = (dept.mgr
__________________________________________
---After T_Var, context is ((emp.name)::text = (dept.mgr
__________________________________________
---After T_Var, context is ((emp.name)::text = (dept.mgr
__________________________________________
---For T_RelabelType ,After get_rule_expr_paren, context is ((emp.name)::text = (dept.mgr)::text
---After T_RelabelType, context is ((emp.name)::text = (dept.mgr)::text
__________________________________________
---After T_OpExpr, context is ((emp.name)::text = (dept.mgr)::text)
__________________________________________
---Before T_OpExpr, context is
---Before T_RelabelType, context is (
---For T_RelabelType ,Before get_rule_expr_paren, context is (
---Before T_Var, context is ((
.......In get_Variable, levelsup is: 0
.......var->varno is: 1
attnum is: 1
-----------------2
-----------------3
______________IN get_variable, attname is :dept_name
---After T_Var, context is ((dept_name
__________________________________________
---For T_RelabelType ,After get_rule_expr_paren, context is ((dept_name)::text
---After T_RelabelType, context is ((dept_name)::text
__________________________________________
---Before T_Const, context is ((dept_name)::text =
---After T_Const, context is ((dept_name)::text = 'shoe'::text
__________________________________________
---After T_OpExpr, context is ((dept_name)::text = 'shoe'::text)
__________________________________________

实际上，explain面对的也是一个二叉树。

对于我的查询，

第一层：

根节点上的是 =，其NodeTag是 T_OpExpr ，其核心就是 = 符号。

左节点是： (emp.name)::text，右节点是：(dept.mgr)::text。

第二层：

对于 (emp.name)::text而言，其NodeTag 是 T_RelableType，其内容核心是 text。

对于(dept.mgr)::text 而言，其NodeTag 是 T_RelableType，其内容核心是 text。

第三层：

对于 (emp.name)而言，其NodeTag是 T_Var，然后会再进入一层，NodeTag仍然是 T_Var。

其核心是 变量 name。它是Outer表的变量。

对于 (dept.mgr)而言，其NodeTag是 T_Var，然后会再进入一层，NodeTag仍然是 T_Var。

其核心是 变量 mgr。它是Inner表的变量。