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。
本文转自健哥的数据花园博客园博客，原文链接：http://www.cnblogs.com/gaojian/archive/2013/06/12/3132581.html，如需转载请自行联系原作者