PostgreSQL多表连接方式

多表连接方式
nested loop join
hash join
NATURAL INNER JOIN
LEFT/RIGHT OUTER JOIN
Nested Loop Join
Nested Loop Join图解
testdb=# EXPLAIN SELECT * FROM tbl_a AS a, tbl_b AS b WHERE a.id = b.id;
-----------------------------------------------------------------------
Join Filter: (a.id = b.id)
-> Materialize (cost=0.00..98.00 rows=5000 width=8)
(5 rows)
Materialize成本估算
(Materialized) Nested Loop成本估算
QUERY PLAN
Nested Loop (cost=0.29..1935.50 rows=5000 width=16)
-> Index Scan using tbl_c_pkey on tbl_c c (cost=0.29..0.36 rows=1 width=8)
（4 rows）
具有外部索引扫描的嵌套循环联接的三种变体
Merge Join连接方式
Merge Join成本估算
testdb=# EXPLAIN SELECT * FROM tbl_a AS a, tbl_b AS b WHERE a.id = b.id AND b.id < 1000;
-------------------------------------------------------------------------
Merge Cond: (a.id = b.id)
Sort Key: a.id
-> Sort (cost=135.33..137.83 rows=1000 width=8)
-> Seq Scan on tbl_b b (cost=0.00..85.50 rows=1000 width=8)
(9 rows)
Materialized Merge Join
testdb=# SET enable_hashjoin TO off;
testdb=# EXPLAIN SELECT * FROM tbl_c AS c, tbl_b AS b WHERE c.id = b.id AND b.id < 1000;
--------------------------------------------------------------------------------------
Merge Cond: (c.id = b.id)
-> Sort (cost=135.33..137.83 rows=1000 width=8)
-> Seq Scan on tbl_b b (cost=0.00..85.50 rows=1000 width=8)
(7 rows)
materialized merge join with outer index scan
testdb=# SET enable_nestloop TO off;
QUERY PLAN
Merge Join (cost=421.84..672.09 rows=4500 width=16)
-> Index Scan using tbl_c_pkey on tbl_c c (cost=0.29..318.29 rows=10000 width=8)
-> Sort (cost=421.55..432.80 rows=4500 width=8)
-> Seq Scan on tbl_b b (cost=0.00..85.50 rows=4500 width=8)
(8 rows)
indexed merge join with outer index scan
testdb=# SET enable_nestloop TO off;
QUERY PLAN
Merge Join (cost=0.57..226.07 rows=1000 width=16)
-> Index Scan using tbl_c_pkey on tbl_c c (cost=0.29..318.29 rows=10000 width=8)
Index Cond: (id < 1000)
Hash Join
In-Memory Hash Join
将内部表的所有元组插入到一个批处理中
将外部表的每个元组与批处理中的内部元组进行比较，如果满足连接条件，则进行连接
Hash Join
预处理
2、向上拉子查询
testdb=# SELECT * FROM tbl_a AS a, (SELECT * FROM tbl_b) as b WHERE a.id = b.id;
3、将外部联接转换为内部联接
优化器可用规则
1、表数量小于12张，应用动态规划得到最优的计划
参数 geqo_threshold指定的阈值(默认值为12)
SGetting the Cheapest Path of a Triple-Table Query
testdb=# WHERE a.id = b.id AND b.id = c.id AND a.data < 40;
{tbl_a,tbl_b,tbl_c}=min({tbl_a,{tbl_b,tbl_c}},{tbl_b,{tbl_a,tbl_c}},{tbl_c,{tbl_a,tbl_b}}).