PostgreSQL子查詢最佳化詳解

1.2.2 PostgreSQL

1.2.2.1 S6 語句

查看查詢執行計畫，子查詢被最佳化（採用物化的方式，使得子查詢結果被緩衝，既子查詢只執行一次）。

postgres=# EXPLAIN SELECT * FROM t3 WHERE b3 >= ANY (SELECT b1 FROM t1);

Nested Loop Semi Join   (cost=0.00..41707.39 rows=680 width=12)

Join Filter: (t3.b3 >= t1.b1)

->   Seq Scan on t3   (cost=0.00..30.40 rows=2040 width=12)

->   Materialize   (cost=0.00..40.60 rows=2040 width=4)

->   Seq Scan on t1   (cost=0.00..30.40 rows=2040 width=4)

1.2.2.2 S7 語句

查看查詢執行計畫，子查詢被最佳化（採用半串連）。

postgres=# EXPLAIN SELECT * FROM t3 WHERE b3 >= ANY (SELECT a1 FROM t1);

Nested Loop Semi Join   (cost=0.15..377.82 rows=680 width=12)

->   Seq Scan on t3   (cost=0.00..30.40 rows=2040 width=12)

->   Index Only Scan using t1_a1_key on t1   (cost=0.15..12.08 rows=680 width=4)

Index Cond: (a1 <= t3.b3)

1.2.2.3 S8 語句

查看查詢執行計畫，子查詢被最佳化（採用半串連）。

postgres=# EXPLAIN SELECT * FROM t3 WHERE b3 <= SOME (SELECT a1 FROM t1);

Nested Loop Semi Join   (cost=0.15..377.82 rows=680 width=12)

->   Seq Scan on t3   (cost=0.00..30.40 rows=2040 width=12)

->   Index Only Scan using t1_a1_key on t1   (cost=0.15..12.08 rows=680 width=4)

Index Cond: (a1 >= t3.b3)

1.2.2.4 S9 語句

查看查詢執行計畫，子查詢被最佳化。

postgres=# EXPLAIN SELECT * FROM t3 WHERE b3 = SOME (SELECT a1 FROM t1);

Hash Semi Join   (cost=55.90..103.00 rows=1020 width=12)

Hash Cond: (t3.b3 = t1.a1)

->   Seq Scan on t3   (cost=0.00..30.40 rows=2040 width=12)

->   Hash   (cost=30.40..30.40 rows=2040 width=4)

->   Seq Sc