PostgreSQL 9.2
Hash Semi Join
Ve just arasındaki farkı anlamaya çalışıyorum Hash Join
.
İşte iki sorgu:
ben
EXPLAIN ANALYZE SELECT * FROM orders WHERE customerid IN (SELECT
customerid FROM customers WHERE state='MD');
Hash Semi Join (cost=740.34..994.61 rows=249 width=30) (actual time=2.684..4.520 rows=120 loops=1)
Hash Cond: (orders.customerid = customers.customerid)
-> Seq Scan on orders (cost=0.00..220.00 rows=12000 width=30) (actual time=0.004..0.743 rows=12000 loops=1)
-> Hash (cost=738.00..738.00 rows=187 width=4) (actual time=2.664..2.664 rows=187 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 7kB
-> Seq Scan on customers (cost=0.00..738.00 rows=187 width=4) (actual time=0.018..2.638 rows=187 loops=1)
Filter: ((state)::text = 'MD'::text)
Rows Removed by Filter: 19813
II
EXPLAIN ANALYZE SELECT * FROM orders o JOIN customers c ON o.customerid = c.customerid WHERE c.state = 'MD'
Hash Join (cost=740.34..1006.46 rows=112 width=298) (actual time=2.831..4.762 rows=120 loops=1)
Hash Cond: (o.customerid = c.customerid)
-> Seq Scan on orders o (cost=0.00..220.00 rows=12000 width=30) (actual time=0.004..0.768 rows=12000 loops=1)
-> Hash (cost=738.00..738.00 rows=187 width=268) (actual time=2.807..2.807 rows=187 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 37kB
-> Seq Scan on customers c (cost=0.00..738.00 rows=187 width=268) (actual time=0.018..2.777 rows=187 loops=1)
Filter: ((state)::text = 'MD'::text)
Rows Removed by Filter: 19813
Görülebileceği gibi, planlardaki tek fark, ilk durumda, hastabl tüketir 7kB
, ancak ikincisinde 37kB
ve düğümün olmasıdır Hash Semi Join
.
Ama hashtable boyutundaki farkı anlamıyorum. Hash
Düğümü ile tam anlamıyla aynı kullanan Seq Scan
aynı olan düğüm Filter
. Neden fark var?
explain (analyze, verbose)
.