synchronize_seqscans

• 17
• 16
• 15
• 14
• 13
• 12
• 11
• 10
• 9.6
• 9.5
• 9.4
• 9.3
• 9.2
• 9.1
• 9.0
• 8.4
• 8.3

此简单示例显示了 synchronize_seqscans 设置为 on（默认值）和 off 时规划器行为的差异。

postgres=# EXPLAIN ANALYZE SELECT * FROM foo WHERE id > 10000 limit 5;
                                                  QUERY PLAN                                                  
--------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..0.25 rows=5 width=4) (actual time=0.136..0.137 rows=5 loops=1)
   ->  Seq Scan on foo  (cost=0.00..185288.50 rows=3761080 width=4) (actual time=0.135..0.136 rows=5 loops=1)
         Filter: (id > 10000)
         Rows Removed by Filter: 2768
 Planning Time: 0.033 ms
 Execution Time: 0.145 ms
(6 rows)

postgres=# SET synchronize_seqscans TO off;
SET

postgres=# EXPLAIN ANALYZE SELECT * FROM foo WHERE id > 10000 limit 5;
                                                  QUERY PLAN                                                  
--------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..0.25 rows=5 width=4) (actual time=0.470..0.471 rows=5 loops=1)
   ->  Seq Scan on foo  (cost=0.00..185288.50 rows=3761080 width=4) (actual time=0.469..0.469 rows=5 loops=1)
         Filter: (id > 10000)
         Rows Removed by Filter: 10000
 Planning Time: 0.035 ms
 Execution Time: 0.484 ms
(6 rows)

• PostgreSQL 文档： synchronize_seqscans