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谁能解释为什么第一个查询比第二个查询快一点？

explain analyze select count(*) from view_recurrence_service_plan_description a
left join (select * from vehicle_roster where cast(schedule_date as date)=current_date) vr on a.recurrence_service_plan_id=vr.recurrence_service_plan_id
left join vehicle_route vroute on vroute.vehicle_route_id=vr.vehicle_route_id  
left join vehicle on vehicle.vehicle_id=vroute.vehicle_id;  
                                                               QUERY PLAN                                                               
----------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=736.29..736.30 rows=1 width=0) (actual time=13.206..13.206 rows=1 loops=1)
   ->  Hash Left Join  (cost=1.15..706.42 rows=11946 width=0) (actual time=0.041..11.401 rows=11946 loops=1)
         Hash Cond: (recurrence_service_plan.recurrence_service_plan_id = vehicle_roster.recurrence_service_plan_id)
         ->  Seq Scan on recurrence_service_plan  (cost=0.00..660.46 rows=11946 width=24) (actual time=0.010..8.170 rows=11946 loops=1)
               Filter: (NOT hide)
         ->  Hash  (cost=1.14..1.14 rows=1 width=16) (actual time=0.021..0.021 rows=0 loops=1)
               Buckets: 1024  Batches: 1  Memory Usage: 0kB
               ->  Seq Scan on vehicle_roster  (cost=0.00..1.14 rows=1 width=16) (actual time=0.021..0.021 rows=0 loops=1)
                     Filter: ((schedule_date)::date = ('now'::cstring)::date)
                     Rows Removed by Filter: 10
 Total runtime: 13.286 ms
(11 rows)



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explain analyze select count(*) from view_recurrence_service_plan_description a
left join vehicle_roster vr on a.recurrence_service_plan_id=vr.recurrence_service_plan_id and cast(vr.schedule_date as date)=current_date
left join vehicle_route vroute on vroute.vehicle_route_id=vr.vehicle_route_id
left join vehicle on vehicle.vehicle_id=vroute.vehicle_id;
                                                               QUERY PLAN                                                               
----------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=736.29..736.30 rows=1 width=0) (actual time=13.355..13.356 rows=1 loops=1)
   ->  Hash Left Join  (cost=1.15..706.42 rows=11946 width=0) (actual time=0.040..11.549 rows=11946 loops=1)
         Hash Cond: (recurrence_service_plan.recurrence_service_plan_id = vr.recurrence_service_plan_id)
         ->  Seq Scan on recurrence_service_plan  (cost=0.00..660.46 rows=11946 width=24) (actual time=0.010..7.905 rows=11946 loops=1)
               Filter: (NOT hide)
         ->  Hash  (cost=1.14..1.14 rows=1 width=16) (actual time=0.020..0.020 rows=0 loops=1)
               Buckets: 1024  Batches: 1  Memory Usage: 0kB
               ->  Seq Scan on vehicle_roster vr  (cost=0.00..1.14 rows=1 width=16) (actual time=0.020..0.020 rows=0 loops=1)
                     Filter: ((schedule_date)::date = ('now'::cstring)::date)
                     Rows Removed by Filter: 10
 Total runtime: 13.436 ms
(11 rows)

我正在使用多个左连接，我正在尝试优化它。每一秒对我来说都很重要。我可以在这里使用左连接横向吗？有什么优化技巧吗？

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1 回答 1

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由于您没有提供整个查询，并且我们不知道您的查询的功能含义，我建议如下：

在不更改查询的情况下，您现在可以识别 where 语句中使用的列并验证您是否有索引。（当然取决于数据量）。如果就索引而言一切似乎对您来说都很好，请尝试更新您的表统计信息。否则，如有必要，请添加索引。

于 2015-11-02T17:54:13.730 回答