以下是一个Postgres查询,似乎比我预期的要花费更长的时间。field_instances表在form_instance_id和field_id上都建有索引,而form_instances表在workflow_state上有索引。所以我认为这应该是一个快速查询,但它却需要很长时间。请问有人可以帮助我解释查询计划以及应该添加哪些索引来加快速度吗?谢谢。
explain analyze
select form_id,form_instance_id,answer,field_id
from form_instances,field_instances
where workflow_state = 'DRqueued'
and form_instance_id = form_instances.id
and field_id = 'Book_EstimatedDueDate';
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Hash Join (cost=8733.85..95692.90 rows=9277 width=29) (actual time=2550.000..15430.000 rows=11431 loops=1)
Hash Cond: (field_instances.form_instance_id = form_instances.id)
-> Bitmap Heap Scan on field_instances (cost=2681.11..89071.72 rows=47567 width=25) (actual time=850.000..13690.000 rows=51726 loops=1)
Recheck Cond: ((field_id)::text = 'Book_EstimatedDueDate'::text)
-> Bitmap Index Scan on index_field_instances_on_field_id (cost=0.00..2669.22 rows=47567 width=0) (actual time=830.000..830.000 rows=51729 loops=1)
Index Cond: ((field_id)::text = 'Book_EstimatedDueDate'::text)
-> Hash (cost=5911.34..5911.34 rows=11312 width=8) (actual time=1590.000..1590.000 rows=11431 loops=1)
-> Bitmap Heap Scan on form_instances (cost=511.94..5911.34 rows=11312 width=8) (actual time=720.000..1570.000 rows=11431 loops=1)
Recheck Cond: ((workflow_state)::text = 'DRqueued'::text)
-> Bitmap Index Scan on index_form_instances_on_workflow_state (cost=0.00..509.11 rows=11312 width=0) (actual time=650.000..650.000 rows=11509 loops=1)
Index Cond: ((workflow_state)::text = 'DRqueued'::text)
Total runtime: 15430.000 ms
(12 rows)
set enable_hashjoin = 0;
,看看是否能得到更快的执行计划。如果是,那么我们将继续检查为什么一开始没有使用该计划。 - sayap