postgresql – postgres查询优化问题

下面是两个几乎相同的postgres查询,但产生了截然不同的查询计划和执行时间.我假设第一个查询很快,因为form_id =’W40’只有196个form_instance记录,而form_id =’W30L’则有7000个.但是为什么从200个记录跳到7000个记录(这对我来说似乎相对较小)导致查询时间如此惊人地增加？我试图以各种方式对数据进行索引以加快速度,但基本上是陷入困境.我怎样才能加快速度呢？ (请注意,两个表的模式都包含在底部).

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 in ('Book_EstimatedDueDate','H_SubmittedDate','H_Ccode','miscarriage','miscarriage_of_multiple','stillbirth','AP_IUFD_of_multiple','maternal_death','birth_includes_transport','newborn_death','H_Pid','H_Mid1','H_Mid2','H_Mid3')
and (form_id = 'W40');

QUERY PLAN                                                                                                                                
 Nested Loop  (cost=0.00..70736.14 rows=4646 width=29) (actual time=0.000..20.000 rows=2399 loops=1)
   ->  Index Scan using form_id_and_workflow_state on form_instances  (cost=0.00..1041.42 rows=507 width=8) (actual time=0.000..0.000 rows=196 loops=1)
         Index Cond: (((form_id)::text = 'W40'::text) AND ((workflow_state)::text = 'DRqueued'::text))
   ->  Index Scan using index_field_instances_on_form_instance_id on field_instances  (cost=0.00..137.25 rows=17 width=25) (actual time=0.000..0.102 rows=12 loops=196)
         Index Cond: (field_instances.form_instance_id = form_instances.id)
         Filter: ((field_instances.field_id)::text = ANY ('{Book_EstimatedDueDate,H_SubmittedDate,H_Ccode,miscarriage,miscarriage_of_multiple,stillbirth,AP_IUFD_of_multiple,maternal_death,birth_includes_transport,newborn_death,H_Pid,H_Mid1,H_Mid2,H_Mid3}'::text[]))
 Total runtime: 30.000 ms
(7 rows)

explain analyze select form_id,field_id 
from form_instances,'H_Mid3') 
and (form_id = 'W30L');

QUERY PLAN                                                                                                                             
 Hash Join  (cost=34300.46..160865.40 rows=31045 width=29) (actual time=65670.000..74960.000 rows=102777 loops=1)
   Hash Cond: (field_instances.form_instance_id = form_instances.id)
   ->  Bitmap Heap Scan on field_instances  (cost=29232.57..152163.82 rows=531718 width=25) (actual time=64660.000..72800.000 rows=526842 loops=1)
         Recheck Cond: ((field_id)::text = ANY ('{Book_EstimatedDueDate,H_Mid3}'::text[]))
         ->  Bitmap Index Scan on index_field_instances_on_field_id  (cost=0.00..29099.64 rows=531718 width=0) (actual time=64630.000..64630.000 rows=594515 loops=1)
               Index Cond: ((field_id)::text = ANY ('{Book_EstimatedDueDate,H_Mid3}'::text[]))
   ->  Hash  (cost=5025.54..5025.54 rows=3388 width=8) (actual time=980.000..980.000 rows=10457 loops=1)
         ->  Bitmap Heap Scan on form_instances  (cost=90.99..5025.54 rows=3388 width=8) (actual time=10.000..950.000 rows=10457 loops=1)
               Recheck Cond: (((form_id)::text = 'W30L'::text) AND ((workflow_state)::text = 'DRqueued'::text))
               ->  Bitmap Index Scan on form_id_and_workflow_state  (cost=0.00..90.14 rows=3388 width=0) (actual time=0.000..0.000 rows=10457 loops=1)
                     Index Cond: (((form_id)::text = 'W30L'::text) AND ((workflow_state)::text = 'DRqueued'::text))
 Total runtime: 75080.000 ms

# d form_instances                                        Table "public.form_instances"     Column      |            Type             |                          Modifiers                          
-----------------+-----------------------------+-------------------------------------------------------------
 id              | integer                     | not null default nextval('form_instances_id_seq'::regclass)
 form_id         | character varying(255)      | 
 created_at      | timestamp without time zone | 
 updated_at      | timestamp without time zone | 
 created_by_id   | integer                     | 
 updated_by_id   | integer                     | 
 workflow        | character varying(255)      | 
 workflow_state  | character varying(255)      | 
 validation_data | text                        | 
Indexes:
    "form_instances_pkey" PRIMARY KEY,btree (id)
    "form_id_and_workflow_state" btree (form_id,workflow_state)
    "index_form_instances_on_form_id" btree (form_id)
    "index_form_instances_on_workflow_state" btree (workflow_state)

# d field_instances
                                        Table "public.field_instances"
      Column      |            Type             |                          Modifiers                           
------------------+-----------------------------+--------------------------------------------------------------
 id               | integer                     | not null default nextval('field_instances_id_seq'::regclass)
 form_instance_id | integer                     | 
 created_at       | timestamp without time zone | 
 updated_at       | timestamp without time zone | 
 created_by_id    | integer                     | 
 updated_by_id    | integer                     | 
 field_id         | character varying(255)      | 
 answer           | text                        | 
 state            | character varying(255)      | 
 explanation      | text                        | 
 idx              | integer                     | not null default 0
Indexes:
    "field_instances_pkey" PRIMARY KEY,btree (id)
    "field_instances__lower_answer" btree (lower(answer))
    "index_field_instances_on_answer" btree (answer)
    "index_field_instances_on_field_id" btree (field_id)
    "index_field_instances_on_field_id_and_answer" btree (field_id,answer)
    "index_field_instances_on_form_instance_id" btree (form_instance_id)
    "index_field_instances_on_idx" btree (idx)