PostgreSQL中用于列表报表的内部选择的SQL性能问题

Question

使用PostgreSQL数据库：

我有一个调查应用程序，用户可以在其中输入活动并回答有关其活动的问题。调查本身称为RECALLS_T，输入的事件为EVENTS_T，答案为ANSWERS_T。答案适用于提供的活动问题，存储在ACTIVITY_QUESTIONS_T中，由Lookup(LOOKUP_T(映射。

然后，我需要运行一个基于事件的报告，其中每一行都是来自EVENTS_T的事件，用于每次召回(所有召回的所有事件组合在一起(。但是，该报告中的某些列需要指示某些答案的值，否则这些单元格为NULL。因此，这是一份表格报告。

示例(首先是简单的平面材料，然后是复杂的列表材料(：

RecallID | RecallDate | Event |..| WalkAlone | WalkWithPartner |..| ExerciseAtGym
256      | 10-01-19   | Exrcs |..| NULL      | NULL            |..| yes
256      | 10-01-19   | Walk  |..| yes       | NULL            |..| NULL
256      | 10-01-19   | Eat   |..| NULL      | NULL            |..| NULL
257      | 10-01-19   | Exrcs |..| NULL      | NULL            |..| yes

我的SQL对基于列表答案的列进行了内部选择，如下所示：

select 
-- Easy flat stuff first
r.id as recallid, r.recall_date as recalldate, ... ,
-- Example of Tabulated Columns:
(select l.description from answers_t ans, activity_questions_t aq, lookup_t l 
where l.id=aq.answer_choice_id and aq.question_id=13 
and aq.id=ans.activity_question_id and aq.activity_id=27 and ans.event_id=e.id) 
as transportationotherintensity,
(select l.description from answers_t ans, activity_questions_t aq, lookup_t l
where l.id=66 and l.id=aq.answer_choice_id and aq.question_id=14
and aq.id=ans.activity_question_id and ans.event_id=e.id) 
as commutework,
(select l.description from answers_t ans, activity_questions_t aq, lookup_t l
where l.id=67 and l.id=aq.answer_choice_id and aq.question_id=14 and aq.id=ans.activity_question_id and ans.event_id=e.id) 
as commuteschool,
(select l.description from answers_t ans, activity_questions_t aq, lookup_t l
where l.id=95 and l.id=aq.answer_choice_id and aq.question_id=14 and aq.id=ans.activity_question_id and ans.event_id=e.id) 
as dropoffpickup,

SQL可以工作，并且可以呈现报告，但性能很差。我验证了它在比例上是糟糕的：没有一个特定项目的灵丹妙药可以修复它。每一个内部选择都会导致糟糕的性能。1000行的结果集需要15秒，但不应超过2-3秒。

请注意，这些索引已经存在：

• ANSWERS_T：在ACTIVITY_QUESTION_ID、EVENT_ID上
• EVENTS_T:在RECALL_ID上
• ACTIVITY_QUESTIONS_T：在ACTIVITY_ID、QUESTION_ID、ANSWER_CHOICE_ID上

我的这些内心选择有错吗？

Answer 1

要总结这些问题，需要使用条件聚合。在PostgreSQL中，您可以使用：

select ans.event_id,
max(l.description) filter (where aq.question_id = 13 and aq.activity_id = 27) as transportationotherintensity
max(l.description) filter (where l.id = 66 and aq.question_id = 14 and aq.activity_id = 67) as commutework,
. . .
from activity_questions_t aq join
lookup_t l
on l.id = aq.answer_choice_id join
answers_t ans
on aq.id = ans.activity_question_id
group by ans.event_id