如何优化大型表的Postgresql数组\ U AGG查询？

我使用PostgreSQL实现其数组功能。以下是我的模式：

CREATE TABLE questions (
  id INTEGER PRIMARY KEY,
  product_id INTEGER UNIQUE NOT NULL,
  body VARCHAR(1000) NOT NULL,
  date_written DATE NOT NULL DEFAULT current_date,
  asker_name VARCHAR(60) NOT NULL,
  asker_email VARCHAR(60) NOT NULL,
  reported BOOLEAN DEFAULT FALSE,
  helpful INTEGER NOT NULL DEFAULT 0
);

CREATE TABLE answers (
  id PRIMARY KEY NOT NULL,
  question_id INTEGER NOT NULL,
  body VARCHAR(1000) NOT NULL,
  date_written DATE NOT NULL DEFAULT current_date,
  answerer_name VARCHAR(60) NOT NULL,
  answerer_email VARCHAR(60) NOT NULL,
  reported BOOLEAN DEFAULT FALSE,
  helpful INTEGER NOT NULL DEFAULT 0
);

CREATE TABLE photos (
  id INTEGER UNIQUE,
  answer_id INTEGER NOT NULL,
  photo VARCHAR(200)
);

我正在尝试查询我的答案表，以获取给定问题id的所有答案的列表，并包含该给定答案id存在的所有照片的数组。结果应按有用性的降序排序。到目前为止，我有一个显示我正在查找的结果的大型查询，但执行时间是729.595毫秒。我正在尝试优化，以将查询时间缩短到200毫秒。我有以下索引来尝试优化我的查询时间：

  indexname    |                                 indexdef                                  
-----------------+---------------------------------------------------------------------------
 answer_id       | CREATE UNIQUE INDEX answer_id ON public.answers USING btree (id)
 question_id     | CREATE INDEX question_id ON public.answers USING btree (question_id)
 idx_reported_id | CREATE INDEX idx_reported_id ON public.answers USING btree (reported, id)
 answers_pkey    | CREATE UNIQUE INDEX answers_pkey ON public.answers USING btree (id)

在我的分析中，我注意到GroupAggregate非常耗时：GroupAggregate（成本=126222.21..126222.71行=25宽度=129）（实际时间=729.497..729.506行=5个循环=1） 组密钥：answers.id

有什么方法可以避免耗时的小组讨论吗？我缺少索引了吗？以下是查询本身：

SELECT answers.id, 
       question_id, 
       body, 
       date_written, 
       answerer_name, 
       answerer_email, 
       reported, 
       helpful, 
       ARRAY_AGG(photo) as photos 
FROM answers 
  LEFT JOIN photos ON answers.id = photos.answer_id 
WHERE reported IS 
false AND answers.id IN (SELECT id 
                         FROM answers 
                         WHERE question_id = 20012) 
GROUP BY answers.id 
ORDER BY helpful DESC;

---

谢谢

我认为您可以跳过子查询：

SELECT answers.id, question_id, body, date_written, answerer_name, answerer_email, reported, helpful, ARRAY_AGG(photo) as photos 
FROM answers 
LEFT JOIN photos ON answers.id = photos.answer_id 
WHERE reported IS false AND question_id = 20012 
GROUP BY answers.id, question_id, body, date_written, answerer_name, answerer_email, reported, helpful
ORDER BY helpful DESC;

您可以在photos.answer\u id上添加btree索引，因为此字段在join子句中使用

---

您丢失了GROUPBY子句上的相同字段

一种通常有效的方法是先聚合，然后在结果上加入（而不是聚合整个结果）。而且你也不需要这样的条件

SELECT a.id, 
       a.question_id, 
       a.body, 
       a.date_written, 
       a.answerer_name, 
       a.answerer_email, 
       a.reported, 
       a.helpful, 
       p.photos 
FROM answers a
  LEFT JOIN (
    select answer_id, array_agg(photo) as photos 
    from photos
    group by answer_id
  ) p ON a.id = p.answer_id 
WHERE reported IS false 
  AND a.question_id = 20012
ORDER BY a.helpful DESC;

---

我将尝试更改WHERE order

WHERE QUOTE_id=20012，报告为false

。首先限制行的数量，但Postgresql可能会自动这样做。

SELECT answers.id, question_id, body, date_written, answerer_name, answerer_email, reported, helpful, ARRAY_AGG(photo) as photos 
FROM answers 
LEFT JOIN photos ON answers.id = photos.answer_id 
WHERE reported IS false AND question_id = 20012 
GROUP BY answers.id, question_id, body, date_written, answerer_name, answerer_email, reported, helpful
ORDER BY helpful DESC;

SELECT a.id, 
       a.question_id, 
       a.body, 
       a.date_written, 
       a.answerer_name, 
       a.answerer_email, 
       a.reported, 
       a.helpful, 
       p.photos 
FROM answers a
  LEFT JOIN (
    select answer_id, array_agg(photo) as photos 
    from photos
    group by answer_id
  ) p ON a.id = p.answer_id 
WHERE reported IS false 
  AND a.question_id = 20012
ORDER BY a.helpful DESC;