postgresql – 与没有函数包装器的查询相比,SQL函数非常慢

发布时间：2020-12-13 16:13:42 所属栏目：百科来源：网络整理

导读：我有这个PostgreSQL 9.4查询运行速度非常快(~12ms)： SELECT auth_web_events.id,auth_web_events.time_stamp,auth_web_events.description,auth_web_events.origin,auth_user.email,customers.name,auth_web_events.client_ipFROM public.auth_web_events,p

我有这个PostgreSQL 9.4查询运行速度非常快(~12ms)：

SELECT 
  auth_web_events.id,auth_web_events.time_stamp,auth_web_events.description,auth_web_events.origin,auth_user.email,customers.name,auth_web_events.client_ip
FROM 
  public.auth_web_events,public.auth_user,public.customers
WHERE 
  auth_web_events.user_id_fk = auth_user.id AND
  auth_user.customer_id_fk = customers.id AND
  auth_web_events.user_id_fk = 2
ORDER BY
  auth_web_events.id DESC;

但是,如果我将它嵌入到一个函数中,查询在所有数据中运行速度非常慢,似乎在运行每条记录,我缺少什么？,我有?1M的数据,我想简化我的数据库层存储大型查询进入功能和观点.

CREATE OR REPLACE FUNCTION get_web_events_by_userid(int) RETURNS TABLE(
    id int,time_stamp timestamp with time zone,description text,origin text,userlogin text,customer text,client_ip inet
     ) AS
$func$
SELECT 
  auth_web_events.id,auth_user.email AS user,customers.name AS customer,public.customers
WHERE 
  auth_web_events.user_id_fk = auth_user.id AND
  auth_user.customer_id_fk = customers.id AND
  auth_web_events.user_id_fk = $1
ORDER BY
  auth_web_events.id DESC;
  $func$LANGUAGE SQL;

查询计划是：

"Sort  (cost=20.94..20.94 rows=1 width=791) (actual time=61.905..61.906 rows=2 loops=1)"
"  Sort Key: auth_web_events.id"
"  Sort Method: quicksort  Memory: 25kB"
"  ->  Nested Loop  (cost=0.85..20.93 rows=1 width=791) (actual time=61.884..61.893 rows=2 loops=1)"
"        ->  Nested Loop  (cost=0.71..12.75 rows=1 width=577) (actual time=61.874..61.879 rows=2 loops=1)"
"              ->  Index Scan using auth_web_events_fk1 on auth_web_events  (cost=0.57..4.58 rows=1 width=61) (actual time=61.860..61.860 rows=2 loops=1)"
"                    Index Cond: (user_id_fk = 2)"
"              ->  Index Scan using auth_user_pkey on auth_user  (cost=0.14..8.16 rows=1 width=524) (actual time=0.005..0.005 rows=1 loops=2)"
"                    Index Cond: (id = 2)"
"        ->  Index Scan using customers_id_idx on customers  (cost=0.14..8.16 rows=1 width=222) (actual time=0.004..0.005 rows=1 loops=2)"
"              Index Cond: (id = auth_user.customer_id_fk)"
"Planning time: 0.369 ms"
"Execution time: 61.965 ms"

我这样称呼这个函数：

SELECT * from get_web_events_by_userid(2)

该函数的查询计划：

"Function Scan on get_web_events_by_userid  (cost=0.25..10.25 rows=1000 width=172) (actual time=279107.142..279107.144 rows=2 loops=1)"
"Planning time: 0.038 ms"
"Execution time: 279107.175 ms"

编辑：我只是更改参数,问题仍然存在.
EDIT2：Erwin答案的查询计划：

"Sort  (cost=20.94..20.94 rows=1 width=791) (actual time=0.048..0.049 rows=2 loops=1)"
"  Sort Key: w.id"
"  Sort Method: quicksort  Memory: 25kB"
"  ->  Nested Loop  (cost=0.85..20.93 rows=1 width=791) (actual time=0.030..0.037 rows=2 loops=1)"
"        ->  Nested Loop  (cost=0.71..12.75 rows=1 width=577) (actual time=0.023..0.025 rows=2 loops=1)"
"              ->  Index Scan using auth_user_pkey on auth_user u  (cost=0.14..8.16 rows=1 width=524) (actual time=0.011..0.012 rows=1 loops=1)"
"                    Index Cond: (id = 2)"
"              ->  Index Scan using auth_web_events_fk1 on auth_web_events w  (cost=0.57..4.58 rows=1 width=61) (actual time=0.008..0.008 rows=2 loops=1)"
"                    Index Cond: (user_id_fk = 2)"
"        ->  Index Scan using customers_id_idx on customers c  (cost=0.14..8.16 rows=1 width=222) (actual time=0.003..0.004 rows=1 loops=2)"
"              Index Cond: (id = u.customer_id_fk)"
"Planning time: 0.541 ms"
"Execution time: 0.101 ms"

用户

在重写函数时,我意识到你在这里添加了列别名：

SELECT 
  ...
  auth_user.email AS user,

..因为这些别名在函数外部是不可见的而在函数内部没有引用,所以它不会做任何事情.所以他们会被忽略.出于文档目的,最好使用注释.

但它也会使您的查询无效,因为用户完全是reserved word,除非双引号,否则不能用作列别名.

奇怪的是,在我的测试中,该函数似乎与无效的别名一起使用.可能是因为它被忽略了(？).但我不确定这不会产生副作用.

您的函数被重写(否则相当于)：

CREATE OR REPLACE FUNCTION get_web_events_by_userid(int)
  RETURNS TABLE(
     id int,time_stamp timestamptz,client_ip inet
  ) AS
$func$
SELECT w.id,w.time_stamp,w.description,w.origin,u.email     -- AS user   -- make this a comment!,c.name      -- AS customer,w.client_ip
FROM   public.auth_user       u
JOIN   public.auth_web_events w ON w.user_id_fk = u.id
JOIN   public.customers       c ON c.id = u.customer_id_fk 
WHERE  u.id = $1   -- reverted the logic here
ORDER  BY w.id DESC
$func$LANGUAGE sql STABLE;

显然,STABLE关键字改变了结果. Function volatility不应该是您描述的测试情况中的问题.该设置通常不会使单个隔离的函数调用受益.阅读details in the manual.此外,标准EXPLAIN不显示内部函数内容的查询计划.您可以使用附加模块自动解释：

> Postgres query plan of a UDF invocation written in pgpsql

你有一个非常奇怪的数据分布：

auth_web_events table has 100000000 records,auth_user->2 records,customers-> 1 record

由于您没有另外定义,因此该函数假定要返回1000行的估计值.但是你的函数实际上只返回2行.如果你的所有调用只返回(在2行附近),只需用增加的ROWS 2声明它.也可以更改VOLATILE变量的查询计划(即使STABLE是正确的选择).

（编辑：李大同）

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