为什么Postgres SQL函数会扫描不应该分区的分区

Question

我偶然发现了我的SQL函数非常奇怪的问题.他们似乎有功能之间不同的执行计划language SQL和language plpgsql,但我不能告诉的执行计划设置为SQL版本,因为它需要这样的:Function's final statement must be SELECT or INSERT/UPDATE/DELETE RETURNING.不会让我用EXPLAIN.

至于我为什么知道他们有不同的计划,这是因为SQL版本执行失败,抱怨它不能连接到当前撤下国外服务器之一.使用外部表进行连接,并且该表按日期(列date_col)进行分区,其中一些分区在物理上位于同一服务器上,一些分区位于外部.函数中使用的日期参数确保它只应扫描一个分区,并且该分区位于同一服务器上.这也显示在explain下面使用plain SQL(不在函数中):

Append  (cost=2.77..39.52 rows=2 width=36)
  CTE ct
    ->  Result  (cost=0.00..0.51 rows=100 width=4)
  InitPlan 2 (returns $1)
    ->  Aggregate  (cost=2.25..2.26 rows=1 width=32)
          ->  CTE Scan on ct  (cost=0.00..2.00 rows=100 width=4)
  ->  Seq Scan on table1  (cost=0.00..0.00 rows=1 width=36)
        Filter: ((date_col = '2017-07-30'::date) AND (some_col = ANY ($1)))
  ->  Seq Scan on "part$_table1_201707"  (cost=0.00..36.75 rows=1 width=36)
        Filter: ((date_col = '2017-07-30'::date) AND (some_col = ANY ($1)))

外部分区是在2017年之前,它表明规划者选择正确的分区,并不打扰扫描任何其他分区.这是真的plain SQL,plpgsql function但不是sql function.为什么会这样,如果不重写我的功能,我可以避免它吗？

根据我的想法,参数的传入方式之间必然存在一些差异SQL function,因为其中的硬编码日期会阻止查询扫描不必要的分区.也许这样的事情发生了:

WITH ct AS (SELECT unnest(array[1,2]) AS arr)
  SELECT col1, col2
    FROM table1
   WHERE date_col = (SELECT '2017-07-30'::date)
     AND some_col = ANY((SELECT array_agg(arr) FROM ct)::int[])

制作这样的EXPLAIN:

Append  (cost=2.78..183.67 rows=3 width=36)
  CTE ct
    ->  Result  (cost=0.00..0.51 rows=100 width=4)
  InitPlan 2 (returns $1)
    ->  Result  (cost=0.00..0.01 rows=1 width=4)
  InitPlan 3 (returns $2)
    ->  Aggregate  (cost=2.25..2.26 rows=1 width=32)
          ->  CTE Scan on ct  (cost=0.00..2.00 rows=100 width=4)
  ->  Seq Scan on table1  (cost=0.00..0.00 rows=1 width=36)
        Filter: ((date_col = $1) AND (some_col = ANY ($2)))
  ->  Seq Scan on "part$_table1_201707"  (cost=0.00..36.75 rows=1 width=36)
        Filter: ((date_col = $1) AND (some_col = ANY ($2)))
  ->  Foreign Scan on "part$_table1_201603"  (cost=100.00..144.14 rows=1 width=36)

作为参考,您可以使用以下代码重现PostgreSQL 9.6.4上的问题:

CREATE SERVER broken_server FOREIGN DATA WRAPPER postgres_fdw
OPTIONS (host 'broken_server', dbname 'postgres',
         port '5432');

CREATE USER MAPPING FOR postgres SERVER broken_server 
OPTIONS (user 'foreign_username', password 'foreign_password');

CREATE TABLE table1 (id serial PRIMARY KEY, date_col date,
                     some_col int, col1 int, col2 text);

CREATE TABLE part$_table1_201707 ()
INHERITS (table1);
ALTER TABLE part$_table1_201707 ADD CONSTRAINT part$_table1_201707_date_chk
        CHECK (date_col BETWEEN '2017-07-01'::date AND '2017-07-31'::date);

CREATE FOREIGN TABLE part$_table1_201603 ()
INHERITS (table1) SERVER broken_server
OPTIONS (schema_name 'public', table_name 'part$_table1_201603');
ALTER TABLE part$_table1_201603 ADD CONSTRAINT part$_table1_201603_date_chk
        CHECK (date_col BETWEEN '2016-03-01'::date AND '2016-03-31'::date);

CREATE OR REPLACE FUNCTION function_plpgsql(param1 date, param2 int[])
 RETURNS TABLE(col1 int, col2 text)
 LANGUAGE plpgsql
 SECURITY DEFINER
AS $function$
BEGIN
  --
  RETURN QUERY
  WITH ct AS (SELECT unnest(param2) AS arr)
  SELECT t.col1, t.col2
    FROM table1 AS t
   WHERE date_col = param1
     AND some_col = ANY((SELECT array_agg(arr) FROM ct)::int[]); --reasons
  --
END;
$function$;

CREATE OR REPLACE FUNCTION function_sql(param1 date, param2 int[])
 RETURNS TABLE(col1 int, col2 text)
 LANGUAGE SQL
 SECURITY DEFINER
AS $function$
  --
  WITH ct AS (SELECT unnest(param2) AS arr)
  SELECT t.col1, t.col2
    FROM table1 AS t
   WHERE date_col = param1
     AND some_col = ANY((SELECT array_agg(arr) FROM ct)::int[])
  --
$function$;

CREATE OR REPLACE FUNCTION function_sql_hardcoded(param1 date, param2 int[])
 RETURNS TABLE(col1 int, col2 text)
 LANGUAGE SQL
 SECURITY DEFINER
AS $function$
  --
  WITH ct AS (SELECT unnest(param2) AS arr)
  SELECT t.col1, t.col2
    FROM table1 AS t
   WHERE date_col = '2017-07-30'::date
     AND some_col = ANY((SELECT array_agg(arr) FROM ct)::int[])
  --
$function$;

EXPLAIN ANALYZE
SELECT * FROM function_sql('2017-07-30'::date, array[1,2]);
-- ERROR: could not connect to server "broken_server"

EXPLAIN ANALYZE
SELECT * FROM function_plpgsql('2017-07-30'::date, array[1,2]);
--works

EXPLAIN ANALYZE
SELECT * FROM function_sql_hardcoded('2017-07-30'::date, array[1,2]);
--works, but useless

Answer 1

https://www.postgresql.org/docs/current/static/ddl-partitioning.html

仅当查询的 WHERE 子句包含常量（或外部提供的参数）时，约束排除才起作用。例如，无法优化与非不可变函数（例如 CURRENT_TIMESTAMP）的比较，因为规划器无法知道函数值在运行时可能落入哪个分区。

这可以解释扫描不必要的分区 - plpgsql 在将查询提供给优化器之前处理查询，我认为，并且具有常量的 sql 函数应该可以工作。我想还有准备好的声明。但是将属性值与函数参数进行比较可能不是合适的情况:)