慢慢查询"UNION ALL"视图

Question

我有一个DB视图,它基本上由两个SELECT查询组成UNION ALL,如下所示:

CREATE VIEW v AS
SELECT time, etc. FROM t1 // #1...
UNION ALL
SELECT time, etc. FROM t2 // #2...

问题是选择表格

SELECT ... FROM v WHERE time >= ... AND time < ...

表现真的很慢.

SELECT#1和#2都非常快,索引正确等等:当我创建视图v1和v2时:

CREATE VIEW v1 AS
SELECT time, etc. FROM t1 // #1...

CREATE VIEW v2 AS
SELECT time, etc. FROM t2 // #2...

并且具有与上述相同的WHERE条件的相同SELECT单独工作正常.

关于哪里可能是问题以及如何解决问题的任何想法？

(仅举这一点,它是最近的Postgres版本之一.)

编辑:添加匿名查询计划(请访问@filiprem以获取指向真棒工具的链接):

V1:

Aggregate  (cost=9825.510..9825.520 rows=1 width=53) (actual time=59.995..59.995 rows=1 loops=1)
  ->  Index Scan using delta on echo alpha  (cost=0.000..9815.880 rows=3850 width=53) (actual time=0.039..53.418 rows=33122 loops=1)
          Index Cond: (("juliet" >= 'seven'::uniform bravo_victor oscar whiskey) AND ("juliet" <= 'november'::uniform bravo_victor oscar whiskey))
          Filter: ((NOT victor) AND ((bravo_sierra five NULL) OR ((bravo_sierra)::golf <> 'india'::golf)))

V2:

Aggregate  (cost=15.470..15.480 rows=1 width=33) (actual time=0.231..0.231 rows=1 loops=1)
  ->  Index Scan using yankee on six charlie  (cost=0.000..15.220 rows=99 width=33) (actual time=0.035..0.186 rows=140 loops=1)
          Index Cond: (("juliet" >= 'seven'::uniform bravo oscar whiskey) AND ("juliet" <= 'november'::uniform bravo oscar whiskey))
          Filter: (NOT victor)

五:

Aggregate  (cost=47181.850..47181.860 rows=1 width=0) (actual time=37317.291..37317.291 rows=1 loops=1)
  ->  Append  (cost=42.170..47132.480 rows=3949 width=97) (actual time=1.277..37304.453 rows=33262 loops=1)
        ->  Nested Loop Left Join  (cost=42.170..47052.250 rows=3850 width=99) (actual time=1.275..37288.465 rows=33122 loops=1)
              ->  Hash Left Join  (cost=42.170..9910.990 rows=3850 width=115) (actual time=1.123..117.797 rows=33122 loops=1)
                      Hash Cond: ((alpha_seven.two)::golf = (quebec_three.two)::golf)
                    ->  Index Scan using delta on echo alpha_seven  (cost=0.000..9815.880 rows=3850 width=132) (actual time=0.038..77.866 rows=33122 loops=1)
                            Index Cond: (("juliet" >= 'seven'::uniform bravo_victor oscar whiskey_two) AND ("juliet" <= 'november'::uniform bravo_victor oscar whiskey_two))
                            Filter: ((NOT victor) AND ((bravo_sierra five NULL) OR ((bravo_sierra)::golf <> 'india'::golf)))
                    ->  Hash  (cost=30.410..30.410 rows=941 width=49) (actual time=1.068..1.068 rows=941 loops=1)
                            Buckets: 1024  Batches: 1  Memory Usage: 75kB
                          ->  Seq Scan on alpha_india quebec_three  (cost=0.000..30.410 rows=941 width=49) (actual time=0.010..0.486 rows=941 loops=1)
              ->  Index Scan using mike on hotel quebec_sierra  (cost=0.000..9.630 rows=1 width=24) (actual time=1.112..1.119 rows=1 loops=33122)
                      Index Cond: ((alpha_seven.zulu)::golf = (quebec_sierra.zulu)::golf)
        ->  Subquery Scan on "*SELECT* 2"  (cost=34.080..41.730 rows=99 width=38) (actual time=1.081..1.951 rows=140 loops=1)
              ->  Merge Right Join  (cost=34.080..40.740 rows=99 width=38) (actual time=1.080..1.872 rows=140 loops=1)
                      Merge Cond: ((quebec_three.two)::golf = (charlie.two)::golf)
                    ->  Index Scan using whiskey_golf on alpha_india quebec_three  (cost=0.000..174.220 rows=941 width=49) (actual time=0.017..0.122 rows=105 loops=1)
                    ->  Sort  (cost=18.500..18.750 rows=99 width=55) (actual time=0.915..0.952 rows=140 loops=1)
                            Sort Key: charlie.two
                            Sort Method:  quicksort  Memory: 44kB
                          ->  Index Scan using yankee on six charlie  (cost=0.000..15.220 rows=99 width=55) (actual time=0.022..0.175 rows=140 loops=1)
                                  Index Cond: (("juliet" >= 'seven'::uniform bravo_victor oscar whiskey_two) AND ("juliet" <= 'november'::uniform bravo_victor oscar whiskey_two))
                                  Filter: (NOT victor)

juliet是time.

Answer 1

这似乎是飞行员错误的情况."v"查询计划从至少5个不同的表中进行选择.

现在,您确定已连接到正确的数据库吗？也许有一些时髦的search_path设置？也许t1和t2实际上是视图(可能在不同的模式中)？也许你是以某种方式从错误的观点中选择？

澄清后编辑:

您正在使用一个名为"join removal"的全新功能:http://wiki.postgresql.org/wiki/What%27s_new_in_PostgreSQL_9.0#Join_Removal

http://rhaas.blogspot.com/2010/06/why-join-removal-is-cool.html

当涉及union all时,该功能似乎没有启动.您可能只需要使用所需的两个表重写视图.

另一个编辑:您似乎正在使用聚合(例如"从v中选择计数(*)"与"select*from v"),这可能会在连接删除时获得截然不同的计划.我想如果没有发布实际查询,视图和表格定义以及使用的计划,我们将不会走得太远......

Answer 2

我相信您的查询正在执行类似于:

(
   ( SELECT time, etc. FROM t1 // #1... )
   UNION ALL
   ( SELECT time, etc. FROM t2 // #2... )
)
WHERE time >= ... AND time < ...

哪个优化器难以优化.即它做的UNION ALL第一应用之前WHERE条款,但是,你希望它适用WHERE条款之前的UNION ALL.

你不能把你的WHERE条款放在CREATE VIEW？

CREATE VIEW v AS
( SELECT time, etc. FROM t1  WHERE time >= ... AND time < ... )
UNION ALL
( SELECT time, etc. FROM t2  WHERE time >= ... AND time < ... )

或者,如果视图不能包含该WHERE子句,那么,也许您可​​以保留两个视图并在需要时UNION ALL使用该WHERE子句:

CREATE VIEW v1 AS
SELECT time, etc. FROM t1 // #1...

CREATE VIEW v2 AS
SELECT time, etc. FROM t2 // #2...

( SELECT * FROM v1 WHERE time >= ... AND time < ... )
UNION ALL
( SELECT * FROM v2 WHERE time >= ... AND time < ... )

Answer 3

我认为我没有太多观点可以将其作为评论发布，所以我将其作为答案发布

我不知道 PostgreSQL 在幕后是如何工作的，我想你可能会知道它是否是 Oracle，所以这就是 Oracle 的工作原理

您的UNION ALL视图速度较慢，因为在幕后，来自SELECT #1和#2的记录首先组合在临时表中，该临时表是动态创建的，然后是您的SELECT ... FROM v WHERE time >= 。 .. AND time < ...在此临时表上执行。由于#1和#2都已建立索引，因此它们按预期单独工作得更快，但该临时表没有建立索引（当然），并且从该临时表中选择最终记录，因此导致响应速度较慢。

现在，至少，我没有看到任何方法可以让它更快+视图+非物化

除了显式运行SELECT #1和#2并 UNION 它们之外，为了使其更快，一种方法是使用应用程序编程语言中的存储过程或函数（如果是这种情况），在这个过程中，您可以单独调用每个索引表，然后合并结果，这并不像SELECT ... FROM v WHERE time >= ... AND time < ... :(