我有一个数据库视图,基本上由两个 SELECT
查询和 UNION ALL
组成,如下所示:
CREATE VIEW v AS
SELECT time, etc. FROM t1 // #1...
UNION ALL
SELECT time, etc. FROM t2 // #2...
问题在于形如以下的 select 语句
SELECT ... FROM v WHERE time >= ... AND time < ...
执行非常缓慢。
当我创建视图v1和v2时,选择语句1和2都相当快,已经正确地建立了索引等:
CREATE VIEW v1 AS
SELECT time, etc. FROM t1 // #1...
CREATE VIEW v2 AS
SELECT time, etc. FROM t2 // #2...
上面提到的相同选择(SELECT)和相同的 WHERE 条件,在它们各自单独执行时都能正常工作。
有任何想法关于可能出现问题的地方以及如何解决它?
(只是提一下,这是最近的 Postgres 版本之一。)
编辑:添加匿名查询计划(感谢 @filiprem 提供的链接到一个很棒的工具):
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)
v:
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
。