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Slow query on "UNION ALL" view

Asked 2012-01-27T09:25:12.200
15

I have a DB view which basically consists of two SELECT queries with UNION ALL, like this:

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

The problem is that selects of the form

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

perform really really slow on it.

Both SELECT #1 and #2 are decently fast, properly indexed and so on: when I create views v1 and v2 like:

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

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

And the same SELECT, with same WHERE condition as the above works OK on them individually.

Any ideas about where might be the problem and how to solve it?

(Just to mention, it's one of the recent Postgres versions.)

Edit: Adding anonymized query plans (thaks to @filiprem for the link to an awesome tool):

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:

Aggreg
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1 Answer

6

I believe your query is being executed similar to:

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

which the optimizer is having difficulty optimizing. i.e. it's doing the UNION ALL first before applying the WHERE clause but, you wish it to apply the WHERE clause before the UNION ALL.

Couldn't you put your WHERE clause in the 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 < ... )

Alternatively if the view cannot have the WHERE clause, then, perhaps you can keep to the two views and do the UNION ALL with the WHERE clause when you need them:

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 < ... )
answered 2012-02-03T20:06:24.100

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