Slow query on “UNION ALL” view

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说谎
说谎 2021-02-13 06:42

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 /         


        
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  • 2021-02-13 07:25

    This seems to be a case of a pilot error. The "v" query plan selects from at least 5 different tables.

    Now, Are You sure You are connected to the right database? Maybe there are some funky search_path settings? Maybe t1 and t2 are actually views (possibly in a different schema)? Maybe You are somehow selecting from the wrong view?

    Edited after clarification:

    You are using a quite new feature called "join removal" : 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

    It appears that the feature does not kick in when union all is involved. You probably have to rewrite the view using only the required two tables.

    another edit: You appear to be using an aggregate (like "select count(*) from v" vs. "select * from v"), which could get vastly different plans in face of join removal. I guess we won't get very far without You posting the actual queries, view and table definitions and plans used...

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