From: | Andreas Pflug <Andreas(dot)Pflug(at)web(dot)de> |
---|---|
To: | pgsql-performance(at)postgresql(dot)org |
Subject: | Re: Do Views offer any performance advantage? |
Date: | 2003-04-15 19:55:20 |
Message-ID: | 3E9C63A8.4040801@web.de |
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Thread: | |
Lists: | pgsql-performance |
Tom Lane wrote:
>
>There is probably some minuscule cost difference involved --- you save
>parsing and parse analysis of a long query string. On the other hand,
>you pay to pull the view definition from the catalogs and merge it into
>the given query. I'd not care to hazard a guess on whether the actual
>net cost is more or less; but in any case these costs will be swamped
>by query planning and execution, if the query is complex.
>
Actually, there are cases when a view can impact performance.
If you are joining a view, it seems to be treated as a subquery, that
might have a much larger result than you would like.
Imagine
SELECT something
FROM A JOIN B JOIN C ...
WHERE A.primaryKeyFoo=1234 ...
where C is a view, containing JOINs itself, I observed a query plan
(7.3.2) like
A JOIN B JOIN (D JOIN E)
instead of
A JOIN B JOIN D JOIN E which would be much more efficient for the
A.primaryKeyFoo restriction.
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