Re: Q on views and performance

On 2008-02-23 05:59, Kynn Jones wrote:
> On Fri, Feb 22, 2008 at 8:48 PM, Dean Gibson (DB Administrator)
> <postgresql(at)ultimeth(dot)com <mailto:postgresql(at)ultimeth(dot)com>> wrote:
>
> ...
>
>
> Since you have experience working with views, let me ask you this.
> The converse strategy to the one I described originally would be to
> create the individual tables T1, T2, T3, ..., T100, but instead of
> keeping around the original (and now redundant) table T, replace it
> with a view V made up of the union of T1, T2, T3, ..., T100. The
> problem with this alternative is that one cannot index V, or define a
> primary key constraint for it, because it's a view. This means that a
> search in V, even for a primary key value, would be *have to be* very
> inefficient (i.e. I don't see how even the very clever PostgreSQL
> implementers could get around this one!), because the engine would
> have to search *all* the underlying tables, T1 through T100, even if
> it found the desired record in T1, since it has no way of knowing that
> the value is unique all across V.
>
> Is there a way around this?
>
> kynn
>
Oh, I wouldn't create separate tables and do a UNION of them, I'd think
that would be inefficient.

I didn't look in detail at your previous eMail, but I will now:

1. You haven't told us the distribution of "zipk", or what the tables
are indexed on, or what type of performance you are expecting. Your
initial examples don't help much unless you actually have performance
numbers or EXPLAIN output for them, since adding the third JOIN
significantly changes the picture, as does changing one of the JOINs to
a LEFT JOIN.

2. In your actual (Q1** and Q2**) examples, why is one JOIN an INNER
JOIN and the other one a LEFT JOIN? Given your description of Q1 at the
top of your message, that doesn't make sense to me.

3. Why not write:

CREATE VIEW txt AS
SELECT a1.word AS word1, a1.type AS type1, a2.word AS word2, a2.type
AS type2
FROM T a1 [LEFT] JOIN T a2 USING( zipk ); -- Use "LEFT" if appropriate
SELECT word1, word1
FROM S JOIN txt ON word = word1
WHERE type1 = <int1> AND type2 = <int2>;

If either of those (either with or without the "LEFT") are not
equivalent to your problem, how about just:

SELECT a1.word AS word1, a2.word AS word2
FROM S JOIN T a1 USING( word)
[LEFT] JOIN T a2 USING( zipk ) -- Use "LEFT" if appropriate
WHERE a1.type = <int1> AND a2.type = <int2>;

Show us (using EXPLAIN) what the query planner thinks of each of these.

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