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Date: Thu, 12 Sep 2013 10:10:08 -0500
From: Jim Nasby <jim@nasby.net>
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To: Bastiaan Olij <bastiaan@basenlily.me>
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Subject: Re: Optimising views
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On 8/29/13 9:22 PM, Bastiaan Olij wrote:
> Work well enough.. But as I'm using the same data in different reports
> and I though a view might be smart. So I created a view:
> ----
> create view v_costs as
>    select dcjobid as costjobid, sum(dcamount) as costamount
>    from directcosts
>    group by dcjobid
> union all
>    select invjobid as costjobid, sum(detamount) as costamount from
> invoiceheader
>    join finvoicedetail on detinvid = invid
>    group by invjobid
> ----
>
> And rewrote my report to:
> ----
> select jobid, jobdesc, sum(costamount)
> from jobs
> join v_costs on costjobid = jobid
> where <some filter for my jobs>
> group by jobid, jobdesc
> ----
>
> Now what I was hoping for was that postgres would start at my jobs
> table, find the records I'm trying to report on and then index scan on
> the related tables and start aggregating the amounts.
> What it seems to do is to first execute the view to get totals for all
> the jobs in the database and join that result set with my 2 or 3 jobs
> that match my filter.
>
> What is it about my view that prevents postgres to effectively use it?
> The group bys? the union?

It's probably either the GROUP BY or the UNION. Try stripping those out one at a time and see if it helps. If it doesn't, please post EXPLAIN ANALYZE (or at least EXPLAIN) output.
-- 
Jim C. Nasby, Data Architect                       jim@nasby.net
512.569.9461 (cell)                         http://jim.nasby.net


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