Message-ID: <4360B375.8000303@krap.dk>
Date: Thu, 27 Oct 2005 13:01:09 +0200
From: Svenne Krap <svenne@krap.dk>
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To: pgsql-performance@postgresql.org
Subject: Re: Perfomance of views
References: <435FA002.4060509@krap.dk> <43609EF4.8060907@archonet.com>
In-Reply-To: <43609EF4.8060907@archonet.com>
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What do you mean exactly but "pushing conditions inside" ?

I don't think I will have the option of testing on the full queries, as 
these take many days to write (the current ones, they are replacing on a 
mssql takes up more that 5kb of query). The current ones are nightmares 
from a maintaince standpoint.

Basicly what the application is doing is selecting some base data from 
the "large" table for a point in time (usually a quarter) and selects 
all matching auxilliare data from the other tables. They are made in a 
time-travel like manner with a first and last useable date.

The ways I have considered was :
1) write a big query in hand (not preferred as it gets hard to manage)
2) write layers of views (still not prefered as I still have to remember 
to put on the right conditions everywhere)
3) write layers of sql-functions (returning the right sets of rows from 
the underlying tables) - which I prefer from a development angel .. it 
gets very clean and I cant forget a parameter anywhere.

But I seem to remember (and I have used PGSQL in production since 7.0) 
that the planner has some problems with solution 3 (i.e. estimating the 
cost and rearranging the query), but frankly that would be the way I 
would like to go.

Based on the current (non-optimal) design and hardware constraints, I 
still have to make sure, the query runs fairly optimal - that means the 
planner must use indexes intelligently and other stuff as if it was 
(well-)written using solution 1.

What do you think of the three solutions ? And is there some ressource 
about the planners capabilites for someone like me (that is very used to 
write reasonably fast and complex sql, can read c-code, but does not 
really want to dig into the source code)

Regards

Svenne

Richard Huxton wrote:

> Svenne Krap wrote:
>
>> Hi there.
>>
>> I am currently building a system, where it would be nice to use 
>> multiple levels of views upon each other (it is a staticstics system, 
>> where traceability is important).
>>
>> Is there any significant performance reduction in say 10 levels of 
>> views instead of one giant, nested sql-statement ? I especially think 
>> exection planner-wise.
>
>
> The planner tries to push conditions "inside" views where it can. It's 
> not perfect though, and if you're writing a big query by hand you 
> might be able to do better than it.
>
> In short, I'd test if you can.