Re: Normalization or Performance

От: Iain
Тема: Re: Normalization or Performance
Дата: ,
Msg-id: 009401c4d8d9$a5fc6160$7201a8c0@mst1x5r347kymb
(см: обсуждение, исходный текст)
Ответ на: Normalization or Performance  (Alvaro Nunes Melo)
Список: pgsql-performance

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Normalization or Performance  (Alvaro Nunes Melo, )
 Re: Normalization or Performance  (Richard Huxton, )
 Re: Normalization or Performance  ("Jim C. Nasby", )
 Re: Normalization or Performance  ("Iain", )

Hi,

without knowing much about your system, it seems to me that the current
status of a client should be represented by a status code on the client
record. History is the list of *past* status codes. The full history,
including the current status of a client would be obtained using a union.

I had a situation which might have some parallels with yours. nthis case the
tables represented orders and receivals. The status of an order item was
held on a code in the receival itemss table (an order item can be received
many times). This is seems to me to be not normalized as the status was
actually the status of the order item, not the receival. The receival just
caused the status of the order item to change. This arrangement required
ridiculously complex sql and resulted in poor performance. Moving the status
code to the order item and implementing a simple trigger on the receival
items table cleaned things up significantly.

To put it simply, if the current status of an order item is a simple
attribute of the order item, then it should be in the order item table. The
same might be said for your client.

This is just my personal opinion though and I'm always open to alternative
opinions, as I think you are.

regards
Iain
----- Original Message -----
From: "Alvaro Nunes Melo" <>
To: "Pgsql-Performance" <>
Sent: Friday, December 03, 2004 2:05 AM
Subject: [PERFORM] Normalization or Performance


> Hi,
>
> Before writing this mail, I'd researched a little about this topic, and
> got some opinions from guys like Fabien Pascal, who argues that logical
> design should be separated from physical design, and other sources. As
> this is not fact, I'm writing to you guys, that make things work in real
> world.
>
> We started our first big (for our company standards) project always
> thinking in normalization. But once we imported legacy data into the DB,
> things got harder.
>
> One example is the clients status. A client might be active, inactive or
> pending (for many reasons). We store all the status a client have since
> it is in the system. To check what is the actual status of a client, we
> get the last status from this historical status table. This take a
> considerable time, so our best results were achieved building  a
> function that checks the status and indexing this function. The problem
> is that indexed functions mus bu immutable, so as you can figure, if the
> status change after the creation of the index, the retunr of the
> function is still the same.
>
> What do you suggest for situations like this? Should I add a field to
> clients table and store its actual status, keep storing data in the
> historical table an control its changes with a trigger?
>
> There are other situations that are making things difficult to us. For
> example, one query must return the total amount a client bought in the
> last 90 days. It's taking too long, when we must know it for many
> clients, many times. So should I create summarization tables to store
> this kind of stuff, update it with a trigger in daily basis (for
> example), and solve this problem with one join?
>
> Our database is not that big. The larger table has about 7.000.000 rows.
> About 50.000 clients, half of them active. All that I'd point out above
> uses indexes for queries, etc. But even with this it's not been fast
> enough. We have a Dell server for this (I know, the Dell issue), a Dual
> Xeon 2.8, SCSI HD, 1 GB mem. Do we need better hardware for our system?
>
> --
> +---------------------------------------------------+
> |  Alvaro Nunes Melo    Atua Sistemas de Informacao |
> |         www.atua.com.br       |
> |    UIN - 42722678            (54) 327-1044        |
> +---------------------------------------------------+
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 8: explain analyze is your friend



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