Re: Database design and triggers... - Mailing list pgsql-general

Actually, I don't think you need Table 2 at all.  Rather, you can fairly
easily incorporate all the functionality of CurrentCountTable into Table
2 and then use a query or a VIEW.

Say you have these two tables.  [I'm not the best at data modeling yet,
so I wouldn't necessarily take these verbatim.  I'm still a newbie.
Listed here is pseudo-SQL.]

TABLE "Transaction"
(
  "TransactionID" serial,
  "OperationID" integer,
  "OperationType" char(15) NOT NULL,
  "ClientID" integer NOT NULL,
  "TransactionDate" date NOT NULL DEFAULT 'epoch',
  "UserID" char(15) NOT NULL,
  PRIMARY KEY ("TransactionID"),
  UNIQUE ("OperationID")
)

TABLE "TransactionItem"
(
  "TransactionItemID" serial,
  "OperationID" integer NOT NULL,
  "PartID" integer NOT NULL,
  "LotID" integer NOT NULL,
  "Qty" integer NOT NULL,
  PRIMARY KEY ("TransItemID"),
  FOREIGN KEY ("OperationID")
      REFERENCES "Transaction" ("OperationID")
      ON UPDATE CASCADE ON DELETE RESTRICT,
  UNIQUE ("OperationID", "PartID")
)

Now, when you store Qty, you store a positive number if the inventory
increases and a negative number if it decreases.

Now, you can use a query or create a VIEW based on this query:

SELECT "OperationID"
   , "ClientID"
   , "TransactionDate"
   , "PartID"
   , "LotID"
   , "Qty"
   , "UserID"
FROM "Transaction" NATURAL JOIN "TransactionItem";

Alternately, you can continue to store the Qty as an unsigned integer
and then use this query/VIEW:

SELECT "OperationID"
   , "ClientID"
   , "TransactionDate"
   , "PartID"
   , "LotID"
   , CASE
        WHEN "OperationType" = 'Incoming' THEN "Qty"
        WHEN "OperationType" = 'Outgoing' THEN (-1 * "Qty")
     END
   , "UserID"
FROM "Transaction" NATURAL JOIN "TransactionItem";

As far as speed, speed is always an issue.  PostgreSQL is going to
perform better than Access, but don't use good performance as a crutch
for bad design.

As far as normalization, it is possible to take it too far.  There is a
time when de-normalizing a database will significantly improve its
performance even if it involves duplicating data.  4NF is not the goal
of DB design, having a usable database is.  Knowing when and how to
de-normalize is much more difficult than learning to design a normalized
data model.


--
Brandon Aiken
CS/IT Systems Engineer

-----Original Message-----
From: pgsql-general-owner@postgresql.org
[mailto:pgsql-general-owner@postgresql.org] On Behalf Of
romantercero@gmail.com
Sent: Thursday, September 07, 2006 7:09 PM
To: pgsql-general@postgresql.org
Subject: Re: [GENERAL] Database design and triggers...

Hi Brandon, thanks for answering.

The information regarding a transaction is stored on two separate
tables due to normalization. The first table stores general information
regarding the transaction like Transaction number, date, customer ID,
type of transaction, userID etc... The second table stores the details
of the purchase like the products and quantities that the transaction
affected. Like this:

Table 1:
Operation#     Type_of_operation    Client#        Date    UserID
1           Inbound                    10        1/1/2000
Paul37
2            Outbound                    10        1/2/2000
Steve04

Table 2:
Operation#    Part#   Lot#           Qty
1        X    a        10
1        Y    ds1        9
1        Z    54ad        7
2        X    a        10

Table 2 has Table 1's Operation field as a Foreign key.

Now, to obtain a current count of  Part X we have to create a temporary
table on which we can use aggregate functions.

CurrentCountTable:

Operation    Client#    Date    Part#    Lot#    Qty    UserID
1    10    1/1/2000    X    a    +10    Paul37
1    10    1/1/2000    Y    ds1    +9    Paul37
1    10    1/1/2000    Z    54as    +7    Paul37
2    10    1/2/2000    X    a    -10    Steve04

Now, on the temporary table called CurrentCountTable we can use an
aggregate function, The problem is that creating this table is slow
with INSERT INTO, and so are aggregate functions (On MS Access). So
Naturally it occurred to me that triggers can keep a permanent version
of the CurrentCountTable up to date every time some one inserts in to
Table 1 and Table 2. But it has to be perfect to avoid inconsistencies.

So, are triggers a safe bet? Is using triggers more advisable over the
temporary table solution because on PSQL speed is not an issue? Or
should I use views?

Thanks!!! (Hope the tables got listed correctly :-/ )



"Brandon Aiken" wrote:
> It's not clear to me how your data is organized or exactly what you're
> counting.  If I understand you correctly, yes, you could use triggers
to
> maintain a table in this manner.  However, why can't you simply use a
> SELECT query using the SUM() or COUNT() aggregate functions?  If the
> queries are slow, do some index tuning.
>
> --
> Brandon Aiken
> CS/IT Systems Engineer
>
> -----Original Message-----
> From: pgsql-general-owner@postgresql.org
> [mailto:pgsql-general-owner@postgresql.org] On Behalf Of
> romantercero@gmail.com
> Sent: Wednesday, September 06, 2006 2:05 PM
> To: pgsql-general@postgresql.org
> Subject: [GENERAL] Database design and triggers...
>
> Hi everybody. Hope I'm posting in the correct group.
>
> My question is part design and part implementation.
>
> Since we are creating an inventory system we need to have the clients
> pull up current inventory. Also, we need to have the past transactions
> stored for reference and billing. In our previous system in MS Access
> we accomplished this by adding up all of the transactions stored in
two
> tables and generating a temporary table with the latest inventory
> count. The problem with this approach is that it is slow because the
> temporary table has to be created every time a user needs to see a
> report or work on a form. Even when instead of creating a temporary
> table we use a query it is still slow. With postgreSQL I found out
> about triggers and I figure that instead of calculating the current
> inventory count and storing it in a table every time a client needs it
> I could have a triggers maintain a table with the current count by
> incrementing or decreasing the amounts each time a transaction is
> stored in the transaction tables. My worry is that if for some reason
a
> trigger were to somehow fail to execute correctly there would be an
> inconsistency between the transactions table and the current inventory
> count table and it would have to be calculated from scratch taking in
> to account all of the past transactions in the transactions table.
>
> Are trigger a very safe way to use in the way I describe? Or should I
> try using views or stick with the temporary table solution we already
> have?
>
> My second part of the question is if there is a tutorial for triggers
> and stored procedures and what is the difference between Procedures
and
> Functions?
>
> Thanks Beforehand!
>
>
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