2011/10/22 David Johnston <pol...@yahoo.com>: > On Oct 22, 2011, at 6:41, Linos <i...@linos.es> wrote: > >> Hi all, >> i need a little of advice on what could be the best way to store this >> information. >> >> We need to calculate the difference in costs for our operations, we are >> already >> storing our vendor invoices in the database so calculate the monetary change >> it >> is a no-brainer but we need to store special attributes for any of the >> invoices >> that we need to compare too, for example: >> -electric provider: total Kw. >> -water provider: total m3. >> -car maintenance: kilometers of the car. >> -mobile phones provider: international call minutes, national minutes, >> number >> of sms, etc.. >> >> And much more types/variables, the number of variables can change, not every >> day >> but still can change, i would like that they can be defined/changed from our >> application, so alter table to add columns don't seem the best way (still an >> option though). We will have "generic" reports that will show us changes in >> costs and specific reports for the types with "extended attributes" that we >> want >> to compare. >> >> To compare values from this "extended attributes" i think we have two ways: >> 1- have them in columns and use standard SQL. >> 2- create the columns with a function that reads this attrs and create >> the columns. >> >> So far i thin we have this options: >> 1- a bunch of columns that would be null except when the type of the >> invoice >> uses them. >> 2- a table related with the vendor invoices table for every type of >> invoice >> with his specifics columns. >> 3- a key/value in a separate table related with the vendor invoices table >> where >> i store the extended attrs of every invoice that needs them. >> 4- use a hstore column in the vendor invoces table to store this attrs. >> >> The first two have the problem of probably changes to the number of >> attributes >> of every type and give a more closed solution, apart from that 1- seems to >> be a >> bit awkward and 2- would need the application that creates the query to know >> with what table should join for every type (other point we will need to >> change >> if we want to create new invoices types). >> >> The last two have his own problems too, with 3 i will need to create a >> function >> that return rows as columns to compare them, with 4- given that i will store >> the >> attrs of every type in the database anyway i can use the operator -> (with a >> CASE using operator ? returning 0 if the searched attr it is not in the >> hstore) >> but still don't seem a clean solution for me. >> >> For me it seems i am missing something, probably any of you have a much more >> elegant (or correct) way to handle this situation, what would be your advice? >> Thanks. >> >> > > Create a table with a single numeric column and multiple category columns. > > ( amount_value, amount_unit, amount_category, vendor_id )
This is EAV model - is good for smaller datasets, for larger datasets is problematic. There is second possibility - using a "hstore" contrib module - that emulates HASH table - It has better for larger datasets. Regards Pavel Stehule > > If necessary each "amount_value" data type should have it's own table since > the processing logic will vary (I.e., you cannot subtract text or Boolean > values). > > You are , in effect, creating multiple tables but combining them into one and > using the category column to distinguish between them. > > David J. > -- > Sent via pgsql-sql mailing list (pgsql-sql@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-sql > -- Sent via pgsql-sql mailing list (pgsql-sql@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-sql
