What is database schema decomposition?
Why do we need decompositions?
Probably what is meant by this is that you start with one general schema for your database and decompose this into more specific schemas.
A good choice of more specific schemas can be determined using FK constraints defined on the schema, like join dependencies etc.
Why do you need it? I believe it significantly helps with normalization and manageability.
Related
I'm dealing with a huge ERP database, hundreds of tables, and am having trouble figuring out how one entity is referenced from another. Let's call them the "sale" table and the "shipment" table. Each has FK relationships with numerous other tables, but no FK links either one to the other, and no obvious associative table is linked to both.
Is there any good way using SQL or the psql command line to discover the chains of relationships that connect the two tables?
Is there any good way to discover the chains that connect a specific row or PK of "sale" and a specific row/PK of "shipment"?
You should look at a tool like schemacrawler. It's good a opensource tool for data modelling retro-engineering.
You can find documentation here.
I've created the Entity-relationship model of my database and the I normalized it, but I've a issue, I don't know if after of the normalization I've to create new relations between news tables derived from normalization and the original tables or not.
When you say "relations", you mean constraints, correct? :-) Normalizing a relational database means you broke the tables up to reduce redundancy and dependency. If you are making more tables, then yes you need to create new constraints (keys). You'll also need to run a conversion on your database to move the data to the new tables appropriately.
I am planning to use ASP.NET MVC2 implemented membership system to manage users. Database that uses application should have tables that are related with these users. Is it possible to use two different databases and make relationships (foreign keys) between them or I will have to merge these two databases into one?
Thanks,
Ile
It is NOT possible to put up relationships between databases. You CAN use triggers to ensure relational integrity.
Otherwise I would say: all in one database, put them into different schemata.
I would put membership/roles in a separate database. I don't think having foreign key constraints is that useful. Its better decoupling if you go through the membership API rather than join with the tables directly. The only thing in the membership database you might need to look up often is the username. If thats becomes a performance problem I'd probably just create an lookup table, either in memory or in a lookup table in the other component's database.
I've been tasked with revisiting a database schema we designed and use internally for various ticketing and reporting systems. Currently there exists about 40 tables in one Oracle database schema supporting perhaps six webapps.
However, there's one unifying relationship amongst them all: a rooms table describing the room. Room name, purpose and other data are thrown into a shared table for each app. My initial idea was to pull each of these applications into a separate database, and perform joins between a given database and the room database. But I've discovered this solution prevents foreign key constraints in SQL Server 2005. It seems silly to duplicate one table for each app and keep those multiple copies synchronized.
Should I just leave everything in one large DB, or is there something else I can do separate the tables without losing FK constraints?
The only way to achieve built-in referential integrity is to have the table inside the database in which it is referenced. You might be able to achieve the equivalent of referential integrity using triggers but it would likely be deathly slow.
You might be able to use SQL Server replication, in it's "Transactional replication" mode/form. http://msdn.microsoft.com/en-us/library/ms151176.aspx
if all the apps truly use and depend on the rooms - then keep them all in one DB.
you can still set privilege on the tables properly, and manage the data sets in the non overlapping areas normally -
is there any task you imagine you will not be able to perform when things are together?
I was thinking of putting staging tables and stored procedures that update those tables into their own schema. Such that when importing data from SomeTable to the datawarehouse, I would run a Initial.StageSomeTable procedure which would insert the data into the Initial.SomeTable table. This way all the procs and tables dealing with the Initial staging are grouped together. Then I'd have a Validation schema for that stage of the ETL, etc.
This seems cleaner than trying to uniquely name all these very similar tables, since each table will have multiple instances of itself throughout the staging process.
Question: Is using a user schema to group tables/procs/views together an appropriate use of user schemas in MS SQL Server? Or are user schemas supposed to be used for security, such as grouping permissions together for objects?
This is actually a recommended practice. Take a look at the Microsoft Business Intelligence ETL Design Practices from the Project Real. You will find (download doc from the first link) that they use quite a few schemata to group and identify objects in the warehouse.
In addition to dbo and etl, they also use admin, audit, part, olap and a few more.
I think it's appropriate enough, it doesn't really matter, you could use another database if you liked which is actually what we do.
I'm not sure why you would want a validation schema though, what are you going to do there?
Both the reasons you list (purpose/intent, security) are valid reasons to use schemas. Once you start using them, you should always specify schema when referencing an object (although I'm lazy and never specify dbo).
One trick we use is to have the same-named table in each of several schemas, combined with table partitioning (available in SQL 2005 and up). Load the data in first schema, then when it's validated "swap" the partition into dbo--after swapping the dbo partition into a "dumpster" schema copy of the table. Net Production downtime is measured in seconds, and it's all carefully wrapped in a declared transaction.