We want to replicate data from one database to several others (on another server). Would it make sense to replicate these tables to a shared database on the other server and have our cross-database queries reference the shared database... or would it make more sense to replicate out to each individual database on the other server? Would cross database joins pose a performance hit? Would cross-database constraints work as expected?
Replicating once to a shared database would help replication performance... I'm trying to evaluate whether or not any performance hit as a result of cross-database queries or constraints would be worth it.
Edit: It looks like cross database constraints are not possible in sql server? If this is true then we would have to replicate to each database
Cross database queries are somewhat slower that within the same DB. Foreign keys work within the same DB only. Usual approach is to create a separate schema in each DB (like ETL) and then replicate those tables to that schema. This approach is actually frequently used when replicating dimension tables between data marts.
When using cross-db approach, use triggers to implement constraints -- may be slow and complicated.
Depending on your application, you may implement foreign keys as "logical only" and run periodic "look for orphans" queries to deal with referential integrity.
Related
I am investigating the possibility to split one DB into multiple. We decided to move some tables into another database, but we have queries with join on these tables. I found a few solutions about how to achieve that:
Azure SQL Database elastic query
EXTERNAL DATA SOURCE
But I don`t know what the difference between them and what to choose.
Thanks for any help!
Azure SQL Database Elastic Queries and External data sources are two names for the same concept.
My suggestion is to avoid cross database queries and avoid splitting one database into multiples because query performance involving external data sources won't be the same no matter what strategy you choose to query those external tables.
If you still want to stick with the plan of splitting the database into multiple databases, then know that cross database queries show good performance when the remote tables are not big. When remote tables are big, this article shows you how to perform joins remotely using table variables and improve performance. This other article shows you also how to push parameterized operations to remote databases and improve performance.
if you are thinking to split your DB into multiple SQL server DB with the different host then you can prefer Linked server which has flexible to join across SQL servers
When our IT department converts Access databases to SQL Server the relationships do not transfer over. In the past, I have provided ERDs that they can use to build the relationships. In this case, I didn't.
What are the possible consequences of defining the table relationships in the MS Access Front End versus on the SQL Server itself?
It would be ideal if I could just create the relationships in Access and avoid submitting a request to IT, but I don't want to risk performance issues now or in the future.
There may be some misconceptions.
A relationship in SQL Server enforces referential integrity (an order cannot have a customer ID that doesn't exist). It does not automatically create an index on the Foreign Key, so it has per se no impact on performance.
But in most cases it is a good idea to define an index on a foreign key, to improve performance.
A relationship that you define in Access on linked tables does neither. It cannot enforce referential integrity (that's the server's job).
It is merely a "hint" that the tables are related via the specified fields, e.g., so that the Query Builder can automatically join the tables if they are added to the query design. (copied from here)
So you should
Create the relationships in SQL Server to avoid inconsistent data. ("But my application logic prevents that!", I hear you say. Well, applications have bugs.)
Create indexes on foreign keys where appropriate to avoid performance problems.
If you are working with queries in the Access frontend, additionally define the relationships there.
Ideally you should have a test server where you can yourself define the relationships, and just send the finished SQL script to IT.
I'm am currently developing one project of many to come which will be using its own database and also data from a central database.
Example:
the database "accountancy" with all accountancy package specific tables.
the database "personelladministration" with its specific tables
But we also use data which is general and will be used in all projects like "countries", "cities", ...
So we have put these tables in a separate database called "general"
We come from a db2 environment where we could create foreign keys between databases.
However, we are switching to MS SQL server where it is not possible to put foreign keys between databases.
I have seen that a workaround would be to use triggers, but I'm not convinced that is a clean solution.
Are we doing something wrong in our setup? Because it seems right to me to put tables with general data in a separate database instead of having a table "countries" in every database, that seams difficult to maintain and inefficiënt.
What could be a good approach to overcome this?
I would say that countries is not a terrible table to reproduce in multiple databases. I would rather duplicate static data like that than use more elaborate techniques. There is one physical schema per database in sql server and the schema can not be shared. That is why people use replication or triggers for shared data.
I can across this problem a while back. We have one database for authentication, however, those users have to be shared across multiple applications some of which have their own database.
Here is my question on this topic.
We resorted to replication and using an custom Authentication/Registration service agent to keep the data up to data.
Using views, in what Sourav_Agasti suggested in his answer, would be the most straight forward approach for static data. You can create views and indexed views and join data from databases on linked servers.
Create a loopback linked server and then create a view(if required, on each database) which accesses the table in this "central database" through this linked server. There will be a minor performance impact but it more than enough compensates by being very simiplistic.
Is it possible to define relationships between tables in different databases in SQL server 2008? And can you recommend an online tutorial for studying it? (I prefer ASP.NET, C#)
No, you can't have foreign keys between databases.
Data integrity is within a single database only. If you need transactional consistency across databases then you should use a single database. The main issue is backups/restores: you will end up with broken data after a restore because your backups are not consistent.
A recent blog article "One Database or Ten?" explains in more details
Saying that, you can use triggers if you need this and are prepared to have broken data
Yes you can but NOT using FOREIGN KEYS:
You can use specific stored procs, which checks the consistency - in
this case you have to make the user to use only these procedures for
all the CRUD operations in both DBS
Triggers, which will check the same
All of the above have to run within properly isolated transaction to
be sure, that your "just checked" values will not be deleted in a
moment
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?