As Reporting Services allows us to implement some quite complex logic, report authors are often tempted to build a lot of it in their reports. Having some simple calculations there does not generally cause harm, as simple + and – operations specific to a report should probably be built in it. However, I have seen aggregations, complex queries and code hacks, which would normally be performed fairly easily in Analysis Services or SQL Server built in Reporting Services.
There are a few distinct benefits we can get from keeping logic out of our reports.
- Firstly, by having our MDX scripts and SQL Queries (stored procedures) in the back-end allows us to use them in different reports with no need to maintain them by painfully synchronizing them with every change. Also, this allows business MDX-ignorant users to use complex measures, increasing the value of the BI solution for them.
- Then, normally, our database server is usually maintained by people who have skills in SQL or MDX, and can analyse and optimise the scripts there instead of relying on report authors to do so. Even if we have BI professionals maintaining the whole solutions, it is far more convenient to have most of the logic stored on the same server. Whether it is a performance bottleneck, or a bug, it is far easier to fix it by tracing either cube calculations or stored procedures than logic stored in a report.
- Sending a large amount of data between the back-end server and SSRS is increasing the stress on the network, which can be critical when multiple requests with millions of rows in results are passed to it.
- Caching is important when different reports use the same stored procedures or MDX calculations.
- And a last point: a database/OLAP server is just faster in calculations that SSRS.
In general, using stored procedures as SQL Server data sources instead of building queries in SSRS; or alternatively, keeping MDX in calculations in the OLAP cubes is the first step towards avoiding excessive code accumulation in SSRS. Then, avoiding anything more complex than simple sums and groupings in the reports and moving them to the stored procedures or MDX calculations is another desirable way to structure our solutions. Finally, even simple things like Actual – Budget variance measure should be built in MDX or SQL rather than in the reports. Following these simple rules and keeping ourselves disciplined when building reporting solutions improves their performance, maintainability and even usability.