Reducing code complexity and optimizing code structure

I have an application whose reporting module has been badly coded. It is a standard Laravel model which holds a collection of reporting methods that each contain a specific database query. To use the reporting module in the application, the end-user selects from a pre-determined set of filters to generate a requested report. Many reports share common functionality and do the majority of high-level logic within the database layer.

I have an ongoing goal of reducing code complexity and optimizing code structure. How can I leverage common OOP design patterns to ensure altering the code in any given report does not negatively affect the other reports?

Also, the reporting module is relatively static in definition and unlikely to change over time. The majority of reports generated by the application involve nested sub-queries as well as standard grouping & filtering options. From a solution architecture perspective (without looking too much into DB structure or table indices) what are the recommendations for scaling the report functionality to ensure optimized performance?

via Chebli Mohamed