So I understand that a data warehouse is used for historic analysis and performance reporting but I don’t really know what it actually is or what the difference is to a database
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A database in the very generic sense means any system for storing and retrieving data. So, a data warehouse is technically a kind of database.
In common usage, database refers to the central store of data that an application uses to be able to run. So Reddit, for example, stores posts, comments, users, upvotes and everything else into its database and constantly updates it in real time as people use the site. When you browse the front page, the content is pulled from that database.
Data warehouse is a storage system that is specifically used for data analysis. It takes inputs from various different sources, which could range from application data to logs to telemetry to just about any other part of the business. End users will never directly interact with this system. Instead employees can run large scale queries against the warehouse to generate reports and charts for their own use. So if a Reddit executive wants to know the average daily users the site had over the last year, they will turn to the warehouse.
Why do you need two different systems at all? A database is optimized for very fast insert and update operations. This is necessary because you don’t want your site’s performance to be slow. A data warehouse on the other hand is able to handle much larger quantities of data, but queries take a lot longer to run.
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