A database that works is not the same as a database that works fast. Once your server is running, the next job of a database administrator is keeping queries quick as the data grows. Indexing & query tuning is where you learn how.
You'll start with how indexes work under the hood, including the B-tree structure that most databases use, and why an index lets the database skip scanning every row. From there the module covers tuning indexes to reduce blocking, whether rebuilding indexes actually helps, and how statistics shape the plans the optimizer chooses. Several lessons use SQL Server, but the ideas apply to any relational database.
Once your queries are fast, you'll be ready for the next module, Transactions & Concurrency, where you learn how a database stays correct when many users work at once.
A deep course on how databases work inside, including RAM vs hard disk, index tables, multi-level indexing, and B-tree visualization. Use it to understand why indexes make queries faster.
How to tune indexes to avoid blocking, reduce contention, and improve database performance, taught by SQL Server expert Brent Ozar.