To be a true database architect, you must understand what happens under the hood when you type SELECT *. SQL Server is not just a spreadsheet; it is a complex engine made of two main parts: the Relational Engine (Query Optimizer) and the Storage Engine.
Everything in SQL Server is stored in 8KB Pages. Eight of these pages form an Extent (64KB). When you ask for data, SQL Server doesn't read a "Row"; it reads the entire Page into the Buffer Pool (RAM). This is why selecting 10 columns takes nearly the same time as selecting 1 column if they are on the same page.
Disk I/O is slow. Memory is fast. SQL Server's primary goal is to keep as much data in the Buffer Pool as possible. If the data is already in RAM, you get a Logical Read (0.01ms). If it has to go to the disk, you get a Physical Read (10ms+). A database architect's job is to minimize logical reads through proper indexing.
When you write data, SQL Server writes to the Transaction Log (.ldf) BEFORE it writes to the data file (.mdf). This is called Write-Ahead Logging. If the power goes out, SQL Server uses the log to "Replay" the changes, ensuring your data is never corrupted.
Q: "Why does SQL Server performance drop when 'Page Life Expectancy' (PLE) is low?"
Architect Answer: "PLE measures how many seconds a data page stays in the Buffer Pool before being kicked out to make room for new data. If PLE is low (e.g., less than 300 seconds), it means your server is under **Memory Pressure**. It is constantly reading from disk, using the page once, and then throwing it away. This results in massive Disk I/O bottlenecks and high CPU usage."