Definition

When processing your query, the RDBMS will actually store an RA tree (like a bunch of labeled nodes and pointers)

After some optimizations, the RA tree is converted into instructions (like a bunch of functions linked together).

Follow FWGHOS, we can think the tuples are “flow up” the tree, getting modified along the way.

Read RA tree from bottom to top

• Bottom → Data Sources
• Top → Query Output

Operators

• Every operator takes 1 (unary) or 2 (binary) relations as input
• If you need to operate with 2 relations (eg. join 3 relations together), you would need to stack the operations
• Every operator outputs a valid relation

$\pi$ (Projection Symbol)

Projection removes unspecified columns, roughly like selection