◄ prev contents next ►

⚠

The web version only has simple instructions since chapter 04, while the full book has detailed explanations and background info.

0502: Expression Evaluation

Interpreter

We can now add computation to SELECT and UPDATE statements:

select a + b, c * d from t where a = 123;
update t set x = x + 1 where a = 123;

In the previous step, expressions were parsed into a tree structure. During evaluation, we must compute subtrees first, then compute the current node. So this is a recursive function that walks a tree:

func evalExpr(schema *Schema, row Row, expr interface{}) (*Cell, error) {
    switch e := expr.(type) {
    case string:
        // TODO
    case *Cell:
        return e, nil
    case *ExprBinOp:
        left, err := evalExpr(schema, row, e.left)
        if err != nil {
            return nil, err
        }
        right, err := evalExpr(schema, row, e.right)
        if err != nil {
            return nil, err
        }
        // TODO
    default:
        panic("unreachable")
    }
}

The evalExpr() function takes column data (schema, row), uses a switch to check the expression type, and returns the computed result (*Cell).

ⓘ

CodeCrafters.io has similar courses in many programming languages, including build your own Redis, SQLite, Docker, etc. It’s worth checking out.

◄ prev contents next ►