Chapter 10: Specialize One Primitive Loop

Work in progress. This course has not yet received the author’s final audit. Chapters, exercises, and commands may still change before the course is marked complete.

Correct generic evaluation comes first. Now you can optimize one common case and prove that every other representation still follows the established path.

Prerequisites: Chapter 9 and basic benchmarking discipline.

By the end of this chapter, you will:

• select an all-valid i32 loop once per batch;
• preserve the general path for nulls, dictionaries, and other operators; and
• compare loop shapes without moving validation into the measured row loop.

cargo x copy-test --chapter 10
cargo test -p type-exercise-starter chapter_10 --locked

The first run should fail on fast-path selection while the earlier general evaluator stays green.

Prove the fast-path preconditions

A checked ColumnViewImpl::try_non_null_array proves once that an array has no null rows and records Nullability::NonNull beside its PhysicalType. A constant value is already one non-null scalar repeated for the batch. These facts permit four dense binary loop shapes over the same primitive array representation:

• array / array;
• array / constant;
• constant / array; and
• constant / constant.

A dictionary, typed null, nullable array, type mismatch, arity mismatch, or length mismatch must return to the general contract or the same structured error.

Checkpoint 1: select once per batch

• Target: type-exercise-starter/src/physical_type.rs::Nullability, type-exercise-starter/src/column.rs::{ColumnViewImpl::nullability, ColumnViewImpl::try_non_null_array}, and type-exercise-starter/src/expression.rs::{Expression::output_nullability, PrimitiveLoop, PrimitiveBinaryExpression::evaluate_with_loop, BinaryExpression::new_with_loop}.
• Change: keep one primitive array representation, establish physical nullability at the column boundary, and choose the dense i32 path only after ordinary validation succeeds. The builtin catalog stores that selection as another whole-batch kernel; it does not reintroduce a scalar callback.
• Preserve: output values, nulls, and errors are identical to evaluate.
• Run: the Chapter 10 focused test.
• Passing means: all four dense shapes report their selected loop and every fallback reports PrimitiveLoop::General.

Checkpoint 2: forward through binding

• Target: type-exercise-starter/src/binder.rs::{BoundExpression::output_nullability, BoundExpression::evaluate_with_loop}.
• Change: delegate nullability propagation and evaluation to the already-selected physical expression.
• Preserve: logical selection does not choose a fast path; batch representation does.
• Run: focused and cumulative tests.
• Passing means: binding and non-primitive catalog entries remain unchanged.

Required and extension work

Representative i32 specialization and semantic fallbacks are required. Fast paths for every numeric family and operator are extensions. Do not duplicate the full evaluator to chase a benchmark.

cargo test -p type-exercise-starter chapter_10 --locked
cargo test -p type-exercise-starter --lib --locked

After the tests pass, you may run the maintained reference benchmark without reading its source:

cargo bench -p type-exercise --bench expression

It reports the four dense shapes and three fallbacks separately. Setup and dictionary validation stay outside the timed row loop. The measurements are machine-specific observations, not a completion gate.

Next: Chapter 11 builds a one-level List column.