query-engine/ROADMAP.md

## Project Roadmap

This document tracks the current state and next steps for the repository.

> [!IMPORTANT]
> The project is still early-stage. Treat this roadmap as directional rather
> than fixed, and do not read unchecked items as implied existing behavior.

### Current Foundation

- [x] Rust crate and binary named `query_engine` / `query-engine`
- [x] Core term representation (`Constant`, `Null`, `Variable`)
- [x] Atom structure (predicate with terms)
- [x] Fact storage via `Instance`
- [x] Tuple-generating dependencies (TGDs)
- [x] Equality-generating dependencies (EGDs)
- [x] Rule and EGD builders
- [x] Variable substitutions and atom unification
- [x] Restricted chase implementation
- [x] Standard chase implementation
- [x] Existential variable support (labeled null generation)
- [x] Multi-atom rule bodies and heads
- [x] Equality merging with union-find support
- [x] REPL, script runner, and local web UI
- [x] Provenance-oriented explanation support
- [x] Relational schema, value, row, and result-set scaffolding
- [x] Predicate-backed catalog inference
- [x] Minimal SQL AST and parser
- [x] Logical plan scaffolding
- [x] Logical-plan execution for the first SQL slice
- [x] `SELECT-FROM-WHERE` support with positional or named columns
- [x] Basic multi-table SQL joins via qualified-column filtering
- [x] Table aliases for self-joins and qualified references
- [x] Basic `ORDER BY` support over output columns

### Near-Term Cleanup

- [ ] Keep all public docs aligned with actual implemented behavior
- [ ] Remove remaining stale terminology in comments and help text
- [ ] Expand examples for the current rule-engine workflow
- [ ] Add rustdoc coverage for the main public types
- [x] Document the current SQL subset and its limits

### Query-Engine Structure

- [x] Introduce a dedicated logical representation module
- [x] Define clear front-end, planning, and execution boundaries
- [ ] Add engine-level abstractions that are not chase-specific
- [x] Establish common schema and typed-value representations
- [ ] Design a source boundary for future scans and pushdown

### Front End and Planning

- [x] Add a parser-oriented module beyond the current rule REPL language
- [x] Add AST types for a structured query front end
- [x] Add logical plan node types
- [x] Add name resolution and schema validation hooks
- [x] Add expression typing and nullability tracking
- [x] Add aliases and richer projection expressions
- [x] Add joins across multiple predicate-backed tables
- [x] Add a catalog path for stable column naming beyond `c0`, `c1`, ...

### Execution and Optimization

- [ ] Introduce physical operator abstractions
- [x] Add a planning step from logical operators to executable operators
- [ ] Add basic rule-based logical rewrites
- [ ] Add statistics and cost-model scaffolding
- [ ] Add indexing and access-path abstractions

### Rule-Engine Evolution

- [x] Restricted chase
- [x] Standard chase
- [ ] Oblivious chase
- [ ] Skolem chase
- [ ] Core chase
- [ ] Negative constraints
- [ ] Stratified negation in rule bodies
- [ ] Disjunctive heads
- [ ] Aggregation support in rule evaluation
- [ ] Semi-naive evaluation
- [ ] Termination analysis helpers

### Data and Interoperability

- [ ] Fact import/export
- [ ] File-backed data source experiments
- [x] Table-like row or batch abstractions
- [ ] Stable script/query file format
- [ ] Integration with external storage or file formats

### Performance and Reliability

- [ ] Predicate indexing for fact lookup
- [ ] Incremental evaluation
- [ ] Benchmarks
- [ ] Fuzzing
- [ ] Profiling-guided optimization passes

### Testing and Tooling

- [x] Unit tests
- [x] Integration tests
- [x] Property-based tests
- [x] Regression tests
- [x] Initial SQL pipeline tests
- [ ] Benchmark coverage
- [ ] Snapshot-style frontend tests
- [ ] More planner/executor tests as those layers are added