storage-engine-playground/crates/query-ops/README.md

## Query Ops

This crate provides a small set of query operators that can be used to implement a simple query-plan executor.
The operators are: atom scan, semijoin, and natural join.

### Public API

| Item                                             | Kind    | Description                                                                                                                                                                                                         |
|--------------------------------------------------|---------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `scan_atom(&Table, &AtomPattern) -> Relation`    | function | Scans the table under the pattern and returns a binding relation with one column per distinct variable in first-occurrence order. Literal positions and repeated variables filter rows during the scan.             |
| `semijoin(&Relation, &Relation) -> Relation`     | function | Returns the rows of `left` whose values on the columns shared with `right` also appear in `right`. The output column list is the same as `left.columns`.                                                            |
| `natural_join(&Relation, &Relation) -> Relation` | function | Returns every pair of `left` and `right` rows that agree on shared columns. Each output row holds the columns of `left` followed by the non-shared columns of `right`.                                              |
| `Table`                                          | struct  | Holds positional input rows of fixed arity and carries no column names. Construct it with `Table::new(arity)` or `Table::from_rows(arity, rows)`.                                                                   |
| `AtomPattern`                                    | struct  | Specifies, for each table column, either a variable to bind or a literal value to match. The pattern is a `Vec<Term>` whose length must equal the table's arity.                                                    |
| `Term`                                           | enum    | Represents one position of an `AtomPattern`. A term is either `Var(String)` to bind the cell to a named variable, or `Lit(Value)` to require the cell to equal a given value.                                       |
| `Relation`                                       | struct  | Holds rows over named columns and is the type produced by every operator. Construct it with `Relation::new(columns)` or `Relation::from_rows(columns, rows)`. Column names within a single relation must be unique. |
| `Value`                                          | enum    | Represents a single cell value stored in a `Table` or `Relation`. A value is either `Int(i64)` or `Str(String)`.                                                                                                    |

Data types and their relationships:

<div align="center">
  <picture>
    <img alt="Types" src="docs/diagrams/types.svg" height="60%" width="60%">
  </picture>
</div>

### Example

The rule below returns the authors of every bestseller along with the book's price.
It uses all three operators:
- `scan_atom` for the three input tables,
- `semijoin` to keep only authors of bestsellers,
- and `natural_join` to attach each book's price.

```prolog
Q(name, book, dollars) :- author(name, book), bestseller(book), price(book, dollars).
```

```rust
use query_ops::atom::{AtomPattern, Term, scan_atom};
use query_ops::join::{natural_join, semijoin};
use query_ops::table::Table;
use query_ops::value::Value;

fn s(x: &str) -> Value {
    Value::Str(x.to_string())
}
fn i(x: i64) -> Value {
    Value::Int(x)
}

fn main() {
    let author = Table::from_rows(
        2,
        vec![
            vec![s("Alice"), s("Foo")],
            vec![s("Bob"), s("Bar")],
            vec![s("Alice"), s("Baz")],
            vec![s("Carol"), s("Qux")],
        ],
    );
    let bestseller = Table::from_rows(1, vec![vec![s("Foo")], vec![s("Baz")]]);
    let price = Table::from_rows(
        2,
        vec![
            vec![s("Foo"), i(25)],
            vec![s("Bar"), i(15)],
            vec![s("Baz"), i(30)],
            vec![s("Qux"), i(20)],
        ],
    );

    let author_rel = scan_atom(
        &author,
        &AtomPattern {
            columns: vec![Term::Var("name".to_string()), Term::Var("book".to_string())],
        },
    );
    let bestseller_rel = scan_atom(
        &bestseller,
        &AtomPattern {
            columns: vec![Term::Var("book".to_string())],
        },
    );
    let price_rel = scan_atom(
        &price,
        &AtomPattern {
            columns: vec![Term::Var("book".to_string()), Term::Var("dollars".to_string())],
        },
    );

    let authors_of_bestsellers = semijoin(&author_rel, &bestseller_rel);
    let result = natural_join(&authors_of_bestsellers, &price_rel);

    assert_eq!(
        result.columns,
        vec!["name".to_string(), "book".to_string(), "dollars".to_string()],
    );
    assert_eq!(
        result.rows,
        vec![
            vec![s("Alice"), s("Foo"), i(25)],
            vec![s("Alice"), s("Baz"), i(30)],
        ],
    );
}
```

How it works:

<div align="center">
  <picture>
    <img alt="Types" src="docs/diagrams/workflow.svg" height="90%" width="90%%">
  </picture>
</div>

### Test

```sh
cargo test -p query-ops
```