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crates/query-ops/README.md
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@ -1,12 +1,12 @@
## 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.
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`. |
@ -28,6 +28,7 @@ Data types and their relationships:
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.
@ -119,3 +120,16 @@ How it works:
```sh
cargo test -p query-ops
```
### Notes
- **Tables versus relations:** A `Table` is positional (fixed arity with no column names), while a `Relation` is keyed by variable names. The atom
scan is the bridge that turns one into the other, and every join after that operates on relations.
- **Joining is by column name:** `semijoin` and `natural_join` find shared columns by matching the strings in `Relation.columns`. Whether two
relations join on a column therefore depends on the variable name you chose in each `AtomPattern`. Picking the same `Term::Var(name)` in two
patterns is what makes them join on that column.
- **No projection operator yet:** `natural_join` always carries forward every column from both inputs, and `scan_atom` keeps every distinct variable
that appears in the pattern. There is no way to drop columns from a relation today, so a result may include more columns than the Datalog rule head
implies.
- **Bulk, not streaming:** Each operator materializes its full output as a new `Relation` and returns it. Operators compose by passing the result of
one as input to the next: `natural_join(&semijoin(&a, &b), &scan_atom(&t, &p))`.