data-sourceslisted

# Data Sources Skill Pick the right data source in the `data` module, respect the schema contract that keeps forward-only sources away from schema-dependent callers, and run the uniqueness checker to find whether a set of columns can serve as a key. ## In-memory vs iterative — pick first Every format ships in two variants: - **`InMemory…`** — loads all rows once (`readAll()`), then runs multiple recursive checks cheaply. Use when the data fits in heap. - **`Iterable…`** — holds one row at a time (tiny heap), but re-reads the source for each recursive pass. Use for large data or streaming. ## The schema contract (don't fight it) - **`IterableDataSource`** — the base forward-only contract: `open()`, `nextRow()`, `hasMoreData()`, `reset()`, `nextRows(int batchSize)`. - **`ColumnarDataSource extends IterableDataSource`** — adds `getColumnNames()` for sources whose columns are known up front. - Forward-only sources that discover keys as they stream — **iterable JSON, YAML, TOON** — deliberately do **not** implement `ColumnarDataSource`. Callers that need the schema (e.g. the uniqueness check) depend on the narrower `ColumnarDataSource`, so a schema-less source can never be handed in and answer with `null`. Don't widen `IterableDataSource` to "fix" a compile error — reach for a columnar source instead. ## Source picker | Format | In-memory | Iterable | Notes | |---|---|---|---| | CSV | `InMemoryCSVDataSource` | `CSVDataSource` | file path or `InputStream` | |