andygrove opened a new issue, #45:
URL: https://github.com/apache/datafusion-java/issues/45
### Is your feature request related to a problem or challenge?
DataFrame introspection is currently limited to `count()`, `show()`,
and `show(int)`. Users wanting to inspect schema, see the planned
query, or materialize an intermediate result have no Java entry point.
### Describe the solution you'd like
- **`DataFrame.schema()`** — return an Arrow `Schema`. Reuse the IPC
round-trip already established for `SessionContext.tableSchema`
(`tableSchemaIpc` pattern in `SessionContext.java`). Non-consuming;
the DataFrame remains usable.
- **`DataFrame.explain(boolean verbose, boolean analyze)`** — wraps
`DataFusion::DataFrame::explain`, returning a `DataFrame` whose rows
are the plan-explanation strings. Caller calls `show()` / `collect()`
on the result. Matches DataFusion's own semantics.
- **`DataFrame.cache()`** — materializes the plan into an in-memory
table and returns a new DataFrame. Async on the Rust side; blocks on
the Tokio runtime, same pattern as `collect`. Caller-closes the
returned DataFrame.
- **`DataFrame.describe()`** — async, returns a DataFrame with summary
stats (count, mean, stddev, min, max) per numeric column. Same
pattern as `cache`.
### Describe alternatives you've considered
For `schema`: `ctx.tableSchema(name)` works only for *registered*
tables. A user who built a DataFrame via `sql("SELECT …")` or chained
transformations has no schema accessor.
For `explain`: `ctx.sql("EXPLAIN <query>")` works but only against a
SQL string.
### Additional context
All four are non-consuming except `cache` and `describe` (which return
new DataFrames the caller owns and closes). The schema/explain pair are
the most-requested and could land first as a smaller PR; `cache` and
`describe` are independent and can follow.
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