docs: Improve streaming section of the user guide (#13750)

docs/_build/API_REFERENCE_LINKS.yml

@@ -169,6 +169,8 @@ rust:
   concat: https://docs.pola.rs/docs/rust/dev/polars_lazy/dsl/functions/fn.concat.html
   SQLContext: https://docs.pola.rs/py-polars/html/reference/sql.html
 
+  explain: https://docs.rs/polars/latest/polars/prelude/struct.LazyFrame.html#method.explain
+
   operators: https://docs.pola.rs/docs/rust/dev/polars_lazy/dsl/enum.Operator.html
 
   Array: https://docs.pola.rs/docs/rust/dev/polars/datatypes/enum.DataType.html#variant.Array

docs/src/python/user-guide/concepts/streaming.py

@@ -1,12 +1,26 @@
+# --8<-- [start:import]
 import polars as pl
+# --8<-- [end:import]
 
 # --8<-- [start:streaming]
-q = (
+q1 = (
     pl.scan_csv("docs/data/iris.csv")
     .filter(pl.col("sepal_length") > 5)
     .group_by("species")
     .agg(pl.col("sepal_width").mean())
 )
-
-df = q.collect(streaming=True)
+df = q1.collect(streaming=True)
 # --8<-- [end:streaming]
+
+# --8<-- [start:example]
+print(q1.explain(streaming=True))
+
+# --8<-- [end:example]
+
+# --8<-- [start:example2]
+q2 = pl.scan_csv("docs/data/iris.csv").with_columns(
+    pl.col("sepal_length").mean().over("species")
+)
+
+print(q2.explain(streaming=True))
+# --8<-- [end:example2]

docs/src/rust/user-guide/concepts/streaming.rs

@@ -2,16 +2,33 @@ use polars::prelude::*;
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
     // --8<-- [start:streaming]
-    let q = LazyCsvReader::new("docs/data/iris.csv")
+    let q1 = LazyCsvReader::new("docs/data/iris.csv")
         .has_header(true)
         .finish()?
         .filter(col("sepal_length").gt(lit(5)))
         .group_by(vec![col("species")])
         .agg([col("sepal_width").mean()]);
 
-    let df = q.with_streaming(true).collect()?;
+    let df = q1.clone().with_streaming(true).collect()?;
     println!("{}", df);
     // --8<-- [end:streaming]
 
+    // --8<-- [start:example]
+    let query_plan = q1.with_streaming(true).explain(true)?;
+    println!("{}", query_plan);
+    // --8<-- [end:example]
+
+    // --8<-- [start:example2]
+    let q2 = LazyCsvReader::new("docs/data/iris.csv")
+        .finish()?
+        .with_columns(vec![col("sepal_length")
+            .mean()
+            .over(vec![col("species")])
+            .alias("sepal_length_mean")]);
+
+    let query_plan = q2.with_streaming(true).explain(true)?;
+    println!("{}", query_plan);
+    // --8<-- [end:example2]
+
     Ok(())
 }

docs/user-guide/concepts/streaming.md

@@ -16,6 +16,30 @@ Streaming is supported for many operations including:
 - `with_columns`,`select`
 - `group_by`
 - `join`
+- `unique`
 - `sort`
 - `explode`,`melt`
 - `scan_csv`,`scan_parquet`,`scan_ipc`
+
+This list is not exhaustive. Polars is in active development, and more operations can be added without explicit notice.
+
+### Example with supported operations
+
+To determine which parts of your query are streaming, use the `explain` method. Below is an example that demonstrates how to inspect the query plan. More information about the query plan can be found in the chapter on the [Lazy API](https://docs.pola.rs/user-guide/lazy/query-plan/).
+
+{{code_block('user-guide/concepts/streaming', 'example',['explain'])}}
+
+```python exec="on" result="text" session="user-guide/streaming"
+--8<-- "python/user-guide/concepts/streaming.py:import"
+--8<-- "python/user-guide/concepts/streaming.py:streaming"
+--8<-- "python/user-guide/concepts/streaming.py:example"
+```
+
+### Example with non-streaming operations
+
+{{code_block('user-guide/concepts/streaming', 'example2',['explain'])}}
+
+```python exec="on" result="text" session="user-guide/streaming"
+--8<-- "python/user-guide/concepts/streaming.py:import"
+--8<-- "python/user-guide/concepts/streaming.py:example2"
+```