Library Guide: Add Using the DataFrame API

docs/source/library-user-guide/using-the-dataframe-api.md

@@ -19,4 +19,129 @@
 
 # Using the DataFrame API
 
-Coming Soon
+## What is a DataFrame
+
+`DataFrame` in `DataFrame` is modeled after the Pandas DataFrame interface, and is a thin wrapper over LogicalPlan that adds functionality for building and executing those plans.
+
+```rust
+pub struct DataFrame {
+    session_state: SessionState,
+    plan: LogicalPlan,
+}
+```
+
+You can build up `DataFrame`s using its methods, similarly to building `LogicalPlan`s using `LogicalPlanBuilder`:
+
+```rust
+let df = ctx.table("users").await?;
+
+// Create a new DataFrame sorted by  `id`, `bank_account`
+let new_df = df.select(vec![col("id"), col("bank_account")])?
+    .sort(vec![col("id")])?;
+
+// Build the same plan using the LogicalPlanBuilder
+let plan = LogicalPlanBuilder::from(&df.to_logical_plan())
+    .project(vec![col("id"), col("bank_account")])?
+    .sort(vec![col("id")])?
+    .build()?;
+```
+
+You can use `collect` or `execute_stream` to execute the query.
+
+## How to generate a DataFrame
+
+You can directly use the `DataFrame` API or generate a `DataFrame` from a SQL query.
+
+For example, to use `sql` to construct `DataFrame`:
+
+```rust
+let ctx = SessionContext::new();
+// Register the in-memory table containing the data
+ctx.register_table("users", Arc::new(create_memtable()?))?;
+let dataframe = ctx.sql("SELECT * FROM users;").await?;
+```
+
+To construct `DataFrame` using the API:
+
+```rust
+let ctx = SessionContext::new();
+// Register the in-memory table containing the data
+ctx.register_table("users", Arc::new(create_memtable()?))?;
+let dataframe = ctx
+  .table("users")
+  .filter(col("a").lt_eq(col("b")))?
+  .sort(vec![col("a").sort(true, true), col("b").sort(false, false)])?;
+```
+
+## Collect / Streaming Exec
+
+DataFusion `DataFrame`s are "lazy", meaning they do not do any processing until they are executed, which allows for additional optimizations.
+
+When you have a `DataFrame`, you can run it in one of three ways:
+
+1.  `collect` which executes the query and buffers all the output into a `Vec<RecordBatch>`
+2.  `streaming_exec`, which begins executions and returns a `SendableRecordBatchStream` which incrementally computes output on each call to `next()`
+3.  `cache` which executes the query and buffers the output into a new in memory DataFrame.
+
+You can just collect all outputs once like:
+
+```rust
+let ctx = SessionContext::new();
+let df = ctx.read_csv("tests/data/example.csv", CsvReadOptions::new()).await?;
+let batches = df.collect().await?;
+```
+
+You can also use stream output to incrementally generate output one `RecordBatch` at a time
+
+```rust
+let ctx = SessionContext::new();
+let df = ctx.read_csv("tests/data/example.csv", CsvReadOptions::new()).await?;
+let mut stream = df.execute_stream().await?;
+while let Some(rb) = stream.next().await {
+    println!("{rb:?}");
+}
+```
+
+# Write DataFrame to Files
+
+You can also serialize `DataFrame` to a file. For now, `Datafusion` supports write `DataFrame` to `csv`, `json` and `parquet`.
+
+When writing a file, DataFusion will execute the DataFrame and stream the results to a file.
+
+For example, to write a csv_file
+
+```rust
+let ctx = SessionContext::new();
+// Register the in-memory table containing the data
+ctx.register_table("users", Arc::new(mem_table))?;
+let dataframe = ctx.sql("SELECT * FROM users;").await?;
+
+dataframe
+    .write_csv("user_dataframe.csv", DataFrameWriteOptions::default(), None)
+    .await;
+```
+
+and the file will look like (Example Output):
+
+```
+id,bank_account
+1,9000
+```
+
+## Transform between LogicalPlan and DataFrame
+
+As shown above, `DataFrame` is just a very thin wrapper of `LogicalPlan`, so you can easily go back and forth between them.
+
+```rust
+// Just combine LogicalPlan with SessionContext and you get a DataFrame
+let ctx = SessionContext::new();
+// Register the in-memory table containing the data
+ctx.register_table("users", Arc::new(mem_table))?;
+let dataframe = ctx.sql("SELECT * FROM users;").await?;
+
+// get LogicalPlan in dataframe
+let plan = dataframe.logical_plan().clone();
+
+// construct a DataFrame with LogicalPlan
+let new_df = DataFrame::new(ctx.state(), plan);
+```