Add examples of how to convert logical plan to/from sql strings (#10558)

* Add examples of how to convert logical plan to/from sql strings

* Fix clippy

* fix comments

* fix comments

datafusion-examples/examples/plan_to_sql.rs

@@ -20,7 +20,7 @@ use datafusion::error::Result;
 use datafusion::prelude::*;
 use datafusion::sql::unparser::expr_to_sql;
 use datafusion_sql::unparser::dialect::CustomDialect;
-use datafusion_sql::unparser::Unparser;
+use datafusion_sql::unparser::{plan_to_sql, Unparser};
 
 /// This example demonstrates the programmatic construction of
 /// SQL using the DataFusion Expr [`Expr`] and LogicalPlan [`LogicalPlan`] API.
@@ -41,6 +41,8 @@ async fn main() -> Result<()> {
     simple_expr_to_sql_demo()?;
     simple_expr_to_sql_demo_no_escape()?;
     simple_expr_to_sql_demo_escape_mysql_style()?;
+    simple_plan_to_sql_parquest_dataframe_demo().await?;
+    round_trip_plan_to_sql_parquest_dataframe_demo().await?;
     Ok(())
 }
 
@@ -77,3 +79,70 @@ fn simple_expr_to_sql_demo_escape_mysql_style() -> Result<()> {
     assert_eq!(sql, r#"((`a` < 5) OR (`a` = 8))"#);
     Ok(())
 }
+
+/// DataFusion can convert a logic plan created using the DataFrames API to read from a parquet file
+/// to SQL, using column name escaping PostgreSQL style.
+async fn simple_plan_to_sql_parquest_dataframe_demo() -> Result<()> {
+    // create local execution context
+    let ctx = SessionContext::new();
+
+    let testdata = datafusion::test_util::parquet_test_data();
+    let df = ctx
+        .read_parquet(
+            &format!("{testdata}/alltypes_plain.parquet"),
+            ParquetReadOptions::default(),
+        )
+        .await?
+        .select_columns(&["id", "int_col", "double_col", "date_string_col"])?;
+
+    let ast = plan_to_sql(df.logical_plan())?;
+
+    let sql = format!("{}", ast);
+
+    assert_eq!(
+        sql,
+        r#"SELECT "?table?"."id", "?table?"."int_col", "?table?"."double_col", "?table?"."date_string_col" FROM "?table?""#
+    );
+
+    Ok(())
+}
+
+// DataFusion could parse a SQL into a DataFrame, adding a Filter, and converting that back to sql.
+async fn round_trip_plan_to_sql_parquest_dataframe_demo() -> Result<()> {
+    // create local execution context
+    let ctx = SessionContext::new();
+
+    let testdata = datafusion::test_util::parquet_test_data();
+
+    // register parquet file with the execution context
+    ctx.register_parquet(
+        "alltypes_plain",
+        &format!("{testdata}/alltypes_plain.parquet"),
+        ParquetReadOptions::default(),
+    )
+    .await?;
+
+    // create a logical plan from a SQL string and then programmatically add new filters
+    let df = ctx
+        .sql(
+            "SELECT int_col, double_col, CAST(date_string_col as VARCHAR) \
+        FROM alltypes_plain",
+        )
+        .await?
+        .filter(
+            col("id")
+                .gt(lit(1))
+                .and(col("tinyint_col").lt(col("double_col"))),
+        )?;
+
+    let ast = plan_to_sql(df.logical_plan())?;
+
+    let sql = format!("{}", ast);
+
+    assert_eq!(
+        sql,
+        r#"SELECT "alltypes_plain"."int_col", "alltypes_plain"."double_col", CAST("alltypes_plain"."date_string_col" AS VARCHAR) FROM "alltypes_plain" WHERE (("alltypes_plain"."id" > 1) AND ("alltypes_plain"."tinyint_col" < "alltypes_plain"."double_col"))"#
+    );
+
+    Ok(())
+}