Differentiate 0-row and 1-row EmptyRelation in EXPLAIN

The `LogicalPlan::EmptyRelation` can produce no rows or exactly 1 null
(placeholder) row of the requested schema. When viewing EXPLAIN output
of a LogicalPlan it's good to know which one is the case.

Author: findepi

datafusion-cli/tests/snapshots/cli_quick_test@can_see_indent_format.snap

@@ -5,7 +5,6 @@ info:
   args:
     - "--command"
     - EXPLAIN FORMAT indent SELECT 123
-snapshot_kind: text
 ---
 success: true
 exit_code: 0
@@ -15,7 +14,7 @@ exit_code: 0
 | plan_type     | plan                                     |
 +---------------+------------------------------------------+
 | logical_plan  | Projection: Int64(123)                   |
-|               |   EmptyRelation                          |
+|               |   EmptyRelation: rows=1                  |
 | physical_plan | ProjectionExec: expr=[123 as Int64(123)] |
 |               |   PlaceholderRowExec                     |
 |               |                                          |

datafusion/core/tests/dataframe/mod.rs

@@ -1213,7 +1213,7 @@ async fn join_on_filter_datatype() -> Result<()> {
         JoinType::Inner,
         Some(Expr::Literal(ScalarValue::Null, None)),
     )?;
-    assert_snapshot!(join.into_optimized_plan().unwrap(), @"EmptyRelation");
+    assert_snapshot!(join.into_optimized_plan().unwrap(), @"EmptyRelation: rows=0");
 
     // JOIN ON expression must be boolean type
     let join = left.join_on(right, JoinType::Inner, Some(lit("TRUE")))?;
@@ -4940,11 +4940,11 @@ async fn test_dataframe_placeholder_missing_param_values() -> Result<()> {
 
     assert_snapshot!(
         actual,
-        @r###"
+        @r"
     Filter: a = $0 [a:Int32]
       Projection: Int32(1) AS a [a:Int32]
-        EmptyRelation []
-    "###
+        EmptyRelation: rows=1 []
+    "
     );
 
     // Executing LogicalPlans with placeholders that don't have bound values
@@ -4973,11 +4973,11 @@ async fn test_dataframe_placeholder_missing_param_values() -> Result<()> {
 
     assert_snapshot!(
         actual,
-        @r###"
+        @r"
     Filter: a = Int32(3) [a:Int32]
       Projection: Int32(1) AS a [a:Int32]
-        EmptyRelation []
-    "###
+        EmptyRelation: rows=1 []
+    "
     );
 
     // N.B., the test is basically `SELECT 1 as a WHERE a = 3;` which returns no results.
@@ -5004,10 +5004,10 @@ async fn test_dataframe_placeholder_column_parameter() -> Result<()> {
 
     assert_snapshot!(
         actual,
-        @r###"
+        @r"
     Projection: $1 [$1:Null;N]
-      EmptyRelation []
-    "###
+      EmptyRelation: rows=1 []
+    "
     );
 
     // Executing LogicalPlans with placeholders that don't have bound values
@@ -5034,10 +5034,10 @@ async fn test_dataframe_placeholder_column_parameter() -> Result<()> {
 
     assert_snapshot!(
         actual,
-        @r###"
+        @r"
     Projection: Int32(3) AS $1 [$1:Null;N]
-      EmptyRelation []
-    "###
+      EmptyRelation: rows=1 []
+    "
     );
 
     assert_snapshot!(
@@ -5073,11 +5073,11 @@ async fn test_dataframe_placeholder_like_expression() -> Result<()> {
 
     assert_snapshot!(
         actual,
-        @r###"
+        @r#"
     Filter: a LIKE $1 [a:Utf8]
       Projection: Utf8("foo") AS a [a:Utf8]
-        EmptyRelation []
-    "###
+        EmptyRelation: rows=1 []
+    "#
     );
 
     // Executing LogicalPlans with placeholders that don't have bound values
@@ -5106,11 +5106,11 @@ async fn test_dataframe_placeholder_like_expression() -> Result<()> {
 
     assert_snapshot!(
         actual,
-        @r###"
+        @r#"
     Filter: a LIKE Utf8("f%") [a:Utf8]
       Projection: Utf8("foo") AS a [a:Utf8]
-        EmptyRelation []
-    "###
+        EmptyRelation: rows=1 []
+    "#
     );
 
     assert_snapshot!(

datafusion/core/tests/execution/logical_plan.rs

@@ -128,7 +128,7 @@ fn inline_scan_projection_test() -> Result<()> {
         @r"
     SubqueryAlias: ?table?
       Projection: a
-        EmptyRelation
+        EmptyRelation: rows=0
     "
     );
 

datafusion/core/tests/optimizer/mod.rs

@@ -62,7 +62,7 @@ fn select_arrow_cast() {
         plan,
         @r#"
     Projection: Float64(1234) AS f64, LargeUtf8("foo") AS large
-      EmptyRelation
+      EmptyRelation: rows=1
     "#
     );
 }

datafusion/expr/src/logical_plan/plan.rs

@@ -1720,7 +1720,10 @@ impl LogicalPlan {
         impl Display for Wrapper<'_> {
             fn fmt(&self, f: &mut Formatter) -> fmt::Result {
                 match self.0 {
-                    LogicalPlan::EmptyRelation(_) => write!(f, "EmptyRelation"),
+                    LogicalPlan::EmptyRelation(EmptyRelation { produce_one_row, schema: _ }) => {
+                        let rows = if *produce_one_row { 1 } else { 0 };
+                        write!(f, "EmptyRelation: rows={rows}")
+                    },
                     LogicalPlan::RecursiveQuery(RecursiveQuery {
                         is_distinct, ..
                     }) => {