fix: incorrect simplification of case expr (#7006)

* fix: incorrect simplification of case expr

* Use pattern to match against None

Author: jonahgao

datafusion/core/tests/sqllogictests/test_files/scalar.slt

@@ -1090,6 +1090,18 @@ FROM t1
 999
 999
 
+# issue: https://github.com/apache/arrow-datafusion/issues/7004
+query B
+select case c1
+    when 'foo' then TRUE
+    when 'bar' then FALSE
+end from t1
+----
+NULL
+NULL
+NULL
+NULL
+
 statement ok
 drop table t1
 

datafusion/optimizer/src/simplify_expressions/expr_simplifier.rs

@@ -34,8 +34,8 @@ use datafusion_common::tree_node::{RewriteRecursion, TreeNode, TreeNodeRewriter}
 use datafusion_common::{DFSchema, DFSchemaRef, DataFusionError, Result, ScalarValue};
 use datafusion_expr::expr::{InList, InSubquery, ScalarFunction};
 use datafusion_expr::{
-    and, expr, lit, or, BinaryExpr, BuiltinScalarFunction, ColumnarValue, Expr, Like,
-    Volatility,
+    and, expr, lit, or, BinaryExpr, BuiltinScalarFunction, Case, ColumnarValue, Expr,
+    Like, Volatility,
 };
 use datafusion_physical_expr::{create_physical_expr, execution_props::ExecutionProps};
 
@@ -1069,17 +1069,20 @@ impl<'a, S: SimplifyInfo> TreeNodeRewriter for Simplifier<'a, S> {
             //
             // Note: the rationale for this rewrite is that the expr can then be further
             // simplified using the existing rules for AND/OR
-            Expr::Case(case)
-                if !case.when_then_expr.is_empty()
-                && case.when_then_expr.len() < 3 // The rewrite is O(n!) so limit to small number
-                && info.is_boolean_type(&case.when_then_expr[0].1)? =>
+            Expr::Case(Case {
+                expr: None,
+                when_then_expr,
+                else_expr,
+            }) if !when_then_expr.is_empty()
+                && when_then_expr.len() < 3 // The rewrite is O(n!) so limit to small number
+                && info.is_boolean_type(&when_then_expr[0].1)? =>
             {
                 // The disjunction of all the when predicates encountered so far
                 let mut filter_expr = lit(false);
                 // The disjunction of all the cases
                 let mut out_expr = lit(false);
 
-                for (when, then) in case.when_then_expr {
+                for (when, then) in when_then_expr {
                     let case_expr = when
                         .as_ref()
                         .clone()
@@ -1090,7 +1093,7 @@ impl<'a, S: SimplifyInfo> TreeNodeRewriter for Simplifier<'a, S> {
                     filter_expr = filter_expr.or(*when);
                 }
 
-                if let Some(else_expr) = case.else_expr {
+                if let Some(else_expr) = else_expr {
                     let case_expr = filter_expr.not().and(*else_expr);
                     out_expr = out_expr.or(case_expr);
                 }
@@ -2819,9 +2822,9 @@ mod tests {
 
     #[test]
     fn simplify_expr_case_when_then_else() {
-        // CASE WHERE c2 != false THEN "ok" == "not_ok" ELSE c2 == true
+        // CASE WHEN c2 != false THEN "ok" == "not_ok" ELSE c2 == true
         // -->
-        // CASE WHERE c2 THEN false ELSE c2
+        // CASE WHEN c2 THEN false ELSE c2
         // -->
         // false
         assert_eq!(
@@ -2836,9 +2839,9 @@ mod tests {
             col("c2").not().and(col("c2")) // #1716
         );
 
-        // CASE WHERE c2 != false THEN "ok" == "ok" ELSE c2
+        // CASE WHEN c2 != false THEN "ok" == "ok" ELSE c2
         // -->
-        // CASE WHERE c2 THEN true ELSE c2
+        // CASE WHEN c2 THEN true ELSE c2
         // -->
         // c2
         //
@@ -2856,7 +2859,7 @@ mod tests {
             col("c2").or(col("c2").not().and(col("c2"))) // #1716
         );
 
-        // CASE WHERE ISNULL(c2) THEN true ELSE c2
+        // CASE WHEN ISNULL(c2) THEN true ELSE c2
         // -->
         // ISNULL(c2) OR c2
         //
@@ -2873,7 +2876,7 @@ mod tests {
                 .or(col("c2").is_not_null().and(col("c2")))
         );
 
-        // CASE WHERE c1 then true WHERE c2 then false ELSE true
+        // CASE WHEN c1 then true WHEN c2 then false ELSE true
         // --> c1 OR (NOT(c1) AND c2 AND FALSE) OR (NOT(c1 OR c2) AND TRUE)
         // --> c1 OR (NOT(c1) AND NOT(c2))
         // --> c1 OR NOT(c2)
@@ -2892,7 +2895,7 @@ mod tests {
             col("c1").or(col("c1").not().and(col("c2").not()))
         );
 
-        // CASE WHERE c1 then true WHERE c2 then true ELSE false
+        // CASE WHEN c1 then true WHEN c2 then true ELSE false
         // --> c1 OR (NOT(c1) AND c2 AND TRUE) OR (NOT(c1 OR c2) AND FALSE)
         // --> c1 OR (NOT(c1) AND c2)
         // --> c1 OR c2