Simplify case when all result expressions are identical

datafusion/optimizer/src/simplify_expressions/expr_simplifier.rs

@@ -1466,6 +1466,21 @@ impl TreeNodeRewriter for Simplifier<'_> {
                 }))
             }
 
+            // CASE [expr] WHEN ... THEN A WHEN ... THEN A ... ELSE A END --> A
+            // All branches (THEN expressions and ELSE) must be identical.
+            // ELSE clause must be present (otherwise unmatched cases return NULL).
+            Expr::Case(Case {
+                expr: _,
+                when_then_expr,
+                else_expr: Some(else_expr),
+            }) if !when_then_expr.is_empty()
+                && when_then_expr
+                    .iter()
+                    .all(|(_, then_expr)| then_expr.as_ref() == else_expr.as_ref()) =>
+            {
+                Transformed::yes(*else_expr)
+            }
+
             // CASE
             //   WHEN X THEN A
             //   WHEN Y THEN B

datafusion/sqllogictest/test_files/cse.slt

@@ -188,12 +188,12 @@ EXPLAIN SELECT
 FROM t1
 ----
 logical_plan
-01)Projection: (__common_expr_1 OR random() = Float64(0)) AND __common_expr_2 AS c1, __common_expr_2 AND random() = Float64(0) OR __common_expr_1 AS c2, CASE WHEN __common_expr_3 = Float64(0) THEN __common_expr_3 ELSE Float64(0) END AS c3, CASE WHEN __common_expr_4 = Float64(0) THEN Int64(0) WHEN CAST(__common_expr_4 AS Boolean) THEN Int64(0) ELSE Int64(0) END AS c4, CASE WHEN __common_expr_5 = Float64(0) THEN Float64(0) WHEN random() = Float64(0) THEN __common_expr_5 ELSE Float64(0) END AS c5, CASE WHEN __common_expr_6 = Float64(0) THEN Float64(0) ELSE __common_expr_6 END AS c6
-02)--Projection: t1.a = Float64(1) AS __common_expr_1, t1.a = Float64(2) AS __common_expr_2, t1.a + Float64(3) AS __common_expr_3, t1.a + Float64(4) AS __common_expr_4, t1.a + Float64(5) AS __common_expr_5, t1.a + Float64(6) AS __common_expr_6
+01)Projection: (__common_expr_1 OR random() = Float64(0)) AND __common_expr_2 AS c1, __common_expr_2 AND random() = Float64(0) OR __common_expr_1 AS c2, CASE WHEN __common_expr_3 = Float64(0) THEN __common_expr_3 ELSE Float64(0) END AS c3, Int64(0) AS c4, CASE WHEN __common_expr_4 = Float64(0) THEN Float64(0) WHEN random() = Float64(0) THEN __common_expr_4 ELSE Float64(0) END AS c5, CASE WHEN __common_expr_5 = Float64(0) THEN Float64(0) ELSE __common_expr_5 END AS c6
+02)--Projection: t1.a = Float64(1) AS __common_expr_1, t1.a = Float64(2) AS __common_expr_2, t1.a + Float64(3) AS __common_expr_3, t1.a + Float64(5) AS __common_expr_4, t1.a + Float64(6) AS __common_expr_5
 03)----TableScan: t1 projection=[a]
 physical_plan
-01)ProjectionExec: expr=[(__common_expr_1@0 OR random() = 0) AND __common_expr_2@1 as c1, __common_expr_2@1 AND random() = 0 OR __common_expr_1@0 as c2, CASE WHEN __common_expr_3@2 = 0 THEN __common_expr_3@2 ELSE 0 END as c3, CASE WHEN __common_expr_4@3 = 0 THEN 0 WHEN CAST(__common_expr_4@3 AS Boolean) THEN 0 ELSE 0 END as c4, CASE WHEN __common_expr_5@4 = 0 THEN 0 WHEN random() = 0 THEN __common_expr_5@4 ELSE 0 END as c5, CASE WHEN __common_expr_6@5 = 0 THEN 0 ELSE __common_expr_6@5 END as c6]
-02)--ProjectionExec: expr=[a@0 = 1 as __common_expr_1, a@0 = 2 as __common_expr_2, a@0 + 3 as __common_expr_3, a@0 + 4 as __common_expr_4, a@0 + 5 as __common_expr_5, a@0 + 6 as __common_expr_6]
+01)ProjectionExec: expr=[(__common_expr_1@0 OR random() = 0) AND __common_expr_2@1 as c1, __common_expr_2@1 AND random() = 0 OR __common_expr_1@0 as c2, CASE WHEN __common_expr_3@2 = 0 THEN __common_expr_3@2 ELSE 0 END as c3, 0 as c4, CASE WHEN __common_expr_4@3 = 0 THEN 0 WHEN random() = 0 THEN __common_expr_4@3 ELSE 0 END as c5, CASE WHEN __common_expr_5@4 = 0 THEN 0 ELSE __common_expr_5@4 END as c6]
+02)--ProjectionExec: expr=[a@0 = 1 as __common_expr_1, a@0 = 2 as __common_expr_2, a@0 + 3 as __common_expr_3, a@0 + 5 as __common_expr_4, a@0 + 6 as __common_expr_5]
 03)----DataSourceExec: partitions=1, partition_sizes=[0]
 
 # Surely only once but also conditionally evaluated subexpressions
@@ -226,8 +226,8 @@ EXPLAIN SELECT
 FROM t1
 ----
 logical_plan
-01)Projection: (random() = Float64(0) OR t1.a = Float64(1)) AND t1.a = Float64(2) AS c1, random() = Float64(0) AND t1.a = Float64(2) OR t1.a = Float64(1) AS c2, CASE WHEN random() = Float64(0) THEN t1.a + Float64(3) ELSE t1.a + Float64(3) END AS c3, CASE WHEN random() = Float64(0) THEN Float64(0) WHEN t1.a + Float64(4) = Float64(0) THEN t1.a + Float64(4) ELSE Float64(0) END AS c4, CASE WHEN random() = Float64(0) THEN Float64(0) WHEN t1.a + Float64(5) = Float64(0) THEN Float64(0) ELSE t1.a + Float64(5) END AS c5, CASE WHEN random() = Float64(0) THEN Float64(0) WHEN random() = Float64(0) THEN t1.a + Float64(6) ELSE t1.a + Float64(6) END AS c6
+01)Projection: (random() = Float64(0) OR t1.a = Float64(1)) AND t1.a = Float64(2) AS c1, random() = Float64(0) AND t1.a = Float64(2) OR t1.a = Float64(1) AS c2, t1.a + Float64(3) AS c3, CASE WHEN random() = Float64(0) THEN Float64(0) WHEN t1.a + Float64(4) = Float64(0) THEN t1.a + Float64(4) ELSE Float64(0) END AS c4, CASE WHEN random() = Float64(0) THEN Float64(0) WHEN t1.a + Float64(5) = Float64(0) THEN Float64(0) ELSE t1.a + Float64(5) END AS c5, CASE WHEN random() = Float64(0) THEN Float64(0) WHEN random() = Float64(0) THEN t1.a + Float64(6) ELSE t1.a + Float64(6) END AS c6
 02)--TableScan: t1 projection=[a]
 physical_plan
-01)ProjectionExec: expr=[(random() = 0 OR a@0 = 1) AND a@0 = 2 as c1, random() = 0 AND a@0 = 2 OR a@0 = 1 as c2, CASE WHEN random() = 0 THEN a@0 + 3 ELSE a@0 + 3 END as c3, CASE WHEN random() = 0 THEN 0 WHEN a@0 + 4 = 0 THEN a@0 + 4 ELSE 0 END as c4, CASE WHEN random() = 0 THEN 0 WHEN a@0 + 5 = 0 THEN 0 ELSE a@0 + 5 END as c5, CASE WHEN random() = 0 THEN 0 WHEN random() = 0 THEN a@0 + 6 ELSE a@0 + 6 END as c6]
+01)ProjectionExec: expr=[(random() = 0 OR a@0 = 1) AND a@0 = 2 as c1, random() = 0 AND a@0 = 2 OR a@0 = 1 as c2, a@0 + 3 as c3, CASE WHEN random() = 0 THEN 0 WHEN a@0 + 4 = 0 THEN a@0 + 4 ELSE 0 END as c4, CASE WHEN random() = 0 THEN 0 WHEN a@0 + 5 = 0 THEN 0 ELSE a@0 + 5 END as c5, CASE WHEN random() = 0 THEN 0 WHEN random() = 0 THEN a@0 + 6 ELSE a@0 + 6 END as c6]
 02)--DataSourceExec: partitions=1, partition_sizes=[0]

datafusion/sqllogictest/test_files/simplify_expr.slt

@@ -113,3 +113,21 @@ logical_plan
 physical_plan
 01)ProjectionExec: expr=[[{x:100}] as a]
 02)--PlaceholderRowExec
+
+# Simplify cases where the end expressions are the same to that expression
+query TT
+EXPLAIN SELECT
+    CASE v when 100 then 1 else 1 end as opt1,
+    CASE v when 200 then 2 when 201 then 2 else 2 end as opt2,
+    CASE v when 300 then 3 when 301 then 3 else 4 end as noopt1,
+    CASE v when 400 then 4 when 401 then 4 end as noopt2
+FROM (VALUES (0), (1), (2)) t(v)
+----
+logical_plan
+01)Projection: Int64(1) AS opt1, Int64(2) AS opt2, CASE t.v WHEN Int64(300) THEN Int64(3) WHEN Int64(301) THEN Int64(3) ELSE Int64(4) END AS noopt1, CASE t.v WHEN Int64(400) THEN Int64(4) WHEN Int64(401) THEN Int64(4) END AS noopt2
+02)--SubqueryAlias: t
+03)----Projection: column1 AS v
+04)------Values: (Int64(0)), (Int64(1)), (Int64(2))
+physical_plan
+01)ProjectionExec: expr=[1 as opt1, 2 as opt2, CASE column1@0 WHEN 300 THEN 3 WHEN 301 THEN 3 ELSE 4 END as noopt1, CASE column1@0 WHEN 400 THEN 4 WHEN 401 THEN 4 END as noopt2]
+02)--DataSourceExec: partitions=1, partition_sizes=[1]