enhance: simplify x=x (apache#15387)

- if x is not nullable, x=x -> true
- else, x=x -> x is NOT NULL

Author: ding-young

datafusion/optimizer/src/simplify_expressions/expr_simplifier.rs

@@ -760,6 +760,17 @@ impl<S: SimplifyInfo> TreeNodeRewriter for Simplifier<'_, S> {
                     None => lit_bool_null(),
                 })
             }
+            // A = A --> A IS NOT NULL
+            // A = A --> true (if A not nullable)
+            Expr::BinaryExpr(BinaryExpr {
+                left,
+                op: Eq,
+                right,
+            }) if left == right => Transformed::yes(match !info.nullable(&right)? {
+                true => lit(true),
+                false => Expr::IsNotNull(left),
+            }),
+
             // Rules for NotEq
             //
 
@@ -2152,6 +2163,17 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_simplify_eq_not_self() {
+        let expr_a = col("c2").eq(col("c2"));
+        let expr_b = col("c2_non_null").eq(col("c2_non_null"));
+        let expected_a = col("c2").is_not_null();
+        let expected_b = lit(true);
+
+        assert_eq!(simplify(expr_a), expected_a);
+        assert_eq!(simplify(expr_b), expected_b);
+    }
+
     #[test]
     fn test_simplify_or_true() {
         let expr_a = col("c2").or(lit(true));

datafusion/sqllogictest/test_files/simplify_expr.slt

@@ -63,5 +63,16 @@ query T
 select b from t where b !~ '.*'
 ----
 
+query TT
+explain select * from t where a = a;
+----
+logical_plan
+01)Filter: t.a IS NOT NULL
+02)--TableScan: t projection=[a, b]
+physical_plan
+01)CoalesceBatchesExec: target_batch_size=8192
+02)--FilterExec: a@0 IS NOT NULL
+03)----DataSourceExec: partitions=1, partition_sizes=[1]
+
 statement ok
 drop table t;