fix concat ws simplify (#14213)

* fix concat ws simplify

* fix

* fix tests

* fix

Author: cht42

datafusion/functions/src/string/concat_ws.rs

@@ -21,6 +21,7 @@ use std::sync::Arc;
 
 use arrow::datatypes::DataType;
 
+use crate::string::concat;
 use crate::string::concat::simplify_concat;
 use crate::string::concat_ws;
 use crate::strings::{ColumnarValueRef, StringArrayBuilder};
@@ -317,7 +318,19 @@ fn simplify_concat_ws(delimiter: &Expr, args: &[Expr]) -> Result<ExprSimplifyRes
             match delimiter {
                 // when the delimiter is an empty string,
                 // we can use `concat` to replace `concat_ws`
-                Some(delimiter) if delimiter.is_empty() => simplify_concat(args.to_vec()),
+                Some(delimiter) if delimiter.is_empty() => {
+                    match simplify_concat(args.to_vec())? {
+                        ExprSimplifyResult::Original(_) => {
+                            Ok(ExprSimplifyResult::Simplified(Expr::ScalarFunction(
+                                ScalarFunction {
+                                    func: concat(),
+                                    args: args.to_vec(),
+                                },
+                            )))
+                        }
+                        expr => Ok(expr),
+                    }
+                }
                 Some(delimiter) => {
                     let mut new_args = Vec::with_capacity(args.len());
                     new_args.push(lit(delimiter));

datafusion/sqllogictest/test_files/expr.slt

@@ -465,6 +465,17 @@ SELECT concat_ws('|','a',NULL,NULL)
 ----
 a
 
+statement ok
+create table foo (a varchar, b varchar) as values ('a', 'b');
+
+query T
+SELECT concat_ws('',a,b,'c') from foo
+----
+abc
+
+statement ok
+drop table foo
+
 query T
 SELECT initcap('')
 ----