[fix](cast)fix cast to char(N) error (#14168)

fe/fe-core/src/main/cup/sql_parser.cup

@@ -5561,7 +5561,23 @@ func_args_def ::=
 
 cast_expr ::=
   KW_CAST LPAREN expr:e KW_AS type_def:targetType RPAREN
-  {: RESULT = new CastExpr(targetType, e); :}
+  {: 
+    CastExpr castExpr = new CastExpr(targetType, e); 
+    if (targetType.getType().getLength() != -1
+        && (targetType.getType().getPrimitiveType() == PrimitiveType.VARCHAR 
+        || targetType.getType().getPrimitiveType() == PrimitiveType.CHAR)) {
+        // transfer cast(xx as char(N)/varchar(N)) to substr(cast(xx as char), 1, N)
+        // this is just a workaround to make the result correct
+        ArrayList<Expr> exprs = new ArrayList<>();
+        exprs.add(castExpr);
+        exprs.add(new IntLiteral(1));
+        exprs.add(new IntLiteral(targetType.getType().getLength()));
+        RESULT = new FunctionCallExpr("substr", new FunctionParams(exprs));
+    }
+    else {
+        RESULT = castExpr;
+    }
+  :}
   ;
 
 case_expr ::=

fe/fe-core/src/main/java/org/apache/doris/analysis/CastExpr.java

@@ -321,8 +321,12 @@ public class CastExpr extends Expr {
         }
 
         if (fn == null) {
-            throw new AnalysisException("Invalid type cast of " + getChild(0).toSql()
+            if (childType.isNull() && Type.canCastTo(childType, type)) {
+                return;
+            } else {
+                throw new AnalysisException("Invalid type cast of " + getChild(0).toSql()
                     + " from " + childType + " to " + type);
+            }
         }
 
         if (PrimitiveType.typeWithPrecision.contains(type.getPrimitiveType())) {

fe/fe-core/src/main/java/org/apache/doris/rewrite/FoldConstantsRule.java

@@ -110,7 +110,7 @@ public class FoldConstantsRule implements ExprRewriteRule {
         if (expr instanceof CastExpr) {
             CastExpr castExpr = (CastExpr) expr;
             if (castExpr.getChild(0) instanceof NullLiteral) {
-                return expr;
+                return castExpr.getChild(0);
             }
         }
 

regression-test/data/query_p0/sql_functions/cast_function/test_cast_function.out

@@ -11,6 +11,12 @@
 -- !sql --
 \N
 
+-- !sql --
+\N
+
+-- !sql --
+20
+
 -- !sql --
 1
 
@@ -23,3 +29,9 @@
 -- !sql --
 \N
 
+-- !sql --
+\N
+
+-- !sql --
+20
+

regression-test/suites/query_p0/sql_functions/cast_function/test_cast_function.groovy

@@ -22,6 +22,8 @@ suite("test_cast_function") {
     qt_sql """ select cast(cast ("11.2" as double) as bigint) """
     qt_sql """ select cast ("0.0101031417" as datetime) """
     qt_sql """ select cast ("0.0000031417" as datetime) """
+    qt_sql """ select cast (NULL AS CHAR(1)); """
+    qt_sql """ select cast ('20190101' AS CHAR(2)); """
 
     sql """ SET enable_vectorized_engine = FALSE; """
 
@@ -29,6 +31,7 @@ suite("test_cast_function") {
     qt_sql """ select cast(cast ("11.2" as double) as bigint) """
     qt_sql """ select cast ("0.0101031417" as datetime) """
     qt_sql """ select cast ("0.0000031417" as datetime) """
-
+    qt_sql """ select cast (NULL AS CHAR(1)); """
+    qt_sql """ select cast ('20190101' AS CHAR(2)); """
 }