[Bug](decimalv3) fix BE crash for function if (#16152)

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

@@ -1193,6 +1193,14 @@ public class FunctionCallExpr extends Expr {
         } else if (fnName.getFunction().equalsIgnoreCase("if")) {
             Type[] childTypes = collectChildReturnTypes();
             Type assignmentCompatibleType = ScalarType.getAssignmentCompatibleType(childTypes[1], childTypes[2], true);
+            if (assignmentCompatibleType.isDecimalV3()) {
+                if (childTypes[1].isDecimalV3() && !((ScalarType) childTypes[1]).equals(assignmentCompatibleType)) {
+                    uncheckedCastChild(assignmentCompatibleType, 1);
+                }
+                if (childTypes[2].isDecimalV3() && !((ScalarType) childTypes[2]).equals(assignmentCompatibleType)) {
+                    uncheckedCastChild(assignmentCompatibleType, 2);
+                }
+            }
             childTypes[1] = assignmentCompatibleType;
             childTypes[2] = assignmentCompatibleType;
             fn = getBuiltinFunction(fnName.getFunction(), childTypes,
@@ -1200,6 +1208,8 @@ public class FunctionCallExpr extends Expr {
             if (assignmentCompatibleType.isDatetimeV2()) {
                 fn.setReturnType(assignmentCompatibleType);
             }
+
+
         } else if (AggregateFunction.SUPPORT_ORDER_BY_AGGREGATE_FUNCTION_NAME_SET.contains(
                 fnName.getFunction().toLowerCase())) {
             // order by elements add as child like windows function. so if we get the

regression-test/data/datatype_p0/decimalv3/test_functions.out

@@ -0,0 +1,5 @@
+-- This file is automatically generated. You should know what you did if you want to edit this
+-- !select_default --
+10001	13.300000000
+10007	13.300000000
+

regression-test/suites/datatype_p0/decimalv3/test_functions.groovy

@@ -0,0 +1,53 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+import org.codehaus.groovy.runtime.IOGroovyMethods
+
+import java.nio.charset.StandardCharsets
+import java.nio.file.Files
+import java.nio.file.Paths
+
+suite("test_functions") {
+    def dbName = "test_functions"
+    sql "CREATE DATABASE IF NOT EXISTS ${dbName}"
+    sql "USE $dbName"
+
+    def tableName = "test_decimal_load"
+    try {
+        sql """ DROP TABLE IF EXISTS ${tableName} """
+        sql """
+        CREATE TABLE IF NOT EXISTS ${tableName} (
+              id int(11) NULL,
+              s_count int(11) NULL,
+              fee DECIMALV3(15,4) NULL
+            ) ENGINE=OLAP
+            DUPLICATE KEY(`id`)
+            COMMENT 'OLAP'
+            DISTRIBUTED BY HASH(`id`) BUCKETS 1
+            PROPERTIES (
+            "replication_allocation" = "tag.location.default: 1"
+        );
+        """
+
+        sql """ INSERT INTO ${tableName} VALUES(10007,26,13.3), (10001,12,13.3) """
+        qt_select_default """ SELECT id, if(1 = 2,
+                                     1.0*s_count - fee,
+                                     fee) FROM ${tableName} t ORDER BY id; """
+    } finally {
+        try_sql("DROP TABLE IF EXISTS ${tableName}")
+    }
+}