okumin commented on code in PR #4090:
URL: https://github.com/apache/hive/pull/4090#discussion_r1199573605


##########
ql/src/test/results/clientpositive/llap/udf_array_except.q.out:
##########
@@ -0,0 +1,134 @@
+PREHOOK: query: DESCRIBE FUNCTION array_except
+PREHOOK: type: DESCFUNCTION
+POSTHOOK: query: DESCRIBE FUNCTION array_except
+POSTHOOK: type: DESCFUNCTION
+array_except(array1, array2) - Returns an array of the elements in array1 but 
not in array2.
+PREHOOK: query: DESCRIBE FUNCTION EXTENDED array_except
+PREHOOK: type: DESCFUNCTION
+POSTHOOK: query: DESCRIBE FUNCTION EXTENDED array_except
+POSTHOOK: type: DESCFUNCTION
+array_except(array1, array2) - Returns an array of the elements in array1 but 
not in array2.
+Example:
+  > SELECT array_except(array(1, 2, 3,4), array(2,3)) FROM src LIMIT 1;
+  [1,4]
+Function class:org.apache.hadoop.hive.ql.udf.generic.GenericUDFArrayExcept
+Function type:BUILTIN
+PREHOOK: query: SELECT array_except(array(1, 2, 3, null,3,4),array(1, 3, null))
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(1, 2, 3, null,3,4),array(1, 3, 
null))
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+[2,4]
+PREHOOK: query: SELECT array_except(array(),array())
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(),array())
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+NULL

Review Comment:
   Should `SELECT array_except(array(), array())` return an empty array? 
PrestoSQL or Spark 3.4 behaves like that. It is up to a Hive committer but I 
prefer their behaviors.
   
   ```
   spark-sql (default)> select array_except(array(), array());
   ...
   []
   Time taken: 0.084 seconds, Fetched 1 row(s)
   ```



##########
ql/src/test/results/clientpositive/llap/udf_array_except.q.out:
##########
@@ -0,0 +1,134 @@
+PREHOOK: query: DESCRIBE FUNCTION array_except
+PREHOOK: type: DESCFUNCTION
+POSTHOOK: query: DESCRIBE FUNCTION array_except
+POSTHOOK: type: DESCFUNCTION
+array_except(array1, array2) - Returns an array of the elements in array1 but 
not in array2.
+PREHOOK: query: DESCRIBE FUNCTION EXTENDED array_except
+PREHOOK: type: DESCFUNCTION
+POSTHOOK: query: DESCRIBE FUNCTION EXTENDED array_except
+POSTHOOK: type: DESCFUNCTION
+array_except(array1, array2) - Returns an array of the elements in array1 but 
not in array2.
+Example:
+  > SELECT array_except(array(1, 2, 3,4), array(2,3)) FROM src LIMIT 1;
+  [1,4]
+Function class:org.apache.hadoop.hive.ql.udf.generic.GenericUDFArrayExcept
+Function type:BUILTIN
+PREHOOK: query: SELECT array_except(array(1, 2, 3, null,3,4),array(1, 3, null))
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(1, 2, 3, null,3,4),array(1, 3, 
null))
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+[2,4]
+PREHOOK: query: SELECT array_except(array(),array())
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(),array())
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+NULL
+PREHOOK: query: SELECT array_except(array(null),array(null))
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(null),array(null))
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+[]
+PREHOOK: query: SELECT array_except(array(1.12, 2.23, 3.34, 
null,1.11,1.12,2.9),array(1.12,3.34,1.11,1.12))
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(1.12, 2.23, 3.34, 
null,1.11,1.12,2.9),array(1.12,3.34,1.11,1.12))
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+[2.23,null,2.9]
+PREHOOK: query: SELECT array_except(array(1.1234567890, 2.234567890, 
3.34567890, null, 3.3456789, 2.234567,1.1234567890),array(1.1234567890, 
3.34567890, null,2.234567))
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(1.1234567890, 2.234567890, 
3.34567890, null, 3.3456789, 2.234567,1.1234567890),array(1.1234567890, 
3.34567890, null,2.234567))
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+[2.23456789]
+PREHOOK: query: SELECT array_except(array(11234567890, 2234567890, 334567890, 
null, 11234567890, 2234567890, 334567890, null),array(11234567890, 2234567890, 
334567890))
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(11234567890, 2234567890, 334567890, 
null, 11234567890, 2234567890, 334567890, null),array(11234567890, 2234567890, 
334567890))
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+[null]
+PREHOOK: query: SELECT 
array_except(array(array("a","b","c","d"),array("a","b","c","d"),array("a","b","c","d","e"),null,array("e","a","b","c","d")),array(array("a","b","c","d"),array("a","b","c","d"),array("a","b","c","d","e"),null))
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT 
array_except(array(array("a","b","c","d"),array("a","b","c","d"),array("a","b","c","d","e"),null,array("e","a","b","c","d")),array(array("a","b","c","d"),array("a","b","c","d"),array("a","b","c","d","e"),null))
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+[["e","a","b","c","d"]]
+PREHOOK: query: create external table test_null_array (id int, value 
Array<String>) ROW FORMAT DELIMITED
+#### A masked pattern was here ####
+PREHOOK: type: CREATETABLE
+#### A masked pattern was here ####
+PREHOOK: Output: database:default
+PREHOOK: Output: default@test_null_array
+POSTHOOK: query: create external table test_null_array (id int, value 
Array<String>) ROW FORMAT DELIMITED
+#### A masked pattern was here ####
+POSTHOOK: type: CREATETABLE
+#### A masked pattern was here ####
+POSTHOOK: Output: database:default
+POSTHOOK: Output: default@test_null_array
+PREHOOK: query: select value from test_null_array
+PREHOOK: type: QUERY
+PREHOOK: Input: default@test_null_array
+#### A masked pattern was here ####
+POSTHOOK: query: select value from test_null_array
+POSTHOOK: type: QUERY
+POSTHOOK: Input: default@test_null_array
+#### A masked pattern was here ####
+["NULL"]
+["null","null"]
+[]
+PREHOOK: query: select array_except(value,value) from test_null_array
+PREHOOK: type: QUERY
+PREHOOK: Input: default@test_null_array
+#### A masked pattern was here ####
+POSTHOOK: query: select array_except(value,value) from test_null_array
+POSTHOOK: type: QUERY
+POSTHOOK: Input: default@test_null_array
+#### A masked pattern was here ####
+NULL
+[]
+[]
+PREHOOK: query: select value, array_except(value,value) from test_null_array
+PREHOOK: type: QUERY
+PREHOOK: Input: default@test_null_array
+#### A masked pattern was here ####
+POSTHOOK: query: select value, array_except(value,value) from test_null_array
+POSTHOOK: type: QUERY
+POSTHOOK: Input: default@test_null_array
+#### A masked pattern was here ####
+[]     NULL
+[]     []
+[]     []
+PREHOOK: query: SELECT array_except(array(1, 2, 3, null,3,4),value) from 
test_null_array
+PREHOOK: type: QUERY
+PREHOOK: Input: default@test_null_array
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(1, 2, 3, null,3,4),value) from 
test_null_array
+POSTHOOK: type: QUERY
+POSTHOOK: Input: default@test_null_array
+#### A masked pattern was here ####
+[1,2,3,null,4]

Review Comment:
   I confirmed at least PrestoSQL and Spark behave like this, meaning 
duplicated entries, `3` in this case, are deduplicated.



##########
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFArrayExcept.java:
##########
@@ -0,0 +1,59 @@
+/*
+ * 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.
+ */
+package org.apache.hadoop.hive.ql.udf.generic;
+
+import org.apache.hadoop.hive.ql.exec.Description;
+import org.apache.hadoop.hive.ql.exec.UDFArgumentException;
+import org.apache.hadoop.hive.ql.metadata.HiveException;
+import org.apache.hadoop.hive.serde2.objectinspector.ListObjectInspector;
+import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspector;
+
+import java.util.List;
+import java.util.stream.Collectors;
+
+/**
+ * GenericUDFArrayExcept
+ */
+@Description(name = "array_except", value = "_FUNC_(array, value) - Returns an 
array of the elements in array1 but not in array2.", extended =
+    "Example:\n" + "  > SELECT _FUNC_(array(1, 2, 3,4), array(2,3)) FROM src 
LIMIT 1;\n"
+        + "  [1,4]") @NDV(maxNdv = 2) public class GenericUDFArrayExcept 
extends AbstractGenericUDFArrayBase {
+  static final int ARRAY2_IDX = 1;
+  private static final String FUNC_NAME = "ARRAY_EXCEPT";
+
+  public GenericUDFArrayExcept() {
+    super(FUNC_NAME, 2, 2, ObjectInspector.Category.LIST);
+  }
+
+  @Override public ObjectInspector initialize(ObjectInspector[] arguments) 
throws UDFArgumentException {
+    ObjectInspector defaultOI = super.initialize(arguments);
+    checkArgCategory(arguments, ARRAY2_IDX, ObjectInspector.Category.LIST, 
FUNC_NAME,
+        org.apache.hadoop.hive.serde.serdeConstants.LIST_TYPE_NAME); //Array1 
is already getting validated in Parent class

Review Comment:
   Thanks. I think we should carefully think of the expected specification 
first. What should the following SQL return?
   
   ```
   SELECT array_except(array(1, 2, 3), array(2.0, 3.3));
   ```
   
   If it should return `array(1, 3)`, meaning type conversion is applied, we 
should add it to our test cases.
   If it should return `array(1, 2, 3)`, meaning the second argument is 
meaningless if types are unmatched, I personally think we should raise a syntax 
error. That's because it happens only when a user misunderstands the types of 
the 1st and 2nd arguments.
   
   For example, Spark 3.4 fails in that case. PrestoSQL returns `[1.0, 3.0]` 
with the same SQL, meaning PrestoSQL applies the type conversion from int to 
float.
   
   ```
   spark-sql (default)> select array_except(array(1, 2, 3), array(2.0, 3.3));
   [DATATYPE_MISMATCH.BINARY_ARRAY_DIFF_TYPES] Cannot resolve 
"array_except(array(1, 2, 3), array(2.0, 3.3))" due to data type mismatch: 
Input to function `array_except` should have been two "ARRAY" with same element 
type, but it's ["ARRAY<INT>", "ARRAY<DECIMAL(2,1)>"].; line 1 pos 7;
   'Project [unresolvedalias(array_except(array(1, 2, 3), array(2.0, 3.3)), 
None)]
   +- OneRowRelation
   ```



##########
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFArrayExcept.java:
##########
@@ -0,0 +1,59 @@
+/*
+ * 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.
+ */
+package org.apache.hadoop.hive.ql.udf.generic;
+
+import org.apache.hadoop.hive.ql.exec.Description;
+import org.apache.hadoop.hive.ql.exec.UDFArgumentException;
+import org.apache.hadoop.hive.ql.metadata.HiveException;
+import org.apache.hadoop.hive.serde2.objectinspector.ListObjectInspector;
+import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspector;
+
+import java.util.List;
+import java.util.stream.Collectors;
+
+/**
+ * GenericUDFArrayExcept
+ */
+@Description(name = "array_except", value = "_FUNC_(array, value) - Returns an 
array of the elements in array1 but not in array2.", extended =
+    "Example:\n" + "  > SELECT _FUNC_(array(1, 2, 3,4), array(2,3)) FROM src 
LIMIT 1;\n"
+        + "  [1,4]") @NDV(maxNdv = 2) public class GenericUDFArrayExcept 
extends AbstractGenericUDFArrayBase {
+  static final int ARRAY2_IDX = 1;
+  private static final String FUNC_NAME = "ARRAY_EXCEPT";
+
+  public GenericUDFArrayExcept() {
+    super(FUNC_NAME, 2, 2, ObjectInspector.Category.LIST);
+  }
+
+  @Override public ObjectInspector initialize(ObjectInspector[] arguments) 
throws UDFArgumentException {
+    ObjectInspector defaultOI = super.initialize(arguments);
+    checkArgCategory(arguments, ARRAY2_IDX, ObjectInspector.Category.LIST, 
FUNC_NAME,
+        org.apache.hadoop.hive.serde.serdeConstants.LIST_TYPE_NAME); //Array1 
is already getting validated in Parent class
+    return defaultOI;
+  }
+
+  @Override public Object evaluate(DeferredObject[] arguments) throws 
HiveException {
+    Object array = arguments[ARRAY_IDX].get();
+    if (array == null || arrayOI.getListLength(array) <= 0) {
+      return null;
+    }
+
+    List<?> retArray3 = ((ListObjectInspector) 
argumentOIs[ARRAY_IDX]).getList(array);
+    retArray3.removeAll(((ListObjectInspector) 
argumentOIs[ARRAY2_IDX]).getList(arguments[ARRAY2_IDX].get()));

Review Comment:
   Thanks. As I commented on a test file, I feel `SELECT array1, 
ARRAY_EXCEPT(array1, array2)` returns an incorrect result.



##########
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFArrayExcept.java:
##########
@@ -0,0 +1,59 @@
+/*
+ * 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.
+ */
+package org.apache.hadoop.hive.ql.udf.generic;
+
+import org.apache.hadoop.hive.ql.exec.Description;
+import org.apache.hadoop.hive.ql.exec.UDFArgumentException;
+import org.apache.hadoop.hive.ql.metadata.HiveException;
+import org.apache.hadoop.hive.serde2.objectinspector.ListObjectInspector;
+import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspector;
+
+import java.util.List;
+import java.util.stream.Collectors;
+
+/**
+ * GenericUDFArrayExcept
+ */
+@Description(name = "array_except", value = "_FUNC_(array1, array2) - Returns 
an array of the elements in array1 but not in array2.", extended =
+    "Example:\n" + "  > SELECT _FUNC_(array(1, 2, 3,4), array(2,3)) FROM src 
LIMIT 1;\n"
+        + "  [1,4]") public class GenericUDFArrayExcept extends 
AbstractGenericUDFArrayBase {

Review Comment:
   nit: I think we should break a line before the declaration of the class.
   
https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFArrayMax.java



##########
ql/src/test/results/clientpositive/llap/udf_array_except.q.out:
##########
@@ -0,0 +1,134 @@
+PREHOOK: query: DESCRIBE FUNCTION array_except
+PREHOOK: type: DESCFUNCTION
+POSTHOOK: query: DESCRIBE FUNCTION array_except
+POSTHOOK: type: DESCFUNCTION
+array_except(array1, array2) - Returns an array of the elements in array1 but 
not in array2.
+PREHOOK: query: DESCRIBE FUNCTION EXTENDED array_except
+PREHOOK: type: DESCFUNCTION
+POSTHOOK: query: DESCRIBE FUNCTION EXTENDED array_except
+POSTHOOK: type: DESCFUNCTION
+array_except(array1, array2) - Returns an array of the elements in array1 but 
not in array2.
+Example:
+  > SELECT array_except(array(1, 2, 3,4), array(2,3)) FROM src LIMIT 1;
+  [1,4]
+Function class:org.apache.hadoop.hive.ql.udf.generic.GenericUDFArrayExcept
+Function type:BUILTIN
+PREHOOK: query: SELECT array_except(array(1, 2, 3, null,3,4),array(1, 3, null))
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(1, 2, 3, null,3,4),array(1, 3, 
null))
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+[2,4]
+PREHOOK: query: SELECT array_except(array(),array())
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(),array())
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+NULL
+PREHOOK: query: SELECT array_except(array(null),array(null))
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(null),array(null))
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+[]
+PREHOOK: query: SELECT array_except(array(1.12, 2.23, 3.34, 
null,1.11,1.12,2.9),array(1.12,3.34,1.11,1.12))
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(1.12, 2.23, 3.34, 
null,1.11,1.12,2.9),array(1.12,3.34,1.11,1.12))
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+[2.23,null,2.9]
+PREHOOK: query: SELECT array_except(array(1.1234567890, 2.234567890, 
3.34567890, null, 3.3456789, 2.234567,1.1234567890),array(1.1234567890, 
3.34567890, null,2.234567))
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(1.1234567890, 2.234567890, 
3.34567890, null, 3.3456789, 2.234567,1.1234567890),array(1.1234567890, 
3.34567890, null,2.234567))
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+[2.23456789]
+PREHOOK: query: SELECT array_except(array(11234567890, 2234567890, 334567890, 
null, 11234567890, 2234567890, 334567890, null),array(11234567890, 2234567890, 
334567890))
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT array_except(array(11234567890, 2234567890, 334567890, 
null, 11234567890, 2234567890, 334567890, null),array(11234567890, 2234567890, 
334567890))
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+[null]
+PREHOOK: query: SELECT 
array_except(array(array("a","b","c","d"),array("a","b","c","d"),array("a","b","c","d","e"),null,array("e","a","b","c","d")),array(array("a","b","c","d"),array("a","b","c","d"),array("a","b","c","d","e"),null))
+PREHOOK: type: QUERY
+PREHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+POSTHOOK: query: SELECT 
array_except(array(array("a","b","c","d"),array("a","b","c","d"),array("a","b","c","d","e"),null,array("e","a","b","c","d")),array(array("a","b","c","d"),array("a","b","c","d"),array("a","b","c","d","e"),null))
+POSTHOOK: type: QUERY
+POSTHOOK: Input: _dummy_database@_dummy_table
+#### A masked pattern was here ####
+[["e","a","b","c","d"]]
+PREHOOK: query: create external table test_null_array (id int, value 
Array<String>) ROW FORMAT DELIMITED
+#### A masked pattern was here ####
+PREHOOK: type: CREATETABLE
+#### A masked pattern was here ####
+PREHOOK: Output: database:default
+PREHOOK: Output: default@test_null_array
+POSTHOOK: query: create external table test_null_array (id int, value 
Array<String>) ROW FORMAT DELIMITED
+#### A masked pattern was here ####
+POSTHOOK: type: CREATETABLE
+#### A masked pattern was here ####
+POSTHOOK: Output: database:default
+POSTHOOK: Output: default@test_null_array
+PREHOOK: query: select value from test_null_array
+PREHOOK: type: QUERY
+PREHOOK: Input: default@test_null_array
+#### A masked pattern was here ####
+POSTHOOK: query: select value from test_null_array
+POSTHOOK: type: QUERY
+POSTHOOK: Input: default@test_null_array
+#### A masked pattern was here ####
+["NULL"]
+["null","null"]
+[]
+PREHOOK: query: select array_except(value,value) from test_null_array
+PREHOOK: type: QUERY
+PREHOOK: Input: default@test_null_array
+#### A masked pattern was here ####
+POSTHOOK: query: select array_except(value,value) from test_null_array
+POSTHOOK: type: QUERY
+POSTHOOK: Input: default@test_null_array
+#### A masked pattern was here ####
+NULL
+[]
+[]
+PREHOOK: query: select value, array_except(value,value) from test_null_array
+PREHOOK: type: QUERY
+PREHOOK: Input: default@test_null_array
+#### A masked pattern was here ####
+POSTHOOK: query: select value, array_except(value,value) from test_null_array
+POSTHOOK: type: QUERY
+POSTHOOK: Input: default@test_null_array
+#### A masked pattern was here ####
+[]     NULL

Review Comment:
   Should the first column retain the original `value`, meaning `["null"]`, 
`["null", "null"]`, or `[]`? It would be surprising that `array_except` mutates 
the value of another expression.



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