timestamp_col NDV intermittently incorrect

 TestDdlStatements.test_truncate_table[exec_option: {'batch_size': 0, 'num_nodes': 0, 'sync_ddl': 0, 'disable_codegen': False, 'abort_on_error': 1, 'exec_single_node_rows_threshold': 0} | table_format: text/none-unique_database0] 
[gw2] linux2 -- Python 2.7.6 /home/ubuntu/tmp.AXxcDDTCpm/bin/../infra/python/env/bin/python
metadata/test_ddl.py:182: in test_truncate_table
    multiple_impalad=self._use_multiple_impalad(vector))
common/impala_test_suite.py:340: in run_test_case
    self.__verify_results_and_errors(vector, test_section, result, use_db)
common/impala_test_suite.py:234: in __verify_results_and_errors
    replace_filenames_with_placeholder)
common/test_result_verifier.py:398: in verify_raw_results
    VERIFIER_MAP[verifier](expected, actual)
common/test_result_verifier.py:231: in verify_query_result_is_equal
    assert expected_results == actual_results
E   assert Comparing QueryTestResults (expected vs actual):
E     Detailed information truncated, use "-vv" to show
---------------------------- Captured stderr setup -----------------------------
SET sync_ddl=True;
-- executing against localhost:21000
DROP DATABASE IF EXISTS `test_truncate_table_d04c0434` CASCADE;

SET sync_ddl=True;
-- executing against localhost:21000
CREATE DATABASE `test_truncate_table_d04c0434`;

MainThread: Created database "test_truncate_table_d04c0434" for test ID "metadata/test_ddl.py::TestDdlStatements::()::test_truncate_table[exec_option: {'batch_size': 0, 'num_nodes': 0, 'sync_ddl': 0, 'disable_codegen': False, 'abort_on_error': 1, 'exec_single_node_rows_threshold': 0} | table_format: text/none-unique_database0]"
----------------------------- Captured stderr call -----------------------------
-- executing against localhost:21000
use test_truncate_table_d04c0434;

SET batch_size=0;
SET num_nodes=0;
SET sync_ddl=0;
SET disable_codegen=False;
SET abort_on_error=False;
SET exec_single_node_rows_threshold=0;
-- executing against localhost:21000
create table t1 like functional.alltypes
location '/test-warehouse/test_truncate_table_d04c0434.db/t1';

-- executing against localhost:21000

insert into t1 partition(year, month) select * from functional.alltypes;

-- executing against localhost:21000

compute incremental stats t1;

-- executing against localhost:21000

show table stats t1;

-- executing against localhost:21000
show column stats t1;

MainThread: Comparing QueryTestResults (expected vs actual):
'bigint_col','BIGINT',10,-1,8,8 == 'bigint_col','BIGINT',10,-1,8,8
'bool_col','BOOLEAN',2,-1,1,1 == 'bool_col','BOOLEAN',2,-1,1,1
'date_string_col','STRING',736,-1,8,8 == 'date_string_col','STRING',736,-1,8,8
'double_col','DOUBLE',10,-1,8,8 == 'double_col','DOUBLE',10,-1,8,8
'float_col','FLOAT',10,-1,4,4 == 'float_col','FLOAT',10,-1,4,4
'id','INT',7505,-1,4,4 == 'id','INT',7505,-1,4,4
'int_col','INT',10,-1,4,4 == 'int_col','INT',10,-1,4,4
'month','INT',12,0,4,4 == 'month','INT',12,0,4,4
'smallint_col','SMALLINT',10,-1,2,2 == 'smallint_col','SMALLINT',10,-1,2,2
'string_col','STRING',10,-1,1,1 == 'string_col','STRING',10,-1,1,1
'timestamp_col','TIMESTAMP',7554,-1,16,16 != 'timestamp_col','TIMESTAMP',7552,-1,16,16
'tinyint_col','TINYINT',10,-1,1,1 == 'tinyint_col','TINYINT',10,-1,1,1
'year','INT',2,0,4,4 == 'year','INT',2,0,4,4

Query: show column stats alltypes
+-----------------+-----------+------------------+--------+----------+----------+
| Column          | Type      | #Distinct Values | #Nulls | Max Size | Avg Size |
+-----------------+-----------+------------------+--------+----------+----------+
| id              | INT       | 7505             | -1     | 4        | 4        |
| bool_col        | BOOLEAN   | 2                | -1     | 1        | 1        |
| tinyint_col     | TINYINT   | 10               | -1     | 1        | 1        |
| smallint_col    | SMALLINT  | 10               | -1     | 2        | 2        |
| int_col         | INT       | 10               | -1     | 4        | 4        |
| bigint_col      | BIGINT    | 10               | -1     | 8        | 8        |
| float_col       | FLOAT     | 10               | -1     | 4        | 4        |
| double_col      | DOUBLE    | 10               | -1     | 8        | 8        |
| date_string_col | STRING    | 736              | -1     | 8        | 8        |
| string_col      | STRING    | 10               | -1     | 1        | 1        |
| timestamp_col   | TIMESTAMP | 7552             | -1     | 16       | 16       |
| year            | INT       | 2                | 0      | 4        | 4        |
| month           | INT       | 12               | 0      | 4        | 4        |
+-----------------+-----------+------------------+--------+----------+----------+