Alter table add column drops all the column stats

[vd0340.halxg.cloudera.com:21000] > compute stats single_node_insert.shipdates;
Query: compute stats single_node_insert.shipdates
+-----------------------------------------+
| summary                                 |
+-----------------------------------------+
| Updated 1 partition(s) and 4 column(s). |
+-----------------------------------------+
Fetched 1 row(s) in 0.72s
[vd0340.halxg.cloudera.com:21000] > show column stats single_node_insert.shipdates;
Query: show column stats single_node_insert.shipdates
+--------------+--------+------------------+--------+----------+-------------------+
| Column       | Type   | #Distinct Values | #Nulls | Max Size | Avg Size          |
+--------------+--------+------------------+--------+----------+-------------------+
| l_shipdate   | STRING | 2629             | -1     | 10       | 9.992899894714355 |
| l_orderkey   | BIGINT | 1                | -1     | 8        | 8                 |
| l_linenumber | BIGINT | 1                | -1     | 8        | 8                 |
| l_partkey    | BIGINT | 1                | -1     | 8        | 8                 |
+--------------+--------+------------------+--------+----------+-------------------+
Fetched 4 row(s) in 0.01s
[vd0340.halxg.cloudera.com:21000] > alter table shipdates add columns (l_custkey bigint);
Query: alter table shipdates add columns (l_custkey bigint)
[vd0340.halxg.cloudera.com:21000] > show column stats single_node_insert.shipdates;
Query: show column stats single_node_insert.shipdates
+--------------+--------+------------------+--------+----------+----------+
| Column       | Type   | #Distinct Values | #Nulls | Max Size | Avg Size |
+--------------+--------+------------------+--------+----------+----------+
| l_shipdate   | STRING | -1               | -1     | -1       | -1       |
| l_orderkey   | BIGINT | -1               | -1     | 8        | 8        |
| l_linenumber | BIGINT | -1               | -1     | 8        | 8        |
| l_partkey    | BIGINT | -1               | -1     | 8        | 8        |
| l_custkey    | BIGINT | -1               | -1     | 8        | 8        |
+--------------+--------+------------------+--------+----------+----------+