Moving a Table Deletes its Statistics

Statistics are important as they help the optimizer to work out the execution plan for a SQL statement. If you move a table, this deletes its statistics so you need to analyze it again afterwards. You can see this in the example below. First I created a table:

SQL> create table object_list

  2  as select * from dba_objects

  3  /

Table created.

SQL>

When you create a table it has no statistics so the num_rows column is null:

SQL> select nvl(to_char(num_rows), 'NULL')

  2  as row_count

  3  from dba_tables

  4  where table_name = 'OBJECT_LIST'

  5  /

ROW_COUNT
----------------------------------------

NULL

                                                  

SQL>

When you calculate statistics, the num_rows column is updated:

SQL> analyze table object_list

  2  compute statistics

  3  /

Table analyzed.

SQL> select nvl(to_char(num_rows), 'NULL')

  2  as row_count

  3  from dba_tables

  4  where table_name = 'OBJECT_LIST'

  5  /

ROW_COUNT

----------------------------------------
7932

SQL>

Moving the table deletes the statistics so num_rows is null afterwards:

SQL> alter table object_list move

  2  /

Table altered.

SQL> select nvl(to_char(num_rows), 'NULL')

  2  as row_count

  3  from dba_tables

  4  where table_name = 'OBJECT_LIST'

  5  /

ROW_COUNT

---------------------------------------- NULL                                       

SQL>

To reinstate the statistics, simply analyze the table again:

SQL> analyze table object_list

  2  compute statistics

  3  /

Table analyzed.

SQL> select nvl(to_char(num_rows), 'NULL')

  2  as row_count

  3  from dba_tables

  4  where table_name = 'OBJECT_LIST'

  5  /

ROW_COUNT
----------------------------------------
7932

SQL>