16.10 - Finding and Fixing Skewed Tables - Teradata Database

The DBC.TableSizeV contains information that can help identify skewed tables. Table skew results from an inappropriate primary index. A query that references a skewed table may try to process more rows on some AMPs than others, and may run out of space:

• SELECT statements may run out of spool space if the accessed object has a defined spool space limit.
• INSERT and UPDATE statements may run out of perm space.

1. Use the following SQL statement to find skewed tables in the DBC.TableSizeV:

   SELECT vproc AS
   "AMP", TableName (FORMAT 'X(20)'), CurrentPerm
   FROM DBC.TableSizeV
   WHERE DatabaseName = 'database'
   ORDER BY TableName, "AMP" ;
   
   AMP     TableName                CurrentPerm
   ---     --------------------     -----------
   0       employee_upi_onempid     18,944
   1       employee_upi_onempid     18,944
   2       employee_upi_onempid     18,944
   3       employee_upi_onempid     19,968
   0       employee_nupi_ondept     4,096
   1       employee_nupi_ondept     30,208
   2       employee_nupi_ondept     15,360
   3       employee_nupi_ondept     12,288

   In this example, the answer set displays space allocations by AMP for two tables. The results show that:

   • CurrentPerm is similar across all vprocs for employee_upi_onempid. Permanent space distribution is relatively even across all AMPs in the system.
   • The table Employee_nupi_ondept is poorly distributed. The CurrentPerm figures range from 4,096 bytes to 30,208 bytes on different AMPs.
2. Redefine the primary index for any skewed tables that you find.

   See Choosing a Primary Index.