15.00 - LOB and XML Subtable Rows and Skew - Teradata Database

Teradata Database Design

Teradata Database
User Guide

LOB and XML Subtable Rows and Skew

Each LOB or XML string is stored in 64 KB sections in a subtable, except for the row that stores the last section (because the number of bytes in a LOB or XML string is typically not an exact multiple of 64 KB). This means that for any LOB or XML string greater than 64 KB, there is not a 1:1 equivalence between the number of base table rows and the number of LOB or XML subtable rows supporting those base table rows. As a result, unless every LOB or XML string in a table has the identical size, it is likely that storage skew will occur.


IF the primary index for a base table is a …



this skew does not affect performance in any way, but it is likely to produce an asymmetric storage effect on storage requirements.


both of the following assertions are true:

  • Performance costs accrued by a skewed distribution of base table rows are likely to be further magnified.
  • Storage skew is even more likely to occur than for tables with a UPI.