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

Teradata Database Design

prodname
Teradata Database
vrm_release
15.00
category
User Guide
featnum
B035-1094-015K

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 …

THEN …

UPI

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

NUPI

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.