16.20 - Migrating LOB Tables to Systems with Different Hash Functions - Teradata Vantage NewSQL Engine

Teradata Vantageā„¢ NewSQL Engine Node Software Migration Guide

Teradata Database
Teradata Vantage NewSQL Engine
March 2019

If the destination system has a different hash function than the source system, all of the data rows are redistributed during the migration. When LOB tables are restored across different hash functions, however the table cannot be used on the destination system until the rehashlobs.pl script has been executed, which rebuilds and redistributes the LOB table correctly.

The rehashlobs.pl script (located in the PUTTools directory) functions as follows:
  1. Create a temporary NoPI table that has the same field listing.
  2. Insert Select the data from the restored table (primary) into the temporary NoPI table.
  3. Delete all data rows from the primary table. (This ensures duplicates are removed.)
  4. Insert Select the data from the temporary table into the primary table.
  5. Drop the temporary table.

Additional changes are needed to NoPI Tables to change the hash bucket unique identifier for large tables with LOBs.

The script is not automatically executed because it is not required on systems with the same hash functions or systems without LOB tables.