How Teradata determines primary index
Primary Index Rules
Rule 1: One Primary Index per table.
Rule 2: A Primary Index value can be unique or non-unique.
Rule 3: The Primary Index value can be NULL.
Rule 4: The Primary Index value can be modified.
Rule 5: The Primary Index of a populated table cannot be modified.
Rule 6: A Primary Index has a limit of 64 columns.
Two Types of Primary Indexes (UPI or NUPI)
Unique Primary Index(UPI) - A Unique Primary Index (UPI) is unique and cannot have any duplicates. If you try and insert a row with a Primary Index value that is already in the table, the row will be rejected. An UPI enforces UNIQUENESS for a column
Non-Unique Primary Index (NUPI) - A Non-Unique Primary Index (NUPI) means that the values for the selected column can be non-unique. Duplicate values can exist.
What happens when user not defined Primary index
Teradata assigns first column of table as Primary index, when not defined during creating a Table.
But always the best practice is need to define primary index during Table creation time. Else, you will get UNIQUENESS violation errors, since the first column of table may not follow uniqueness rules.