Check out example codes for "postgresql truncate cascade restart identity". It will help you in understanding the concepts better.

Code Example 1

TRUNCATE bigtable, fattable RESTART IDENTITY;

Code Example 2

TRUNCATE <table_name> RESTART IDENTITY CASCADE;
--RESTART will reset the identity sequence
--CASCADE all tables that have a foreign key reference to 
--table_name will be truncated as well

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