Explain Primary Index

It is used for data distribution among the AMPs. There is always one index in a table and it is must for every table (even if you don't define it explicitly).

A primary index should be defined to provide a nearly uniform distribution of rows among the  AMPs, the more unique the index, the more even the distribution of rows and the better  space utilization.
The index should be defined on as few columns as possible. 
Primary index can be either Unique or non-unique. A unique index must have a unique value in the corresponding fields of every row.

A non-unique index permits the insertion of duplicate field values. The unique primary index is more efficient. Once created, the primary index cannot be dropped or modified, the  index must be changed by recreating the table. 

Once created, the primary index cannot be dropped or modified, the index must be changed by recreating the table. 

Creating index with create table command
create table organic (serial_No integer,
                                   organic_name char(15),
                                   Carbon_number smallint,
                                    amount smallint)  
unique primary index (serial_No);

create table inorganic (serial_No integer,
                                      inorgnic_name char(15),
                                      anion char(5),
                                      cation char(6),
                                      amount smallint)  
unique primary index (serial_No); 

non unique primary index:
create table student (student_ID char(15),
                                  student_name char(20),
                                  Department char(10))  
primary index (student_ID);