ERROR: index row size exceeds maximum 2712 for index

Dear All,

while inserting data in database(my own created DB) i'm getting this error

org.postgresql.util.PSQLException: ERROR: index row size 3176 exceeds maximum 2712 for index "description_department_of_aeronautics_and_astronautics_5_pkey"
at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:1592)
at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1327)
at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:192)
at org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:451)
at org.postgresql.jdbc2.AbstractJdbc2Statement.executeWithFlags(AbstractJdbc2Statement.java:336)
at org.postgresql.jdbc2.AbstractJdbc2Statement.executeUpdate(AbstractJdbc2Statement.java:282)

i searched but could not understand how to resolve it.
Do you have any idea how can i get rid of this error?
if postgres does not allow to increase index row size then how DSpace is maintaining data in its db because it is saving a lot of rows..???

Thanks for your time & consideration... 
 
Best regards:
Amjad Usman