ORA-01555 while running impdp

Hi,

While importing a schema, we could see snapshot too old error has been written in the alert log. But, the job is getting successful. We don't see ORA-01555 has been written in import log as well..But in alert log it is writing continuosuly.

1.ORA-01555 is captured only in alert log and not in import log.

2.Import is getting successful, would like to know the reason why ORA-01555 is thrown in alert log when import job gets sucessful.

ORA-01555 caused by SQL statement below (SQL ID: gsz2k73wzq102, Query Duration=4578 sec, SCN: 0x0001.01463560):

SELECT process_order, flags, xml_clob, NVL(dump_fileid, :1), NVL(dump_position, :2), dump_length, dump_allocation, NVL(value_n, 0), grantor, object_row, object_schema, object_long_name, partition_name, subpartition_name, processing_status, processing_state, base_object_type, base_object_schema, base_object_name, base_process_order, property, size_estimate, in_progress, original_object_schema, original_object_name, creation_level, object_int_oid, metadata_io, option_tag FROM "SYS"."SYS_IMPORT_FULL_02" WHERE  process_order between