Identify Blocking Sessions

select
x.inst_id WAITING_INSTANCE,
x.sid WAITING_SID,
w.username WAITING_USER,
w.machine WAITING_MACHINE,
w.program WAITING_PROGRAM,
w.module WAITING_MODULE,
(select distinct sql_text from v$sql where sql_id=w.sql_id) WAITING_SQL,
w.sql_exec_start WAITING_SQL_START,
w.seconds_in_wait WAITING_SECONDS,
w.blocking_session BLOCKING_SID,
b.inst_id BLOCKING_INSTANCE,
b.username BLOCKING_USER,
b.machine BLOCKING_MACHINE,
b.program BLOCKING_PROGRAM,
b.module BLOCKING_MODULE,
(select distinct sql_text from v$sql where sql_id=b.prev_sql_id) BLOCKING_SQL,
b.prev_exec_start BLOCKING_SQL_START
from
GV$lock x, -- waiting lock
gv$lock y, -- blocking lock
gv$session w, -- waiting session
gv$session b -- blocking session
where
x.request>0
and w.sid=x.sid
and y.block>0
and b.sid=y.sid
and x.id1=y.id1
and x.id2=y.id2
and w.blocking_session=b.sid
and y.addr=b.taddr

Preview dump file before importing

If using datapump, the option is SQLFILE.

The file_name specifies where the import job will write the DDL that would be
executed during the job. The SQL is not actually executed, and the target system
remains unchanged. The file is written to the directory object specified in the
DIRECTORY parameter, unless another directory_object is explicitly specified
here. Any existing file that has a name matching the one specified with this parameter
is overwritten.

Example:
impdp hr DIRECTORY=dpump_dir1 DUMPFILE=expfull.dmp SQLFILE=dpump_dir2:filename

If using original import the option is SHOW=Y.

The SHOW parameter can be used only with the FULL=y, FROMUSER, TOUSER, or
TABLES parameter.