Session Blocking

select s1.username || '@' || s1.machine || ' ( SID=' || s1.sid || ' )  is blocking '
    || s2.username || '@' || s2.machine || ' ( SID=' || s2.sid || ' ) ' AS blocking_status
    from v$lock l1, v$session s1, v$lock l2, v$session s2
    where s1.sid=l1.sid and s2.sid=l2.sid
    and l1.BLOCK=1 and l2.request > 0
    and l1.id1 = l2.id1
    and l2.id2 = l2.id2 ;


Identifying the locked row:
select do.object_name,
    row_wait_obj#, row_wait_file#, row_wait_block#, row_wait_row#,
    dbms_rowid.rowid_create ( 1, ROW_WAIT_OBJ#, ROW_WAIT_FILE#, ROW_WAIT_BLOCK#, ROW_WAIT_ROW# )
    from v$session s, dba_objects do
    where sid=pid_del_que_ha_sido_bloqueado
    and s.ROW_WAIT_OBJ# = do.OBJECT_ID ;

CONSISTENT GET and DB BLOCK GET

Oracle accesses blocks in one of two modes, current or consistent.


A db block get/Current block get is a current mode get. That is, it's the most up-to-date copy of the data in that block, as it is right now, or currently. There can only be one current copy of a block in the buffer cache at any time.
Db block gets generally are used when DML changes data in the database.
In that case, row-level locks are implicitly taken on the updated rows.
There is also at least one well-known case where a select statement does a db block get, and does not take a lock. That is, when it does a full table scan or fast full index scan, Oracle will read the segment header in current mode (multiple times, the number varies based on Oracle version).

A consistent get/Consistent Get/CR block get is when Oracle gets the data in a block which is consistent with a given point in time, or SCN. The consistent get is at the heart of Oracle's read consistency mechanism. When blocks are fetched in order to satisfy a query result set, they are fetched in consistent mode. If no block in the buffer cache is consistent to the correct point in time, Oracle will (attempt to) reconstruct that block using the information in the rollback
segments. If it fails to do so, that's when a query errors out with the much dreaded, much feared, and much misunderstood ORA-1555 "snapshot too old".