We found this beauty in statspack report. This query was executed 7 times during 15 minutes period. Each execution had 23592 LIO, 23372 PIO and Cost 2144. As we later found, that query was used to check validity of database link. 


SQL> SELECT COUNT(*) FROM SCHEMA_NAME.SOME_BIG_TABLE A1;

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=2144 Card=1)
   1    0   SORT (AGGREGATE)
   2    1     INDEX (FAST FULL SCAN) OF 'PK_ID_SOME_BIG_TABLE' (UNIQUE
          ) (Cost=2144 Card=2725982)

Statistics
----------------------------------------------------------
        760  recursive calls
          0  db block gets
      23592  consistent gets
      23372  physical reads
      12932  redo size
        200  bytes sent via SQL*Net to client
        240  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
         28  sorts (memory)
          0  sorts (disk)
          1  rows processed
What can I say? Don't try this at work...