Query sometimes slow

Question

Have anybody idea why this query sometimes running very slow?
But only sometimes!
Thanks for any ideas on this.
SQL> SELECT a, b
  2    FROM X_TAB
  3 WHERE A_KL = '2ABRY'
  4   AND A_JL = '2ABRY'
  5   AND YEAR = 2006
  6   AND ID_DB = 906858
  7  ORDER BY a;
Execution Plan
   0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=1 Card=1 Bytes=41)
   1    0   TABLE ACCESS (BY INDEX ROWID) OF 'X_TAB' (TABLE) (Cost=1 Card=1 Bytes=41)
   2    1     INDEX (RANGE SCAN) OF 'ACG_UK3' (INDEX (UNIQUE)) (Cost=1 Card=3)
Statistics
          0  recursive calls
          0  db block gets
         13  consistent gets
          0  physical reads
          0  redo size
       3555  bytes sent via SQL*Net to client
        295  bytes received via SQL*Net from client
          5  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         58  rows processedDefinition of index:
CREATE UNIQUE INDEX acg_uk3 ON x_tab
    ID_DB                    ASC,
    A_KL                     ASC,
    YEAR                     ASC,
    A                        ASC
  PCTFREE     10
  INITRANS    2
  MAXTRANS    255
  TABLESPACE  xx000_w
  STORAGE   (
    INITIAL     131072
    NEXT        131072
    PCTINCREASE 0
    MINEXTENTS  1
    MAXEXTENTS  2147483645
  )Should we reconfigure this index or someone see problem somewhere else?

Answer 1

Have anybody idea why this query sometimes running
very slow?
But only sometimes!It seems to me there is some process which cause to degrade yours query execution ,for that a wait event occurs because something is being processed where other processes waiting for its completion are dependent on the result of the event causing the wait. When one process waits for another to complete a wait event is recorded in database statistics.
For detail info
http://download-west.oracle.com/docs/cd/B19306_01/server.102/b14211/instance_tune.htm#i18202Khurram