Help required in Index Question

Question

Hi,
This is an interview question that i faced.
Theres a table Statistics having more than 300 columns. It has normal indexes on say five columns col1, col2, col3, col4, col5.
Queries involving the first four columns give excellent performance but the query involving col5 gives poor performance.
COL5 has around 5 million data and 1 normal index ix created on it and theres just one value in it say 'STATS'.(He said the query can be simple select queries or join queries, the queries may select all the rows of few rows may be 5%, 10% etc).
The question was what could be the possible reason?
I was unable to answer it but i think BITMAP index might be useful in this scenario.
Any help or suggestions will be highly appreciated.

Answer 1

Is it a good idea to creat a normal index on a column that have just one value as 'STATS' and the total no of rows is more than 5 million??
No
See the I/O
SQL> CREATE TABLE bitmap_table AS SELECT ROWNUM rn FROM all_objects
  2  /
Table created.
SQL> ALTER TABLE bitmap_table ADD (status VARCHAR2(1))
  2  /
Table altered.
SQL> SELECT COUNT(*) FROM bitmap_table
  2  /
COUNT(*)
    64177
SQL> UPDATE bitmap_table SET status='Y'
  2  /
64177 rows updated.
SQL> COMMIT
  2  /
Commit complete.
SQL> CREATE TABLE btree_table AS SELECT * FROM bitmap_table
  2  /
Table created.
SQL> CREATE BITMAP INDEX bitmap_table_idx ON bitmap_table (status)
  2  /
Index created.
SQL> CREATE INDEX btree_table_idx ON btree_table (status)
  2  /
Index created.
SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS('SCOTT','BITMAP_TABLE')
PL/SQL procedure successfully completed.
SQL> EXEC DBMS_STATS.GATHER_INDEX_STATS('SCOTT','BITMAP_TABLE_IDX')
PL/SQL procedure successfully completed.
SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS('SCOTT','BTREE_TABLE')
PL/SQL procedure successfully completed.
SQL> EXEC DBMS_STATS.GATHER_INDEX_STATS('SCOTT','BTREE_TABLE_IDX')
PL/SQL procedure successfully completed.
SQL> SET AUTOTRACE TRACEONLY
SQL> SELECT status
  2    FROM bitmap_table
  3   WHERE status='Y'
  4  /
64177 rows selected.
Execution Plan
   0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=2 Card=64177 Bytes
          =128354)
   1    0   BITMAP CONVERSION (TO ROWIDS) (Cost=2 Card=64177 Bytes=128
          354)
   2    1     BITMAP INDEX (FAST FULL SCAN) OF 'BITMAP_TABLE_IDX' (IND
          EX (BITMAP))
Statistics
        171  recursive calls
          0  db block gets
         31  consistent gets
          0  physical reads
          0  redo size
     804546  bytes sent via SQL*Net to client
     475281  bytes received via SQL*Net from client
       4280  SQL*Net roundtrips to/from client
          4  sorts (memory)
          0  sorts (disk)
      64177  rows processed
SQL> DROP INDEX BITMAP_TABLE_IDX
  2  /
Index dropped.
SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS('SCOTT','BITMAP_TABLE')
PL/SQL procedure successfully completed.
SQL> SET AUTOTRACE TRACEONLY
SQL> SELECT status
  2    FROM bitmap_table
  3   WHERE status='Y'
  4  /
64177 rows selected.
Execution Plan
   0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=26 Card=64202 Byte
          s=128404)
   1    0   TABLE ACCESS (FULL) OF 'BITMAP_TABLE' (TABLE) (Cost=26 Car
          d=64202 Bytes=128404)
Statistics
        192  recursive calls
          0  db block gets
       4405  consistent gets
          0  physical reads
          0  redo size
     804546  bytes sent via SQL*Net to client
     475281  bytes received via SQL*Net from client
       4280  SQL*Net roundtrips to/from client
          4  sorts (memory)
          0  sorts (disk)
      64177  rows processed
SQL> SELECT status
  2    FROM btree_table
  3   WHERE status='Y'
  4  /
64177 rows selected.
Execution Plan
   0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=27 Card=64177 Byte
          s=128354)
   1    0   INDEX (FAST FULL SCAN) OF 'BTREE_TABLE_IDX' (INDEX) (Cost=
          27 Card=64177 Bytes=128354)
Statistics
        171  recursive calls
          0  db block gets
       4429  consistent gets
          0  physical reads
          0  redo size
     804546  bytes sent via SQL*Net to client
     475281  bytes received via SQL*Net from client
       4280  SQL*Net roundtrips to/from client
          4  sorts (memory)
          0  sorts (disk)
      64177  rows processed
SQL> SELECT index_name,blevel,num_rows
  2    FROM user_indexes
  3   WHERE index_name='BTREE_TABLE_IDX'
  4  /
INDEX_NAME                        BLEVEL  NUM_ROWS
BTREE_TABLE_IDX                        1     64177
SQL> SELECT index_name,blevel,num_rows
  2    FROM user_indexes
  3*  WHERE index_name='BITMAP_TABLE_IDX'
SQL> /
INDEX_NAME                        BLEVEL  NUM_ROWS
BITMAP_TABLE_IDX                       1         3Khurram