Column_name in user_ind_columns when descending??

Question

When I create an indexe with a descending column, the value in user_ind_columns.column_name is not the actual column_name, it's something like SYS_NC00002$. I'm trying to build a Select statement to identify the columns in an index, whether ascending or descending. Everything is fine if ascending, but if descending I cannot figure out how to identify the actual column_name. What table should I be joining to?

Answer 1

Interesting. I notice from analyzing descending index that it appears to include NULL rows - I am not sure why, since B*Tree and FBI do not. Not sure whether that explains the difference times Laurent is getting for his range scan.
Can't think of a reason to use a descending index on a single column. Tom Kyte points out (in his book, available on Amazon etc. blah blah) that multi-column descending/ascending indexes can be used to sort queries with different sort orders on different columns (providing they match those of the index presumably).
Anyway, back to NULLs...
Oracle Database 10g Enterprise Edition Release 10.1.0.2.0 - 64bit Production
With the Partitioning, OLAP and Data Mining options
SQL> SELECT DECODE (GROUPING (comm), 1, 'Total:', comm) comm, COUNT (*)
  2  FROM   emp
  3  GROUP BY ROLLUP (comm);
COMM                                       COUNT(*)
0                                                 1
300                                               1
500                                               1
1400                                              1
                                                 10
Total:                                           14
6 rows selected.
SQL> -- create normal B*Tree
SQL> CREATE INDEX index_name ON emp (comm);
Index created.
SQL> ANALYZE INDEX index_name COMPUTE STATISTICS;
Index analyzed.
SQL> SELECT index_name, index_type, num_rows
  2  FROM   user_indexes
  3  WHERE  index_name = 'INDEX_NAME';
INDEX_NAME                     INDEX_TYPE                    NUM_ROWS
INDEX_NAME                     NORMAL                               4
SQL> DROP INDEX index_name;
Index dropped.
SQL> -- create descending FBI
SQL> CREATE INDEX index_name ON emp (comm DESC);
Index created.
SQL> ANALYZE INDEX index_name COMPUTE STATISTICS;
Index analyzed.
SQL> SELECT index_name, index_type, num_rows
  2  FROM   user_indexes
  3  WHERE  index_name = 'INDEX_NAME';
INDEX_NAME                     INDEX_TYPE                    NUM_ROWS
INDEX_NAME                     FUNCTION-BASED NORMAL               14 <---
SQL> DROP INDEX index_name;
Index dropped.
SQL> -- create normal FBI
SQL> CREATE INDEX index_name ON emp (comm + 0);
Index created.
SQL> ANALYZE INDEX index_name COMPUTE STATISTICS;
Index analyzed.
SQL> SELECT index_name, index_type, num_rows
  2  FROM   user_indexes
  3  WHERE  index_name = 'INDEX_NAME';
INDEX_NAME                     INDEX_TYPE                    NUM_ROWS
INDEX_NAME                     FUNCTION-BASED NORMAL                4
SQL>