423,551 Invalid Indexes

A friend received an alarming email from a commendably proactive DBA who was concerned that he had discovered a potentially serious performance issue on the production system:

The following result shows there are many invalid indexes. For example, in the first row we see that there are nearly 423551 invalid indexes for just one row of a table. So if we rebuilt the index it would improve the performance.

Wow, that sure is a lot of invalid indexes for just one row of a table. For the sake of anonymity, I've run his query against the SCOTT/TIGER schema, where fortunately there are only 14 invalid indexes (or perhaps 10, if that's how you count them). Perhaps you can see what the problem might be:

SELECT DISTINCT
       t.table_name
     , i.index_name
     , t.num_rows table_rows
     , i.num_rows index_rows
FROM   dba_tables t
     , dba_indexes i
WHERE  t.table_name = 'DEPT'
AND    t.owner = 'SCOTT'
AND    i.owner = t.owner
AND    i.index_type = 'NORMAL';

TABLE_NAME                     INDEX_NAME                     TABLE_ROWS INDEX_ROWS
------------------------------ ------------------------------ ---------- ----------
DEPT                           PK_EMP                                  4         14
DEPT                           EMP_DEPT_IX                             4         14
DEPT                           PK_DEPT                                 4          4
DEPT                           PK_SALGRADE                             4          5

4 rows selected.

There is worse to come. It's not just DEPT that has invalid indexes. He goes on:

The following result shows there are many invalid indexes, so if we rebuild the index, it would improve the performance. At the same time from the first and last rows, indexes are not being used, so we need to look into it.

TABLE_NAME                     INDEX_NAME                     TABLE_ROWS INDEX_ROWS
------------------------------ ------------------------------ ---------- ----------
SALGRADE                       PK_DEPT                                 5          4
SALGRADE                       PK_EMP                                  5         14
SALGRADE                       EMP_DEPT_IX                             5         14
SALGRADE                       PK_SALGRADE                             5          5

TABLE_NAME                     INDEX_NAME                     TABLE_ROWS INDEX_ROWS
------------------------------ ------------------------------ ---------- ----------
EMP                            PK_DEPT                                14          4
EMP                            PK_EMP                                 14         14
EMP                            EMP_DEPT_IX                            14         14
EMP                            PK_SALGRADE                            14          5

I just can't help wondering what he thinks an index is.