I have a database table 'TABLE_A' with a nullable string field 'field_x' having non-clustered index on it. A query

Code:
SELECT * FROM TABLE_A WHERE field_x = 'some value'
executes quickly in most environments. In one client's environment all values of this field are NULL and this query runs very slowly.

Question #1: Is there a way in SYBASE to optimize for this specific scenario? It seems like something that SYBASE should already handle.

Question #2: Is there a way to optimize this query? Filtering out NULLS (as in

Code:
SELECT * FROM TABLE_A WHERE field_x IS NOT NULL
) does NOT help with the performance.

I gathered performance statistics by looking at Plan Viewer inside Interactive SQL. When there is some data in this table for field_x then Logical I/O, Physical I/O, and CPU are very close to zero. When all values are NULL then I/O and CPU values are similar to scanning the whole table.

The query execution plan shows that the index is being used:

Code:
QUERY PLAN FOR STATEMENT 1 (at line 1).

STEP 1
    The type of query is SELECT.

1 operator(s) under root

   |ROOT:EMIT Operator (VA = 1)
   |
   |   |SCAN Operator (VA = 0)
   |   |  FROM TABLE
   |   |  TABLE_A
   |   |  Index :   INDEX_X
   |   |  Forward Scan.
   |   |  Positioning by key.
   |   |  Keys are:
   |   |    field_x ASC
   |   |  Using I/O Size 16 Kbytes for index leaf pages.
   |   |  With LRU Buffer Replacement Strategy for index leaf pages.
   |   |  Using I/O Size 16 Kbytes for data pages.
   |   |  With MRU Buffer Replacement Strategy for data pages.
Also, I'm not sure if this problem is reproducible for other databases, but I observed it on SYBASE ASE 15.

Thank you for your help.