vbdpss.hlp (Table of Contents; Topic list)
Article Q83857
                                                 Contents  Index  Back
─────────────────────────────────────────────────────────────────────────────
                           Knowledge Base Contents  Knowledge Base Index
 
 How an ISAM SEEK Works with Multiple Indexes - Q83857
 
 When seeking on multiple index values, the ISAM SEEK statement
 combines the binary images of all indexed columns to generate a key to
 SEEK upon; the ISAM engine does not SEEK on each index value
 individually or consecutively. If you expect the ISAM engine to SEEK
 on multiple index values by SEEKing on the first value, then SEEKing
 on the subset of the records in the table that match this SEEK using
 the next index value, the SEEK routine may appear to work
 inconsistently.
 
 This behavior is by design and is only evident when using the SEEKGT
 or SEEKGE statement, but not SEEKEQ. To find a specific record among
 the subset of records matching a SEEK, you can SEEK on a single
 indexed column and then use the MOVENEXT statement to move to the
 desired record.
 
 More Information:
 
 When seeking on multiple indexes, ISAM concatenates the binary
 representations of the indexes together to make one value, and then
 does a bit-by-bit comparison to satisfy the SEEK statement. Therefore,
 if you are doing a SEEKGT or a SEEKGE, the end result may be a record
 that satisfies the first index, but not the second one.
 
 The following is a simplified example that uses the binary
 representations of three records in an ISAM database file:
 
             Field1     Field2
             ------     ------
 
 Decimal       1          1
            --------   --------
 Binary    |00000001| |00000001|
            --------   --------
 
 Decimal       1          2
            --------   --------
 Binary    |00000001| |00000010|
            --------   --------
 
 Decimal       1          3
            --------   --------
 Binary    |00000001| |00000011|
            --------   --------
 
 In this example database file, when performing a SEEKGT with the value
 1 for both the first and second key values, you may expect to SEEK to
 a record with a number greater than 1 in both the first and second
 indexed columns. The actual current record after such a SEEK is the
 second record in this list. The following is also a simplified example
 of the way the ISAM database would see these concatenated records:
 
            Field1 and Field2
            -----------------
 
 Decimal          257
            ----------------
 Binary    |0000000100000001|
            ----------------
 
 Decimal          258
            ----------------
 Binary    |0000000100000010|
            ----------------
 
 Decimal          259
            ----------------
 Binary    |0000000100000011|
            ----------------
 
 As you can see, 258 is greater than 257; therefore, the record that
 ISAM would select for a statement such as
 
    SEEKGT <filenumber>, 1, 1
 
 is the second record in the sample database.
 
 The following example program demonstrates this functionality.
 
    See Example