Q62460: ISAM INSERT with Duplicate Key Incorrectly Allocates Record

Article: Q62460
Product(s): See article
Version(s): 7.00 7.10 | 7.00 7.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | buglist7.00 buglist7.10 | mspl13_basic
Last Modified: 8-AUG-1990

When you specify a unique key with CREATEINDEX using Microsoft BASIC
Professional Development System (PDS) version 7.00 ISAM, inserting
records with duplicate keys causes a trappable error (run-time error
86). However, the ISAM engine still inserts one record into the ISAM
file per attempt to insert a duplicate key record. The records
inserted with the duplicate keys are marked as deleted. The BASIC
function LOF, which returns the total number of records in a BASIC PDS
7.00 ISAM file, will show the actual number of successful writes to
the file, showing that the records written with duplicate keys are not
valid. Since these records are marked as deleted, using the ISAMPACK
utility will remove the records and reduce the file to its appropriate
size.

Microsoft has confirmed this to be a problem in Microsoft BASIC
Professional Development System (PDS) versions 7.00 and 7.10 for
MS-DOS. We are researching this problem and will post new information
here as it becomes available.

The sample program below demonstrates the problem. If you execute the
code below, a record will be inserted into the ISAM file, and then 999
more attempts will be made to insert that record. Since the program
specifies unique keys in the CREATEINDEX statement, run-time error 86,
"Duplicate key," will occur. The program will trap the "Duplicate key"
error and resume execution. When the program terminates, the LOF
function will show there is only one legitimate record in the file.
The file size, however, will be approximately 1.6 MB. Using
ISAMPACK.EXE on the ISAM file created by the program will reduce the
file to 64K in size.

To execute the program below, you must load the BASIC PDS 7.00 or 7.10
ISAM support by executing the PROISAMD.EXE TSR (terminate-and-stay-resident)
program. After PROISAMD has been loaded, you can execute the program
within the QBX.EXE environment.

Code Example
------------

'WARNING: Even though this program only successfully
'         writes one ISAM record, it creates a 1.6 MB file.
TYPE NewRec
   AString AS STRING * 8
END TYPE

40 ON ERROR GOTO errorhandler
50 DIM NewBuf AS NewRec
60 OPEN "output.mdb" FOR ISAM NewRec "NewRec" AS #1

70 CREATEINDEX #1, "X", 1, "AString"    '* Specify unique key *

80 NewBuf.AString = "ABCDEFGH"
90 FOR i = 1 TO 1000
100     INSERT #1, NewBuf      '* Causes 999 "Duplicate key" errors *
110    PRINT i
120 NEXT
130 PRINT "Size of database (should be 1): "; LOF(1)
135 DELETEINDEX #1, "X" ' Delete the index so the program
                        ' can be run over.
140 END

errorhandler:
150     SELECT CASE ERR
            CASE 86: PRINT "duplicate key attempted":
            RESUME NEXT
        CASE ELSE
            CLS
            PRINT "An error occurred. The number is: "; ERR
            PRINT "It occurred on line: "; ERL
            DELETEINDEX #1, "X"
            END
        END SELECT

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