[freetds] Null values in result record are overwritten with previous non-null data when using a prepared query more than once

Buurman, H.A. (Herbert) h.a.buurman at interlogica.nl
Tue Jun 16 04:17:16 EDT 2009


Hello all,

  This issue seems to happen from 0.82 (stable) onwards to today's nightly build.
  Software used:
    Perl 5.10.0 x86_64-linux-gnu-thread-multi
    DBI v1.607
    DBI::DBD v12.010405
    DBD::ODBC v1.17
    UnixODBC 2.2.11
    FreeTDS 0.82 (stable)
    TDS protocol version 8.0
    Microsoft SQL Server Enterprise Edition v9.0.2047 (x86)
  When using the above-mentioned software on the following table (DDL):
	CREATE TABLE [dbo].[freetds_test] (
	  [identifier] char(1) COLLATE SQL_Latin1_General_CP850_CI_AS NOT NULL,
	  [value] int NULL,
	  PRIMARY KEY CLUSTERED ([identifier])
	)
	ON [PRIMARY]
	GO
  With data:
	identifier	value
	1		17
	2		Null

  And this code:
    my $db_options = {
        PrintError => 1,
        RaiseError => 0,
        LongReadLen => 200,
        LongTruncOk => 1,
        AutoCommit => 0,
    };
    my $db = DBI->connect ("dbi:ODBC:$DSNName",$db_username,$db_password,$db_options);
    my @Identifiers = ('1', '2');
    my $FreeTDS_Query = $db->prepare('SELECT Identifier, Value FROM freetds_test with(nolock) WHERE Identifier = ?') or
      print("Error preparing query: " . $db->errstr . "\n");
    my $Item_ref;
    foreach (@Identifiers) {
        $FreeTDS_Query->execute($_);
        $Item_ref = $FreeTDS_Query->fetchrow_hashref();
        foreach my $key (keys(%{$Item_ref})) {
            print("$key => $Item_ref->{$key}, ");
        }
        print("\n");
    }
    $FreeTDS_Query->finish();
    $db->disconnect();

  Which prints the following output:
	Value => 17, Identifier => 1,
	Value => 17, Identifier => 2,

  As you can see, the Null-value for the 2nd identifier is missing...
  instead, the previous non-null value is returned.
  
  This doesn't happen if the query is prepared and finished inside the
  loop, but that's not really what I want :)

  Is this a known issue or is there something to work around this?
  (While still preparing the query outside the loop and with the use of
  placeholders).

Kind regards,
H. Buurman

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