UPDATE tablename SET foo = foo + 'bar'

This is atomic and should not cause a race condition.

Thanks grep,

It looks like simple concatenation with + won't work with Text columns. See + (String Concatenation) on MSDN.


--
sgifford's Web page

Sorry about that - PostgreSQL does let you concatenate ('||') text fields. It's been awhile since I had to deal with SQLServer weirdness.

Well then you're either left with running a transaction around your read/append/write or CAST the text field to varchar then concat (assuming the data can be cast).

I would go for the transaction approach, you'll be sure it'll work with any kind of data.

As for the ugliness it should be easy to make a method to hide the $dbh->{AutoCommit} = 0 (or $dbh->begin_work), read, append, update, $dbh->commit. Don't forget to turn AutoCommit off if it's on. In fact I would just add that in there to be safe.

local $dbh->{AutoCommit} = 0;
local $dbh->{RaiseError} = 1; 
# You can RaiseError and eval the transaction
# or you can check $sth->err each time and rollback

eval {
      ##Do your read and write here
      $dbh->commit; 
};
if ($@) {
      warn "append failed $@";
      $dbh->rollback;
}
[download]

UPDATE: something similar to this

sub append_to_text {
    my $dbh   = shift;
    my $table = shift;
    my $id    = shift;
    my $col   = shift;
    my $data  = shift;

    local $dbh->{RaiseError} = 1;
    eval {
        $dbh->begin_work();
        my $read = qq| 
            SELECT $col FROM $table 
                         WHERE id = ? 
             |;

        my $update = qq|
            UPDATE $table SET $col = ?
                         WHERE id = ?
            |;
        
        my $sth_read = $dbh->prepare($read);
        $sth_read->execute($id);
        my $row = $sth_read->fetchrow_hashref();
        $sth_read->finish();
        my $current = $row->{$col};
        $current .= $data;
        
        my $rtn = $dbh->do($update,undef,$current,$id);
        $dbh->commit() if ($rtn);
    };    
    if ($@) {
          warn "Append Failed: $@\nStatement: $DBI::lasth->{Statement}
+";
          eval { $dbh->rollback };
        }    
    
}
[download]



grep

One dead unjugged rabbit fish later

Something like this:

DECLARE @ptrval binary(16)
SELECT @ptrval = TEXTPTR(item) 
   FROM table 
   WHERE whatever criteria

UPDATETEXT table.item @ptrval NULL 0 'new text';
[download]

Hi imp,

Any pointers on how to do that with Perl/DBI? I can't seem to find any examples, and haven't been able to figure out the right syntax...

Thanks!


--
sgifford's Web page

I used a stored procedure for encapsulating that behaviour when I had a similar problem in the past, but if you don't have permission to add stored procedures you should be able to run the sequence of commands inline. I don't have access to a server to test with currently though, so your mileage may vary.

Something like this:

my $sql = 'DECLARE @ptrval binary(16);'
        . ' SELECT @ptrval = TEXTPTR(item)'
        . ' FROM table '
        . ' WHERE whatever criteria;'
        . ' UPDATETEXT table.item @ptrval NULL 0 ?';
$dbh->do($sql,{},"new text");
[download]

But of course that's horribly ugly.

So, a stored procedure to encapsulate the behaviour:

CREATE PROCEDURE dbo.APPEND_THINGY
@id int,
@new_text varchar(8000)
AS

DECLARE @ptrval binary(16)
SELECT @ptrval = TEXTPTR(item) 
   FROM table 
   WHERE id = @id

UPDATETEXT table.item @ptrval NULL 0 @new_text;

GO
[download]

And then in your code:

my $sql = q{exec append_thingy @new_text = ?};
$dbh->do($sql,{},"new text");
[download]