MySQL silently truncating your data: Update

After my entry yesterday about MySQL truncating data, several people
have pointed out that MySQL 4.1 or later gives you a warning. Yes, this is true. You
can even see it in the example I gave:

Query OK, 1 row affected, 1 warning (0.00 sec)

I ignored mentioning this, but perhaps should have said something
about it. I reason I didn’t mention it was because I didn’t feel that a
warning really helped anyone. Developers have enough problems
remembering to check for errors, let along remembering to check in case
there was a warning as well. Plus, they’d then have to work out if the
warning was something serious or something they could ignore. There’s
also the question of how well the language bindings present this
information. Take for example, PHP. The mysqli extension gained support
for checking for warnings in PHP5 and gives the following
as an
example of getting warnings:

mysqli_query($link, $query);

if (mysqli_warning_count($link)) {
   if ($result = mysqli_query($link, "SHOW WARNINGS")) {
      $row = mysqli_fetch_row($result);
      printf("%s (%d): %sn", $row[0], $row[1], $row[2]);

Hardly concise code. As of 5.1.0, there is also mysqli_get_warnings(),
but is undocumented beyond noting its existence. The MySQL extension
does not support getting warning information. The PDO wrapper doesn’t
provide any way to get this information.

In perl, DBD::mysql has a mysql_warning_count()
function, but presumably would have to call "SHOW WARNINGS"
like in the PHP example. Seems Python’s MYSQLdb module will raise an
exception on warnings in certain cases. Mostly using the Cursor

In java, you can set the jdbcCompliantTruncation connection
parameter to make the driver throw java.sql.DataTruncation
exceptions, as per the JDBC spec, which makes you wonder why this isn’t
set by default. Unfortunately this setting is usually outside the
programmer’s control. There is also the
java.sql.Statement.getWarnings(), but once again, you need to
check this after every statement. Not sure if ORM tools like Hibernate
check this or not.

So, yes MySQL does give you a warning, but in practice is useless.

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