Re: PERL DBI documentatio -- how to check for errors

Nitzan,

> (A bit long)
> (Bottom line: is it, or is it not, enough to always check DBI::err)

I'd correct the documentation, if I only knew what documentation you're 
referring to. :-) In the MySQL Reference Manual, we have this:

   http://dev.mysql.com/doc/refman/5.0/en/perl.html

But that's nothing but a very short overview.

At the moment, the Perl DBI documentation isn't maintained by us, 
although there are plans to do that.

/Stefan


> The documentation for how to check the success of various DBI calls, and in
> particular do(), selectrow_foo() and selectall_foo() is unclear and not
> precise, at least for me. Actually I am not sure, after scrutinizing, how
> exactly to check for errors.
> 
> Specific problems:
> 
> Looking at the doc for "err", we see: "The DBI resets $h->err to undef
> before most DBI method calls", note the "most". Since the documentation for
> specific methods does not say which do *not* reset err, I have no way of
> knowing which do and which don't.
> 
> Also: looking at (for example) the doc for selectall_hashref(), we can find:
> "If any method except fetchrow_hashref fails, and "RaiseError" is not set,
> selectall_hashref will return undef. If fetchrow_hashref fails and
> "RaiseError" is not set, then it will return with whatever data it has
> fetched thus far. $DBI::err should be checked to catch that".
> selectall_arrayref() and selectcol_arrayref() have similar documentation.
> 
> To me this looks like the correct way to check selectall_hashref() is then:
> my $hr = $dbh->selectall_hashref( ... );
> (!defined $hr || defined $DBI::err) && die( ... );
> 
> Justification: error returns undef, so we need to see whether the result is
> defined at all. Checking DBI::err is to test "that" -- only some rows were
> returned. It does *not* say explicitly that err is reset before hand, so I
> can't be sure that testing err alone is enough!
> 
> Looking at selectrow_hashref() says that error is indicated by undef. It
> does *not* say err will be set. So I can only assume I need to:
> 
> my $hr = $dbh->selectrow_hashref( ... );
> !defined $hr && die( "..." );
> 
> Again it does not say err is reset before, so I cannot assume I can trust
> err for telling me about errors. The docs only explicitly mention the return
> value.
> 
> To summarize:
> 
> My understand of the docs as written is that errors for selectall_ and
> selectcol_ methods should be checked as:
> 
> (!defined $return_value || defined $DBI::err) && die( ... );
> 
> ... that errors for selectrow_ methods should be checked as:
> 
> (defined $return_value) || die( ... );
> 
> ... and that errors for do() should be checked as:
> 
> ($return_value) || die( ... );
> 
> Is that correct? Is it really not possible to only check for (defined
> DBI::err) always?
> 
> Can someone verify, or clarify the docs?
> 
> Much obligied,
> Nitzan
> 
> 
> 


Regards,

Stefan
-- 
Stefan Hinz   <[email protected]>
MySQL AB Documentation Team Lead
Skype: stefanhinz      SIP: 4429
Desk: +49308270294-0     Fax: -1
TZ: Berlin Mobile: +491777841069

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