[DBD-SQLite] Could someone explain the pod re sqlite_unicode and blobs?

[Martin J. Evans]

Hi,

 From 
http://search.cpan.org/~adamk/DBD-SQLite-1.33/lib/DBD/SQLite.pm#Database_Handle_Attributes

=====
sqlite_unicode

     If set to a true value, DBD::SQLite will turn the UTF-8 flag on for 
all text strings coming out of the database (this feature is currently 
disabled for perl < 5.8.5). For more details on the UTF-8 flag see 
perlunicode. The default is for the UTF-8 flag to be turned off.

     Also note that due to some bizarreness in SQLite's type system (see 
http://www.sqlite.org/datatype3.html), if you want to retain blob-style 
behavior for some columns under $dbh->{sqlite_unicode} = 1 (say, to 
store images in the database), you have to state so explicitly using the 
3-argument form of "bind_param" in DBI when doing updates:

       use DBI qw(:sql_types);
       $dbh->{sqlite_unicode} = 1;
       my $sth = $dbh->prepare("INSERT INTO mytable (blobcolumn) VALUES 
(?)");

       # Binary_data will be stored as is.
       $sth->bind_param(1, $binary_data, SQL_BLOB);

     Defining the column type as BLOB in the DDL is not sufficient.
=====

I don't understand this and wondered if someone could explain it better 
to me. I don't get a) the bolded "some" in "for some columns" and when 
it says "in DBI when doing updates:" it is followed with an example 
which does an insert.

What I'm struggling to understand is what are the precise cases when you 
need to bind a blob as a SQL_BLOB when sqlite_unicode is on? What I've 
found if that if you do an insert or update of a png file into a blob 
field and select it back the UTF8 flag is set on but when the blob is 
inserted with TYPE => SQL_BLOB it is not - is this the problem?

Here is an example:

use strict;
use warnings;
use DBI qw(:sql_types);
use Data::Dumper;
use Encode;

my $euro = "\x{20ac}";

my $h = DBI->connect("dbi:SQLite:dbname=test.db", '', '',
     {RaiseError => 1,
      # enable the following of you won't get unicode back:
      sqlite_unicode => 1});
eval {
     $h->do(q/drop table test1/);
};
$h->do(q/create table test1 (a varchar(50), b blob)/);

my $s = $h->prepare(q/insert into test1 values(?, ?)/);

open(my $ifh, "<:raw", "in.png");
my $png;
{
     local $/ = undef;
     $png = <$ifh>;
}
# If you uncomment the following 3 lines and comment the 4th
# then blob data does not come back with UTF8 flag on
#$s->bind_param(1, $euro);
#$s->bind_param(2, $png, {TYPE => SQL_BLOB});
#$s->execute;
$s->execute($euro, $png);

my $row = $h->selectrow_arrayref(q/select * from test1/);
##print Dumper($row);
#
print "UTF8 flag ", Encode::is_utf8($row->[0]), "\n";
print "UTF8 flag ", Encode::is_utf8($row->[1]), "\n";

$s = $h->prepare(q/update test1 set b = ? where a = ?/);
$s->execute($euro, $png);

$row = $h->selectrow_arrayref(q/select * from test1/);
#print Dumper($row);
#
print "UTF8 flag ", Encode::is_utf8($row->[0]), "\n";
print "UTF8 flag ", Encode::is_utf8($row->[1]), "\n";

open(my $ofh, ">:raw", "out.png");
print $ofh $row->[1];
close $ofh;

which produces:

UTF8 flag 1
UTF8 flag 1
UTF8 flag 1
UTF8 flag 1

but when you make the change (see comments) you get:

UTF8 flag 1
UTF8 flag
UTF8 flag 1
UTF8 flag

Thanks

Martin