db_file.pm

关于Berkelay数据库的共享源码

    }

    # specify the Perl sub that will do the comparison
    $DB_BTREE->{'compare'} = \&Compare ;

    unlink "tree" ;
    tie %h, "DB_File", "tree", O_RDWR|O_CREAT, 0666, $DB_BTREE 
        or die "Cannot open file 'tree': $!\n" ;

    # Add a key/value pair to the file
    $h{'Wall'} = 'Larry' ;
    $h{'Smith'} = 'John' ;
    $h{'mouse'} = 'mickey' ;
    $h{'duck'}  = 'donald' ;

    # Delete
    delete $h{"duck"} ;

    # Cycle through the keys printing them in order.
    # Note it is not necessary to sort the keys as
    # the btree will have kept them in order automatically.
    foreach (keys %h)
      { print "$_\n" }

    untie %h ;

Here is the output from the code above.

    mouse
    Smith
    Wall

There are a few point to bear in mind if you want to change the
ordering in a BTREE database:

=over 5

=item 1.

The new compare function must be specified when you create the database.

=item 2.

You cannot change the ordering once the database has been created. Thus
you must use the same compare function every time you access the
database.

=item 3

Duplicate keys are entirely defined by the comparison function.
In the case-insensitive example above, the keys: 'KEY' and 'key'
would be considered duplicates, and assigning to the second one
would overwrite the first. If duplicates are allowed for (with the
R_DUP flag discussed below), only a single copy of duplicate keys
is stored in the database --- so (again with example above) assigning
three values to the keys: 'KEY', 'Key', and 'key' would leave just
the first key: 'KEY' in the database with three values. For some
situations this results in information loss, so care should be taken
to provide fully qualified comparison functions when necessary.
For example, the above comparison routine could be modified to
additionally compare case-sensitively if two keys are equal in the
case insensitive comparison:

    sub compare {
        my($key1, $key2) = @_;
        lc $key1 cmp lc $key2 ||
        $key1 cmp $key2;
    }

And now you will only have duplicates when the keys themselves
are truly the same. (note: in versions of the db library prior to
about November 1996, such duplicate keys were retained so it was
possible to recover the original keys in sets of keys that
compared as equal).


=back 

=head2 Handling Duplicate Keys 

The BTREE file type optionally allows a single key to be associated
with an arbitrary number of values. This option is enabled by setting
the flags element of C<$DB_BTREE> to R_DUP when creating the database.

There are some difficulties in using the tied hash interface if you
want to manipulate a BTREE database with duplicate keys. Consider this
code:

    use warnings ;
    use strict ;
    use DB_File ;

    my ($filename, %h) ;

    $filename = "tree" ;
    unlink $filename ;

    # Enable duplicate records
    $DB_BTREE->{'flags'} = R_DUP ;

    tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE 
	or die "Cannot open $filename: $!\n";

    # Add some key/value pairs to the file
    $h{'Wall'} = 'Larry' ;
    $h{'Wall'} = 'Brick' ; # Note the duplicate key
    $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
    $h{'Smith'} = 'John' ;
    $h{'mouse'} = 'mickey' ;

    # iterate through the associative array
    # and print each key/value pair.
    foreach (sort keys %h)
      { print "$_  -> $h{$_}\n" }

    untie %h ;

Here is the output:

    Smith   -> John
    Wall    -> Larry
    Wall    -> Larry
    Wall    -> Larry
    mouse   -> mickey

As you can see 3 records have been successfully created with key C<Wall>
- the only thing is, when they are retrieved from the database they
I<seem> to have the same value, namely C<Larry>. The problem is caused
by the way that the associative array interface works. Basically, when
the associative array interface is used to fetch the value associated
with a given key, it will only ever retrieve the first value.

Although it may not be immediately obvious from the code above, the
associative array interface can be used to write values with duplicate
keys, but it cannot be used to read them back from the database.

The way to get around this problem is to use the Berkeley DB API method
called C<seq>.  This method allows sequential access to key/value
pairs. See L<THE API INTERFACE> for details of both the C<seq> method
and the API in general.

Here is the script above rewritten using the C<seq> API method.

    use warnings ;
    use strict ;
    use DB_File ;

    my ($filename, $x, %h, $status, $key, $value) ;

    $filename = "tree" ;
    unlink $filename ;

    # Enable duplicate records
    $DB_BTREE->{'flags'} = R_DUP ;

    $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE 
	or die "Cannot open $filename: $!\n";

    # Add some key/value pairs to the file
    $h{'Wall'} = 'Larry' ;
    $h{'Wall'} = 'Brick' ; # Note the duplicate key
    $h{'Wall'} = 'Brick' ; # Note the duplicate key and value
    $h{'Smith'} = 'John' ;
    $h{'mouse'} = 'mickey' ;

    # iterate through the btree using seq
    # and print each key/value pair.
    $key = $value = 0 ;
    for ($status = $x->seq($key, $value, R_FIRST) ;
         $status == 0 ;
         $status = $x->seq($key, $value, R_NEXT) )
      {  print "$key -> $value\n" }

    undef $x ;
    untie %h ;

that prints:

    Smith   -> John
    Wall    -> Brick
    Wall    -> Brick
    Wall    -> Larry
    mouse   -> mickey

This time we have got all the key/value pairs, including the multiple
values associated with the key C<Wall>.

To make life easier when dealing with duplicate keys, B<DB_File> comes with 
a few utility methods.

=head2 The get_dup() Method

The C<get_dup> method assists in
reading duplicate values from BTREE databases. The method can take the
following forms:

    $count = $x->get_dup($key) ;
    @list  = $x->get_dup($key) ;
    %list  = $x->get_dup($key, 1) ;

In a scalar context the method returns the number of values associated
with the key, C<$key>.

In list context, it returns all the values which match C<$key>. Note
that the values will be returned in an apparently random order.

In list context, if the second parameter is present and evaluates
TRUE, the method returns an associative array. The keys of the
associative array correspond to the values that matched in the BTREE
and the values of the array are a count of the number of times that
particular value occurred in the BTREE.

So assuming the database created above, we can use C<get_dup> like
this:

    use warnings ;
    use strict ;
    use DB_File ;

    my ($filename, $x, %h) ;

    $filename = "tree" ;

    # Enable duplicate records
    $DB_BTREE->{'flags'} = R_DUP ;

    $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE 
	or die "Cannot open $filename: $!\n";

    my $cnt  = $x->get_dup("Wall") ;
    print "Wall occurred $cnt times\n" ;

    my %hash = $x->get_dup("Wall", 1) ;
    print "Larry is there\n" if $hash{'Larry'} ;
    print "There are $hash{'Brick'} Brick Walls\n" ;

    my @list = sort $x->get_dup("Wall") ;
    print "Wall =>	[@list]\n" ;

    @list = $x->get_dup("Smith") ;
    print "Smith =>	[@list]\n" ;

    @list = $x->get_dup("Dog") ;
    print "Dog =>	[@list]\n" ;


and it will print:

    Wall occurred 3 times
    Larry is there
    There are 2 Brick Walls
    Wall =>	[Brick Brick Larry]
    Smith =>	[John]
    Dog =>	[]

=head2 The find_dup() Method

    $status = $X->find_dup($key, $value) ;

This method checks for the existence of a specific key/value pair. If the
pair exists, the cursor is left pointing to the pair and the method 
returns 0. Otherwise the method returns a non-zero value.

Assuming the database from the previous example:

    use warnings ;
    use strict ;
    use DB_File ;

    my ($filename, $x, %h, $found) ;

    $filename = "tree" ;

    # Enable duplicate records
    $DB_BTREE->{'flags'} = R_DUP ;

    $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE 
	or die "Cannot open $filename: $!\n";

    $found = ( $x->find_dup("Wall", "Larry") == 0 ? "" : "not") ; 
    print "Larry Wall is $found there\n" ;

    $found = ( $x->find_dup("Wall", "Harry") == 0 ? "" : "not") ; 
    print "Harry Wall is $found there\n" ;

    undef $x ;
    untie %h ;

prints this

    Larry Wall is  there
    Harry Wall is not there


=head2 The del_dup() Method

    $status = $X->del_dup($key, $value) ;

This method deletes a specific key/value pair. It returns
0 if they exist and have been deleted successfully.
Otherwise the method returns a non-zero value.

Again assuming the existence of the C<tree> database

    use warnings ;
    use strict ;
    use DB_File ;

    my ($filename, $x, %h, $found) ;

    $filename = "tree" ;

    # Enable duplicate records
    $DB_BTREE->{'flags'} = R_DUP ;

    $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE 
	or die "Cannot open $filename: $!\n";

    $x->del_dup("Wall", "Larry") ;

    $found = ( $x->find_dup("Wall", "Larry") == 0 ? "" : "not") ; 
    print "Larry Wall is $found there\n" ;

    undef $x ;
    untie %h ;

prints this

    Larry Wall is not there

=head2 Matching Partial Keys 

The BTREE interface has a feature which allows partial keys to be
matched. This functionality is I<only> available when the C<seq> method
is used along with the R_CURSOR flag.

    $x->seq($key, $value, R_CURSOR) ;

Here is the relevant quote from the dbopen man page where it defines
the use of the R_CURSOR flag with seq:

    Note, for the DB_BTREE access method, the returned key is not
    necessarily an exact match for the specified key. The returned key
    is the smallest key greater than or equal to the specified key,
    permitting partial key matches and range searches.

In the example script below, the C<match> sub uses this feature to find
and print the first matching key/value pair given a partial key.

    use warnings ;
    use strict ;
    use DB_File ;
    use Fcntl ;

    my ($filename, $x, %h, $st, $key, $value) ;

    sub match
    {
        my $key = shift ;
        my $value = 0;
        my $orig_key = $key ;
        $x->seq($key, $value, R_CURSOR) ;
        print "$orig_key\t-> $key\t-> $value\n" ;
    }

    $filename = "tree" ;
    unlink $filename ;

    $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE
        or die "Cannot open $filename: $!\n";

    # Add some key/value pairs to the file
    $h{'mouse'} = 'mickey' ;
    $h{'Wall'} = 'Larry' ;
    $h{'Walls'} = 'Brick' ; 
    $h{'Smith'} = 'John' ;


    $key = $value = 0 ;
    print "IN ORDER\n" ;
    for ($st = $x->seq($key, $value, R_FIRST) ;
	 $st == 0 ;
         $st = $x->seq($key, $value, R_NEXT) )

      {  print "$key	-> $value\n" }

    print "\nPARTIAL MATCH\n" ;

    match "Wa" ;
    match "A" ;
    match "a" ;

    undef $x ;
    untie %h ;

Here is the output:

    IN ORDER
    Smith -> John
    Wall  -> Larry
    Walls -> Brick
    mouse -> mickey

    PARTIAL MATCH
    Wa -> Wall  -> Larry
    A  -> Smith -> John
    a  -> mouse -> mickey

=head1 DB_RECNO

DB_RECNO provides an interface to flat text files. Both variable and
fixed length records are supported.

In order to make RECNO more compatible with Perl, the array offset for
all RECNO arrays begins at 0 rather than 1 as in Berkeley DB.

As with normal Perl arrays, a RECNO array can be accessed using
negative indexes. The index -1 refers to the last element of the array,
-2 the second last, and so on. Attempting to access an element before
the start of the array will raise a fatal run-time error.

=head2 The 'bval' Option

The operation of the bval option warrants some discussion. Here is the
definition of bval from the Berkeley DB 1.85 recno manual page:

    The delimiting byte to be used to mark  the  end  of  a
    record for variable-length records, and the pad charac-
    ter for fixed-length records.  If no  value  is  speci-
    fied,  newlines  (``\n'')  are  used to mark the end of
    variable-length records and  fixed-length  records  are
    padded with spaces.

The second sentence is wrong. In actual fact bval will only default to
C<"\n"> when the openinfo parameter in dbopen is NULL. If a non-NULL
openinfo parameter is used at all, the value that happens to be in bval
will be used. That means you always have to specify bval when making
use of any of the options in the openinfo parameter. This documentation
error will be fixed in the next release of Berkeley DB.

That clarifies the situation with regards Berkeley DB itself. What
about B<DB_File>? Well, the behavior defined in the quote above is
quite useful, so B<DB_File> conforms to it.

That means that you can specify other options (e.g. cachesize) and
still have bval default to C<"\n"> for variable length records, and
space for fixed length records.

Also note that the bval option only allows you to specify a single byte
as a delimiter.

=head2 A Simple Example

Here is a simple example that uses RECNO (if you are using a version 
of Perl earlier than 5.004_57 this example won't work -- see 
L<Extra RECNO Methods> for a workaround).

    use warnings ;
    use strict ;
    use DB_File ;