DBD::DBM(3) User Contributed Perl Documentation DBD::DBM(3)
NAME
DBD::DBM - a DBI driver for DBM & MLDBM files
SYNOPSIS
use DBI;
$dbh = DBI->connect('dbi:DBM:'); # defaults to SDBM_File
$dbh = DBI->connect('DBI:DBM(RaiseError=1):'); # defaults to SDBM_File
$dbh = DBI->connect('dbi:DBM:type=GDBM_File'); # defaults to GDBM_File
$dbh = DBI->connect('dbi:DBM:mldbm=Storable'); # MLDBM with SDBM_File
# and Storable
or
$dbh = DBI->connect('dbi:DBM:', undef, undef);
$dbh = DBI->connect('dbi:DBM:', undef, undef, { dbm_type => 'ODBM_File' });
and other variations on connect() as shown in the DBI docs and with the dbm_ attributes shown below
... and then use standard DBI prepare, execute, fetch, placeholders, etc., see "QUICK START" for an
example
DESCRIPTION
DBD::DBM is a database management sytem that can work right out of the box. If you have a standard
installation of Perl and a standard installation of DBI, you can begin creating, accessing, and
modifying database tables without any further installation. You can also add some other modules to
it for more robust capabilities if you wish.
The module uses a DBM file storage layer. DBM file storage is common on many platforms and files can
be created with it in many languges. That means that, in addition to creating files with DBI/SQL,
you can also use DBI/SQL to access and modify files created by other DBM modules and programs. You
can also use those programs to access files created with DBD::DBM.
DBM files are stored in binary format optimized for quick retrieval when using a key field. That
optimization can be used advantageously to make DBD::DBM SQL operations that use key fields very
fast. There are several different "flavors" of DBM - different storage formats supported by
different sorts of perl modules such as SDBM_File and MLDBM. This module supports all of the flavors
that perl supports and, when used with MLDBM, supports tables with any number of columns and
insertion of Perl objects into tables.
DBD::DBM has been tested with the following DBM types: SDBM_File, NDBM_File, ODBM_File, GDBM_File,
DB_File, BerekeleyDB. Each type was tested both with and without MLDBM.
QUICK START
DBD::DBM operates like all other DBD drivers - it's basic syntax and operation is specified by DBI.
If you're not familiar with DBI, you should start by reading DBI and the documents it points to and
then come back and read this file. If you are familiar with DBI, you already know most of what you
need to know to operate this module. Just jump in and create a test script something like the one
shown below.
You should be aware that there are several options for the SQL engine underlying DBD::DBM, see
"Supported SQL syntax". There are also many options for DBM support, see especially the section on
"Adding multi-column support with MLDBM".
But here's a sample to get you started.
use DBI;
my $dbh = DBI->connect('dbi:DBM:');
$dbh->{RaiseError} = 1;
for my $sql( split /;\n+/,"
CREATE TABLE user ( user_name TEXT, phone TEXT );
INSERT INTO user VALUES ('Fred Bloggs','233-7777');
INSERT INTO user VALUES ('Sanjay Patel','777-3333');
INSERT INTO user VALUES ('Junk','xxx-xxxx');
DELETE FROM user WHERE user_name = 'Junk';
UPDATE user SET phone = '999-4444' WHERE user_name = 'Sanjay Patel';
SELECT * FROM user
"){
my $sth = $dbh->prepare($sql);
$sth->execute;
$sth->dump_results if $sth->{NUM_OF_FIELDS};
}
$dbh->disconnect;
USAGE
Specifiying Files and Directories
DBD::DBM will automatically supply an appropriate file extension for the type of DBM you are using.
For example, if you use SDBM_File, a table called "fruit" will be stored in two files called
"fruit.pag" and "fruit.dir". You should never specify the file extensions in your SQL statements.
However, I am not aware (and therefore DBD::DBM is not aware) of all possible extensions for various
DBM types. If your DBM type uses an extension other than .pag and .dir, you should set the dbm_ext
attribute to the extension. And you should write me with the name of the implementation and extension
so I can add it to DBD::DBM! Thanks in advance for that :-).
$dbh = DBI->connect('dbi:DBM:ext=.db'); # .db extension is used
$dbh = DBI->connect('dbi:DBM:ext='); # no extension is used
or
$dbh->{dbm_ext}='.db'; # global setting
$dbh->{dbm_tables}->{'qux'}->{ext}='.db'; # setting for table 'qux'
By default files are assumed to be in the current working directory. To have the module look in a
different directory, specify the f_dir attribute in either the connect string or by setting the
database handle attribute.
For example, this will look for the file /foo/bar/fruit (or /foo/bar/fruit.pag for DBM types that use
that extension)
my $dbh = DBI->connect('dbi:DBM:f_dir=/foo/bar');
my $ary = $dbh->selectall_arrayref(q{ SELECT * FROM fruit });
And this will too:
my $dbh = DBI->connect('dbi:DBM:');
$dbh->{f_dir} = '/foo/bar';
my $ary = $dbh->selectall_arrayref(q{ SELECT x FROM fruit });
You can also use delimited identifiers to specify paths directly in SQL statements. This looks in
the same place as the two examples above but without setting f_dir:
my $dbh = DBI->connect('dbi:DBM:');
my $ary = $dbh->selectall_arrayref(q{
SELECT x FROM "/foo/bar/fruit"
});
If you have SQL::Statement installed, you can use table aliases:
my $dbh = DBI->connect('dbi:DBM:');
my $ary = $dbh->selectall_arrayref(q{
SELECT f.x FROM "/foo/bar/fruit" AS f
});
See the "GOTCHAS AND WARNINGS" for using DROP on tables.
Table locking and flock()
Table locking is accomplished using a lockfile which has the same name as the table's file but with
the file extension '.lck' (or a lockfile extension that you suppy, see belwo). This file is created
along with the table during a CREATE and removed during a DROP. Every time the table itself is
opened, the lockfile is flocked(). For SELECT, this is an shared lock. For all other operations, it
is an exclusive lock.
Since the locking depends on flock(), it only works on operating systems that support flock(). In
cases where flock() is not implemented, DBD::DBM will not complain, it will simply behave as if the
flock() had occurred although no actual locking will happen. Read the documentation for flock() if
you need to understand this.
Even on those systems that do support flock(), the locking is only advisory - as is allways the case
with flock(). This means that if some other program tries to access the table while DBD::DBM has the
table locked, that other program will *succeed* at opening the table. DBD::DBM's locking only
applies to DBD::DBM. An exception to this would be the situation in which you use a lockfile with
the other program that has the same name as the lockfile used in DBD::DBM and that program also uses
flock() on that lockfile. In that case, DBD::DBM and your other program will respect each other's
locks.
If you wish to use a lockfile extension other than '.lck', simply specify the dbm_lockfile attribute:
$dbh = DBI->connect('dbi:DBM:lockfile=.foo');
$dbh->{dbm_lockfile} = '.foo';
$dbh->{dbm_tables}->{qux}->{lockfile} = '.foo';
If you wish to disable locking, set the dbm_lockfile equal to 0.
$dbh = DBI->connect('dbi:DBM:lockfile=0');
$dbh->{dbm_lockfile} = 0;
$dbh->{dbm_tables}->{qux}->{lockfile} = 0;
Specifying the DBM type
Each "flavor" of DBM stores its files in a different format and has different capabilities and
different limitations. See AnyDBM_File for a comparison of DBM types.
By default, DBD::DBM uses the SDBM_File type of storage since SDBM_File comes with Perl itself. But
if you have other types of DBM storage available, you can use any of them with DBD::DBM also.
You can specify the DBM type using the "dbm_type" attribute which can be set in the connection string
or with the $dbh->{dbm_type} attribute for global settings or with the
$dbh->{dbm_tables}->{$table_name}->{type} attribute for per-table settings in cases where a single
script is accessing more than one kind of DBM file.
In the connection string, just set type=TYPENAME where TYPENAME is any DBM type such as GDBM_File,
DB_File, etc. Do not use MLDBM as your dbm_type, that is set differently, see below.
my $dbh=DBI->connect('dbi:DBM:'); # uses the default SDBM_File
my $dbh=DBI->connect('dbi:DBM:type=GDBM_File'); # uses the GDBM_File
You can also use $dbh->{dbm_type} to set global DBM type:
$dbh->{dbm_type} = 'GDBM_File'; # set the global DBM type
print $dbh->{dbm_type}; # display the global DBM type
If you are going to have several tables in your script that come from different DBM types, you can
use the $dbh->{dbm_tables} hash to store different settings for the various tables. You can even use
this to perform joins on files that have completely different storage mechanisms.
my $dbh->('dbi:DBM:type=GDBM_File');
#
# sets global default of GDBM_File
my $dbh->{dbm_tables}->{foo}->{type} = 'DB_File';
#
# over-rides the global setting, but only for the table called "foo"
print $dbh->{dbm_tables}->{foo}->{type};
#
# prints the dbm_type for the table "foo"
Adding multi-column support with MLDBM
Most of the DBM types only support two columns. However a CPAN module called MLDBM overcomes this
limitation by allowing more than two columns. It does this by serializing the data - basically it
puts a reference to an array into the second column. It can also put almost any kind of Perl object
or even Perl coderefs into columns.
If you want more than two columns, you must install MLDBM. It's available for many platforms and is
easy to install.
MLDBM can use three different modules to serialize the column - Data::Dumper, Storable, and
FreezeThaw. Data::Dumper is the default, Storable is the fastest. MLDBM can also make use of user-defined userdefined
defined serialization methods. All of this is available to you through DBD::DBM with just one
attribute setting.
To use MLDBM with DBD::DBM, you need to set the dbm_mldbm attribute to the name of the serialization
module.
Some examples:
$dbh=DBI->connect('dbi:DBM:mldbm=Storable'); # use MLDBM with Storable
$dbh=DBI->connect(
'dbi:DBM:mldbm=MySerializer' # use MLDBM with a user defined module
);
$dbh->{dbm_mldbm} = 'MySerializer'; # same as above
print $dbh->{dbm_mldbm} # show the MLDBM serializer
$dbh->{dbm_tables}->{foo}->{mldbm}='Data::Dumper'; # set Data::Dumper for table "foo"
print $dbh->{dbm_tables}->{foo}->{mldbm}; # show serializer for table "foo"
MLDBM works on top of other DBM modules so you can also set a DBM type along with setting dbm_mldbm.
The examples above would default to using SDBM_File with MLDBM. If you wanted GDBM_File instead,
here's how:
$dbh = DBI->connect('dbi:DBM:type=GDBM_File;mldbm=Storable');
#
# uses GDBM_File with MLDBM and Storable
SDBM_File, the default file type is quite limited, so if you are going to use MLDBM, you should
probably use a different type, see AnyDBM_File.
See below for some "GOTCHAS AND WARNINGS" about MLDBM.
Support for Berkeley DB
The Berkeley DB storage type is supported through two different Perl modules - DB_File (which
supports only features in old versions of Berkeley DB) and BerkeleyDB (which supports all versions).
DBD::DBM supports specifying either "DB_File" or "BerkeleyDB" as a dbm_type, with or without MLDBM
support.
The "BerkeleyDB" dbm_type is experimental and its interface is likely to chagne. It currently
defaults to BerkeleyDB::Hash and does not currently support ::Btree or ::Recno.
With BerkeleyDB, you can specify initialization flags by setting them in your script like this:
my $dbh = DBI->connect('dbi:DBM:type=BerkeleyDB;mldbm=Storable');
use BerkeleyDB;
my $env = new BerkeleyDB::Env -Home => $dir; # and/or other Env flags
$dbh->{dbm_berkeley_flags} = {
'DB_CREATE' => DB_CREATE # pass in constants
, 'DB_RDONLY' => DB_RDONLY # pass in constants
, '-Cachesize' => 1000 # set a ::Hash flag
, '-Env' => $env # pass in an environment
};
Do not set the -Flags or -Filename flags, those are determined by the SQL (e.g. -Flags => DB_RDONLY
is set automatically when you issue a SELECT statement).
Time has not permitted me to provide support in this release of DBD::DBM for further Berkeley DB
features such as transactions, concurrency, locking, etc. I will be working on these in the future
and would value suggestions, patches, etc.
See DB_File and BerkeleyDB for further details.
Supported SQL syntax
DBD::DBM uses a subset of SQL. The robustness of that subset depends on what other modules you have
installed. Both options support basic SQL operations including CREATE TABLE, DROP TABLE, INSERT,
DELETE, UPDATE, and SELECT.
Option #1: By default, this module inherits its SQL support from DBI::SQL::Nano that comes with DBI.
Nano is, as its name implies, a *very* small SQL engine. Although limited in scope, it is faster
than option #2 for some operations. See DBI::SQL::Nano for a description of the SQL it supports and
comparisons of it with option #2.
Option #2: If you install the pure Perl CPAN module SQL::Statement, DBD::DBM will use it instead of
Nano. This adds support for table aliases, for functions, for joins, and much more. If you're going
to use DBD::DBM for anything other than very simple tables and queries, you should install
SQL::Statement. You don't have to change DBD::DBM or your scripts in any way, simply installing
SQL::Statement will give you the more robust SQL capabilities without breaking scripts written for
DBI::SQL::Nano. See SQL::Statement for a description of the SQL it supports.
To find out which SQL module is working in a given script, you can use the dbm_versions() method or,
if you don't need the full output and version numbers, just do this:
print $dbh->{sql_handler};
That will print out either "SQL::Statement" or "DBI::SQL::Nano".
Optimizing use of key fields
Most "flavors" of DBM have only two physical columns (but can contain multiple logical columns as
explained below). They work similarly to a Perl hash with the first column serving as the key. Like
a Perl hash, DBM files permit you to do quick lookups by specifying the key and thus avoid looping
through all records. Also like a Perl hash, the keys must be unique. It is impossible to create two
records with the same key. To put this all more simply and in SQL terms, the key column functions as
the PRIMARY KEY.
In DBD::DBM, you can take advantage of the speed of keyed lookups by using a WHERE clause with a
single equal comparison on the key field. For example, the following SQL statements are optimized
for keyed lookup:
CREATE TABLE user ( user_name TEXT, phone TEXT);
INSERT INTO user VALUES ('Fred Bloggs','233-7777');
# ... many more inserts
SELECT phone FROM user WHERE user_name='Fred Bloggs';
The "user_name" column is the key column since it is the first column. The SELECT statement uses the
key column in a single equal comparision - "user_name='Fred Bloggs' - so the search will find it very
quickly without having to loop through however many names were inserted into the table.
In contrast, thes searches on the same table are not optimized:
1. SELECT phone FROM user WHERE user_name < 'Fred';
2. SELECT user_name FROM user WHERE phone = '233-7777';
In #1, the operation uses a less-than (<) comparison rather than an equals comparison, so it will not
be optimized for key searching. In #2, the key field "user_name" is not specified in the WHERE
clause, and therefore the search will need to loop through all rows to find the desired result.
Specifying Column Names
DBM files don't have a standard way to store column names. DBD::DBM gets around this issue with a
DBD::DBM specific way of storing the column names. If you are working only with DBD::DBM and not
using files created by or accessed with other DBM programs, you can ignore this section.
DBD::DBM stores column names as a row in the file with the key _metadata \0. So this code
my $dbh = DBI->connect('dbi:DBM:');
$dbh->do("CREATE TABLE baz (foo CHAR(10), bar INTEGER)");
$dbh->do("INSERT INTO baz (foo,bar) VALUES ('zippy',1)");
Will create a file that has a structure something like this:
_metadata \0 | foo,bar
zippy | 1
The next time you access this table with DBD::DBM, it will treat the _metadata row as a header rather
than as data and will pull the column names from there. However, if you access the file with
something other than DBD::DBM, the row will be treated as a regular data row.
If you do not want the column names stored as a data row in the table you can set the
dbm_store_metadata attribute to 0.
my $dbh = DBI->connect('dbi:DBM:store_metadata=0');
or
$dbh->{dbm_store_metadata} = 0;
or, for per-table setting
$dbh->{dbm_tables}->{qux}->{store_metadata} = 0;
By default, DBD::DBM assumes that you have two columns named "k" and "v" (short for "key" and
"value"). So if you have dbm_store_metadata set to 1 and you want to use alternate column names, you
need to specify the column names like this:
my $dbh = DBI->connect('dbi:DBM:store_metadata=0;cols=foo,bar');
or
$dbh->{dbm_store_metadata} = 0;
$dbh->{dbm_cols} = 'foo,bar';
To set the column names on per-table basis, do this:
$dbh->{dbm_tables}->{qux}->{store_metadata} = 0;
$dbh->{dbm_tables}->{qux}->{cols} = 'foo,bar';
#
# sets the column names only for table "qux"
If you have a file that was created by another DBM program or created with dbm_store_metadata set to
zero and you want to convert it to using DBD::DBM's column name storage, just use one of the methods
above to name the columns but *without* specifying dbm_store_metadata as zero. You only have to do
that once - thereafter you can get by without setting either dbm_store_metadata or setting dbm_cols
because the names will be stored in the file.
Statement handle ($sth) attributes and methods
Most statement handle attributes such as NAME, NUM_OF_FIELDS, etc. are available only after an
execute. The same is true of $sth->rows which is available after the execute but does not require a
fetch.
The $dbh->dbm_versions() method
The private method dbm_versions() presents a summary of what other modules are being used at any
given time. DBD::DBM can work with or without many other modules - it can use either SQL::Statement
or DBI::SQL::Nano as its SQL engine, it can be run with DBI or DBI::PurePerl, it can use many kinds
of DBM modules, and many kinds of serializers when run with MLDBM. The dbm_versions() method reports
on all of that and more.
print $dbh->dbm_versions; # displays global settings
print $dbh->dbm_versions($table_name); # displays per table settings
An important thing to note about this method is that when called with no arguments, it displays the
*global* settings. If you over-ride these by setting per-table attributes, these will not be shown
unless you specifiy a table name as an argument to the method call.
Storing Objects
If you are using MLDBM, you can use DBD::DBM to take advantage of its serializing abilities to
serialize any Perl object that MLDBM can handle. To store objects in columns, you should (but don't
absolutely need to) declare it as a column of type BLOB (the type is *currently* ignored by the SQL
engine, but heh, it's good form).
You *must* use placeholders to insert or refer to the data.
GOTCHAS AND WARNINGS
Using the SQL DROP command will remove any file that has the name specified in the command with
either '.pag' or '.dir' or your {dbm_ext} appended to it. So this be dangerous if you aren't sure
what file it refers to:
$dbh->do(qq{DROP TABLE "/path/to/any/file"});
Each DBM type has limitations. SDBM_File, for example, can only store values of less than 1,000
characters. *You* as the script author must ensure that you don't exceed those bounds. If you try
to insert a value that is bigger than the DBM can store, the results will be unpredictable. See the
documentation for whatever DBM you are using for details.
Different DBM implementations return records in different orders. That means that you can not depend
on the order of records unless you use an ORDER BY statement. DBI::SQL::Nano does not currently
support ORDER BY (though it may soon) so if you need ordering, you'll have to install SQL::Statement.
DBM data files are platform-specific. To move them from one platform to another, you'll need to do
something along the lines of dumping your data to CSV on platform #1 and then dumping from CSV to DBM
on platform #2. DBD::AnyData and DBD::CSV can help with that. There may also be DBM conversion
tools for your platforms which would probably be quickest.
When using MLDBM, there is a very powerful serializer - it will allow you to store Perl code or
objects in database columns. When these get de-serialized, they may be evaled - in other words MLDBM
(or actually Data::Dumper when used by MLDBM) may take the values and try to execute them in Perl.
Obviously, this can present dangers, so if you don't know what's in a file, be careful before you
access it with MLDBM turned on!
See the entire section on "Table locking and flock()" for gotchas and warnings about the use of
flock().
GETTING HELP, MAKING SUGGESTIONS, AND REPORTING BUGS
If you need help installing or using DBD::DBM, please write to the DBI users mailing list at
dbi-users@perl.org or to the comp.lang.perl.modules newsgroup on usenet. I'm afraid I can't always
answer these kinds of questions quickly and there are many on the mailing list or in the newsgroup
who can.
If you have suggestions, ideas for improvements, or bugs to report, please write me directly at the
email shown below.
When reporting bugs, please send the output of $dbh->dbm_versions($table) for a table that exhibits
the bug and, if possible, as small a sample as you can make of the code that produces the bug. And
of course, patches are welcome too :-).
ACKNOWLEDGEMENTS
Many, many thanks to Tim Bunce for prodding me to write this, and for copious, wise, and patient
suggestions all along the way.
AUTHOR AND COPYRIGHT
This module is written and maintained by
Jeff Zucker < jzucker AT cpan.org >
Copyright (c) 2004 by Jeff Zucker, all rights reserved.
You may freely distribute and/or modify this module under the terms of either the GNU General Public
License (GPL) or the Artistic License, as specified in the Perl README file.
SEE ALSO
DBI, SQL::Statement, DBI::SQL::Nano, AnyDBM_File, MLDBM
perl v5.8.8 2005-04-10 DBD::DBM(3)
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