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DBD::DBM(3pm) User Contributed Perl Documentation DBD::DBM(3pm)
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:dbm_type=DB_File'); # defaults to DB_File
$dbh = DBI->connect('dbi:DBM:dbm_mldbm=Storable'); # MLDBM with SDBM_File
# or
$dbh = DBI->connect('dbi:DBM:', undef, undef);
$dbh = DBI->connect('dbi:DBM:', undef, undef, {
f_ext => '.db/r',
f_dir => '/path/to/dbfiles/',
f_lockfile => '.lck',
dbm_type => 'BerkeleyDB',
dbm_mldbm => 'FreezeThaw',
dbm_store_metadata => 1,
dbm_berkeley_flags => {
'-Cachesize' => 1000, # set a ::Hash flag
},
});
and other variations on connect() as shown in the DBI docs, DBD::File metadata and
"Metadata" shown below.
Use standard DBI prepare, execute, fetch, placeholders, etc., see "QUICK START" for an
example.
DESCRIPTION
DBD::DBM is a database management system that works right out of the box. If you have a
standard installation of Perl and DBI you can begin creating, accessing, and modifying
simple database tables without any further modules. You can add other modules (e.g.,
SQL::Statement, DB_File etc) for improved functionality.
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 programming languages using different APIs. That
means, 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 and vice versa. Note that in those
cases it might be necessary to use a common subset of the provided features.
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 which use
different storage formats supported by 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, BerkeleyDB. Each type was tested both with and without MLDBM and with
the Data::Dumper, Storable, FreezeThaw, YAML and JSON serializers using the DBI::SQL::Nano
or the SQL::Statement engines.
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
This section will explain some usage cases in more detail. To get an overview about the
available attributes, see "Metadata".
Specifying 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.
DBD::DBM recognizes following default extensions for following types:
.pag/r
Chosen for dbm_type "SDBM_File", "ODBM_File" and "NDBM_File" when an implementation is
detected which wraps "-ldbm" for "NDBM_File" (e.g. Solaris, AIX, ...).
For those types, the ".dir" extension is recognized, too (for being deleted when
dropping a table).
.db/r
Chosen for dbm_type "NDBM_File" when an implementation is detected which wraps
BerkeleyDB 1.x for "NDBM_File" (typically BSD's, Darwin).
"GDBM_File", "DB_File" and "BerkeleyDB" don't usually use a file extension.
If your DBM type uses an extension other than one of the recognized types of extensions,
you should set the f_ext attribute to the extension and file a bug report as described in
DBI with the name of the implementation and extension so we can add it to DBD::DBM.
Thanks in advance for that :-).
$dbh = DBI->connect('dbi:DBM:f_ext=.db'); # .db extension is used
$dbh = DBI->connect('dbi:DBM:f_ext='); # no extension is used
# or
$dbh->{f_ext}='.db'; # global setting
$dbh->{f_meta}->{'qux'}->{f_ext}='.db'; # setting for table 'qux'
By default files are assumed to be in the current working directory. To use other
directories 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');
# and this will too:
my $dbh = DBI->connect('dbi:DBM:');
$dbh->{f_dir} = '/foo/bar';
# but this is recommended
my $dbh = DBI->connect('dbi:DBM:', undef, undef, { f_dir => '/foo/bar' } );
# now you can do
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"
});
You can also tell DBD::DBM to use a specified path for a specific table:
$dbh->{dbm_tables}->{f}->{file} = q(/foo/bar/fruit);
Please be aware that you cannot specify this during connection.
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 basename as the table's
file but with the file extension '.lck' (or a lockfile extension that you supply, see
below). This lock file is created 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 a shared lock. For all other operations, it is an exclusive lock (except when you
specify something different using the f_lock attribute).
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 simply behave as if the
flock() had occurred although no actual locking will happen. Read the documentation for
flock() for more information.
Even on those systems that do support flock(), locking is only advisory - as is always the
case with flock(). This means that if another program tries to access the table file
while DBD::DBM has the table locked, that other program will *succeed* at opening unless
it is also using flock on the '.lck' file. As a result DBD::DBM's locking only really
applies to other programs using DBD::DBM or other program written to cooperate with
DBD::DBM locking.
Specifying the DBM type
Each "flavor" of DBM stores its files in a different format and has different capabilities
and 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. If you have other types of DBM storage available, you can use any of them
with DBD::DBM. It is strongly recommended to use at least "DB_File", because "SDBM_File"
has quirks and limitations and "ODBM_file", "NDBM_File" and "GDBM_File" are not always
available.
You can specify the DBM type using the dbm_type attribute which can be set in the
connection string or with "$dbh->{dbm_type}" and "$dbh->{f_meta}->{$table_name}->{type}"
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 "dbm_type=TYPENAME" where "TYPENAME" is any DBM type
such as GDBM_File, DB_File, etc. Do not use MLDBM as your dbm_type as that is set
differently, see below.
my $dbh=DBI->connect('dbi:DBM:'); # uses the default SDBM_File
my $dbh=DBI->connect('dbi:DBM:dbm_type=GDBM_File'); # uses the GDBM_File
# You can also use $dbh->{dbm_type} to set the DBM type for the connection:
$dbh->{dbm_type} = 'DB_File'; # set the global DBM type
print $dbh->{dbm_type}; # display the global DBM type
If you have several tables in your script that use 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.
# sets global default of GDBM_File
my $dbh->('dbi:DBM:type=GDBM_File');
# overrides the global setting, but only for the tables called
# I<foo> and I<bar>
my $dbh->{f_meta}->{foo}->{dbm_type} = 'DB_File';
my $dbh->{f_meta}->{bar}->{dbm_type} = 'BerkeleyDB';
# prints the dbm_type for the table "foo"
print $dbh->{f_meta}->{foo}->{dbm_type};
Note that you must change the dbm_type of a table before you access it for first time.
Adding multi-column support with MLDBM
Most of the DBM types only support two columns and even if it would support more, DBD::DBM
would only use two. However a CPAN module called MLDBM overcomes this limitation by
allowing more than two columns. MLDBM 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 is by default distributed with three serializers - Data::Dumper, Storable, and
FreezeThaw. Data::Dumper is the default and Storable is the fastest. MLDBM can also make
use of user-defined serialization methods or other serialization modules (e.g. YAML::MLDBM
or MLDBM::Serializer::JSON. You select the serializer using the dbm_mldbm attribute.
Some examples:
$dbh=DBI->connect('dbi:DBM:dbm_mldbm=Storable'); # use MLDBM with Storable
$dbh=DBI->connect(
'dbi:DBM:dbm_mldbm=MySerializer' # use MLDBM with a user defined module
);
$dbh=DBI->connect('dbi::dbm:', undef,
undef, { dbm_mldbm => 'YAML' }); # use 3rd party serializer
$dbh->{dbm_mldbm} = 'YAML'; # same as above
print $dbh->{dbm_mldbm} # show the MLDBM serializer
$dbh->{f_meta}->{foo}->{dbm_mldbm}='Data::Dumper'; # set Data::Dumper for table "foo"
print $dbh->{f_meta}->{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:
# uses DB_File with MLDBM and Storable
$dbh = DBI->connect('dbi:DBM:', undef, undef, {
dbm_type => 'DB_File',
dbm_mldbm => 'Storable',
});
SDBM_File, the default dbm_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 it's interface is likely to change. 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:
use BerkeleyDB;
my $env = new BerkeleyDB::Env -Home => $dir; # and/or other Env flags
$dbh = DBI->connect('dbi:DBM:', undef, undef, {
dbm_type => 'BerkeleyDB',
dbm_mldbm => 'Storable',
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 as those are determined and overwritten by the
SQL (e.g. -Flags => DB_RDONLY is set automatically when you issue a SELECT statement).
Time has not permitted us to provide support in this release of DBD::DBM for further
Berkeley DB features such as transactions, concurrency, locking, etc. We will be working
on these in the future and would value suggestions, patches, etc.
See DB_File and BerkeleyDB for further details.
Optimizing the use of key fields
Most "flavors" of DBM have only two physical columns (but can contain multiple logical
columns as explained above in "Adding multi-column support with MLDBM"). 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 (supported by DBI::SQL::Nano only). Also like a Perl hash, the keys must be
unique. It is impossible to create two records with the same key. To put this more simply
and in SQL terms, the key column functions as the PRIMARY KEY or UNIQUE INDEX.
In DBD::DBM, you can take advantage of the speed of keyed lookups by using DBI::SQL::Nano
and 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 comparison - "user_name='Fred Bloggs'" -
so the search will find it very quickly without having to loop through all the names which
were inserted into the table.
In contrast, these 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 requested row(s).
Note that the underlying DBM storage needs to loop over all key/value pairs when the
optimized fetch is used. SQL::Statement has a massively improved where clause evaluation
which costs around 15% of the evaluation in DBI::SQL::Nano - combined with the loop in the
DBM storage the speed improvement isn't so impressive.
Even if lookups are faster by around 50%, DBI::SQL::Nano and SQL::Statement can benefit
from the key field optimizations on updating and deleting rows - and here the improved
where clause evaluation of SQL::Statement might beat DBI::SQL::Nano every time the where
clause contains not only the key field (or more than one).
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 (especially single primary key
lookups). 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, functions, 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}, "\n";
That will print out either "SQL::Statement" or "DBI::SQL::Nano".
Baring the section about optimized access to the DBM storage in mind, comparing the
benefits of both engines:
# DBI::SQL::Nano is faster
$sth = $dbh->prepare( "update foo set value='new' where key=15" );
$sth->execute();
$sth = $dbh->prepare( "delete from foo where key=27" );
$sth->execute();
$sth = $dbh->prepare( "select * from foo where key='abc'" );
# SQL::Statement might faster (depending on DB size)
$sth = $dbh->prepare( "update foo set value='new' where key=?" );
$sth->execute(15);
$sth = $dbh->prepare( "update foo set value=? where key=15" );
$sth->execute('new');
$sth = $dbh->prepare( "delete from foo where key=?" );
$sth->execute(27);
# SQL::Statement is faster
$sth = $dbh->prepare( "update foo set value='new' where value='old'" );
$sth->execute();
# must be expressed using "where key = 15 or key = 27 or key = 42 or key = 'abc'"
# in DBI::SQL::Nano
$sth = $dbh->prepare( "delete from foo where key in (15,27,42,'abc')" );
$sth->execute();
# must be expressed using "where key > 10 and key < 90" in DBI::SQL::Nano
$sth = $dbh->prepare( "select * from foo where key between (10,90)" );
$sth->execute();
# only SQL::Statement can handle
$sth->prepare( "select * from foo,bar where foo.name = bar.name" );
$sth->execute();
$sth->prepare( "insert into foo values ( 1, 'foo' ), ( 2, 'bar' )" );
$sth->execute();
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 | <dbd_metadata><schema></schema><col_names>foo,bar</col_names></dbd_metadata>
zippy | 1
The next time you access this table with DBD::DBM, it will treat the _metadata \0 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:', undef, undef, { dbm_store_metadata => 0 });
# or
$dbh->{dbm_store_metadata} = 0;
# or for per-table setting
$dbh->{f_meta}->{qux}->{dbm_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:', undef, undef, {
dbm_store_metadata => 0,
dbm_cols => [ qw(foo bar) ],
});
# or
$dbh->{dbm_store_metadata} = 0;
$dbh->{dbm_cols} = 'foo,bar';
# or to set the column names on per-table basis, do this:
# sets the column names only for table "qux"
$dbh->{f_meta}->{qux}->{dbm_store_metadata} = 0;
$dbh->{f_meta}->{qux}->{col_names} = [qw(foo bar)];
If you have a file that was created by another DBM program or created with
dbm_store_metadata set to zero and you want to convert it to using DBD::DBM's column name
storage, just use one of the methods above to name the columns but *without* specifying
dbm_store_metadata as zero. You only have to do that once - thereafter you can get by
without setting either dbm_store_metadata or setting dbm_cols because the names will be
stored in the file.
DBI database handle attributes
Metadata
Statement handle ($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 $sth->rows which is available after the execute but
does not require a fetch.
Driver handle ($dbh) attributes
It is not supported anymore to use dbm-attributes without the dbm_-prefix. Currently, if
an DBD::DBM private attribute is accessed without an underscore in it's name, dbm_ is
prepended to that attribute and it's processed further. If the resulting attribute name is
invalid, an error is thrown.
dbm_cols
Contains a comma separated list of column names or an array reference to the column names.
dbm_type
Contains the DBM storage type. Currently known supported type are "ODBM_File",
"NDBM_File", "SDBM_File", "GDBM_File", "DB_File" and "BerkeleyDB". It is not recommended
to use one of the first three types - even if "SDBM_File" is the most commonly available
dbm_type.
dbm_mldbm
Contains the serializer for DBM storage (value column). Requires the CPAN module MLDBM
installed. Currently known supported serializers are:
Data::Dumper
Default serializer. Deployed with Perl core.
Storable
Faster serializer. Deployed with Perl core.
FreezeThaw
Pure Perl serializer, requires FreezeThaw to be installed.
YAML Portable serializer (between languages but not architectures). Requires
YAML::MLDBM installation.
JSON Portable, fast serializer (between languages but not architectures). Requires
MLDBM::Serializer::JSON installation.
dbm_store_metadata
Boolean value which determines if the metadata in DBM is stored or not.
dbm_berkeley_flags
Hash reference with additional flags for BerkeleyDB::Hash instantiation.
dbm_version
Readonly attribute containing the version of DBD::DBM.
f_meta
In addition to the attributes DBD::File recognizes, DBD::DBM knows about the (public)
attributes "col_names" (Note not dbm_cols here!), "dbm_type", "dbm_mldbm",
"dbm_store_metadata" and "dbm_berkeley_flags". As in DBD::File, there are undocumented,
internal attributes in DBD::DBM. Be very careful when modifying attributes you do not
know; the consequence might a destroyed or corrupted table.
dbm_tables
This attribute provides restricted access to the table meta data. See f_meta and "f_meta"
in DBD::File for attribute details.
dbm_tables is a tied hash providing the internal table names as keys (accessing unknown
tables might create an entry) and their meta data as another tied hash. The table meta
storage is obtained via the "get_table_meta" method from the table implementation (see
DBD::File::Developers). Attribute setting and getting within the table meta data is
handled via the methods "set_table_meta_attr" and "get_table_meta_attr".
Following attributes are no longer handled by DBD::DBM:
dbm_ext
This attribute is silently mapped to DBD::File's attribute f_ext. Later versions of DBI
might show a depreciated warning when this attribute is used and eventually it will be
removed.
dbm_lockfile
This attribute is silently mapped to DBD::File's attribute f_lockfile. Later versions of
DBI might show a depreciated warning when this attribute is used and eventually it will be
removed.
DBI database handle methods
The $dbh->dbm_versions() method
The private method dbm_versions() returns 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 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 it called with no arguments, it
displays the *global* settings. If you override these by setting per-table attributes,
these will not be shown unless you specify 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 it's good form).
EXTENSIBILITY
"SQL::Statement"
Improved SQL engine compared to the built-in DBI::SQL::Nano - see "Supported SQL
syntax".
"DB_File"
Berkeley DB version 1. This database library is available on many systems without
additional installation and most systems are supported.
"GDBM_File"
Simple dbm type (comparable to "DB_File") under the GNU license. Typically not
available (or requires extra installation) on non-GNU operating systems.
"BerkeleyDB"
Berkeley DB version up to v4 (and maybe higher) - requires additional installation
but is easier than GDBM_File on non-GNU systems.
db4 comes with a many tools which allow repairing and migrating databases. This
is the recommended dbm type for production use.
"MLDBM" Serializer wrapper to support more than one column for the files. Comes with
serializers using "Data::Dumper", "FreezeThaw" and "Storable".
"YAML::MLDBM"
Additional serializer for MLDBM. YAML is very portable between languages.
"MLDBM::Serializer::JSON"
Additional serializer for MLDBM. JSON is very portable between languages, probably
more than YAML.
GOTCHAS AND WARNINGS
Using the SQL DROP command will remove any file that has the name specified in the command
with either '.pag' and '.dir', '.db' or your {f_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 larger than 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
should not rely on the order of records unless you use an ORDER BY 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 quicker.
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 eval'ed -
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 do not know
what is 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().
BUGS AND LIMITATIONS
This module uses hash interfaces of two column file databases. While none of supported SQL
engines have support for indices, the following statements really do the same (even if
they mean something completely different) for each dbm type which lacks "EXISTS" support:
$sth->do( "insert into foo values (1, 'hello')" );
# this statement does ...
$sth->do( "update foo set v='world' where k=1" );
# ... the same as this statement
$sth->do( "insert into foo values (1, 'world')" );
This is considered to be a bug and might change in a future release.
Known affected dbm types are "ODBM_File" and "NDBM_File". We highly recommended you use a
more modern dbm type such as "DB_File".
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 AT perl.org or to the comp.lang.perl.modules newsgroup on usenet. I cannot
always answer every question quickly but there are many on the mailing list or in the
newsgroup who can.
DBD developers for DBD's which rely on DBD::File or DBD::DBM or use one of them as an
example are suggested to join the DBI developers mailing list at dbi-dev AT perl.org and
strongly encouraged to join our IRC channel at <irc://irc.perl.org/dbi>.
If you have suggestions, ideas for improvements, or bugs to report, please report a bug as
described in DBI. Do not mail any of the authors directly, you might not get an answer.
When reporting bugs, please send the output of $dbh->dbm_versions($table) for a table that
exhibits the bug and as small a sample as you can make of the code that produces the bug.
And of course, patches are welcome, too :-).
If you need enhancements quickly, you can get commercial support as described at
<http://dbi.perl.org/support/> or you can contact Jens Rehsack at rehsack AT cpan.org for
commercial support in Germany.
Please don't bother Jochen Wiedmann or Jeff Zucker for support - they handed over further
maintenance to H.Merijn Brand and Jens Rehsack.
ACKNOWLEDGEMENTS
Many, many thanks to Tim Bunce for prodding me to write this, and for copious, wise, and
patient suggestions all along the way. (Jeff Zucker)
I send my thanks and acknowledgements to H.Merijn Brand for his initial refactoring of
DBD::File and his strong and ongoing support of SQL::Statement. Without him, the current
progress would never have been made. And I have to name Martin J. Evans for each laugh
(and correction) of all those funny word creations I (as non-native speaker) made to the
documentation. And - of course - I have to thank all those unnamed contributors and
testers from the Perl community. (Jens Rehsack)
AUTHOR AND COPYRIGHT
This module is written by Jeff Zucker < jzucker AT cpan.org >, who also maintained it till
2007. After that, in 2010, Jens Rehsack & H.Merijn Brand took over maintenance.
Copyright (c) 2004 by Jeff Zucker, all rights reserved.
Copyright (c) 2010-2013 by Jens Rehsack & H.Merijn Brand, 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, DB_File, BerkeleyDB, MLDBM, YAML::MLDBM,
MLDBM::Serializer::JSON
perl v5.34.0 2022-02-06 DBD::DBM(3pm)
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