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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: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 useage 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 \_.  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 \_ 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 languanges.

       "MLDBM::Serializer::JSON"
               Additional serializer for MLDBM. JSON is very portable between languanges, 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@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@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@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 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 v5.16.2                                     2010-12-21                                      DBD::DBM(3)