NAME

Tie::Hash::DBD - tie a plain hash to a database table

SYNOPSIS

use DBI;
use Tie::Hash::DBD;

my $dbh = DBI->connect ("dbi:Pg:", ...);

tie my %hash, "Tie::Hash::DBD", "dbi:SQLite:dbname=db.tie";
tie my %hash, "Tie::Hash::DBD", $dbh;
tie my %hash, "Tie::Hash::DBD", $dbh, {
    tbl => "t_tie_analysis",
    key => "h_key",
    fld => "h_value",
    str => "Storable",
    trh => 0,
    };

$hash{key} = $value;  # INSERT
$hash{key} = 3;       # UPDATE
delete $hash{key};    # DELETE
$value = $hash{key};  # SELECT
%hash = ();           # CLEAR

DESCRIPTION

This module has been created to act as a drop-in replacement for modules that tie straight perl hashes to disk, like DB_File. When the running system does not have enough memory to hold large hashes, and disk-tieing won't work because there is not enough space, it works quite well to tie the hash to a database, which preferable runs on a different server.

This module ties a hash to a database table using only a key and a value field. If no tables specification is passed, this will create a temporary table with h_key for the key field and a h_value for the value field.

I think it would make sense to merge the functionality that this module provides into Tie::DBI.

tie

The tie call accepts two arguments:

Database

The first argument is the connection specifier. This is either and open database handle or a DBI_DSN string.

If this argument is a valid handle, this module does not open a database all by itself, but uses the connection provided in the handle.

If the first argument is a scalar, it is used as DSN for DBI->connect ().

Supported DBD drivers include DBD::Pg, DBD::SQLite, DBD::CSV, DBD::MariaDB, DBD::mysql, DBD::Oracle, DBD::Unify, and DBD::Firebird. Note that due to limitations they won't all perform equally well. Firebird is not tested anymore.

DBD::Pg and DBD::SQLite have an unexpected great performance when server is the local system. DBD::SQLite is even almost as fast as DB_File.

The current implementation appears to be extremely slow for CSV, as expected, MariaDB/mysql, and Unify. For Unify and MariaDB/mysql that is because these do not allow indexing on the key field so they cannot be set to be primary key.

When using DBD::CSV with Text::CSV_XS version 1.02 or newer, it might be wise to disable utf8 encoding (only supported as of DBD::CSV-0.48):

"dbi:CSV:f_ext=.csv/r;csv_null=1;csv_decode_utf8=0"

Options

The second argument is optional and should - if passed - be a hashref to options. The following options are recognized:

tbl

Defines the name of the table to be used. If none is passed, a new table is created with a unique name like t_tie_dbdh_42253_1. When possible, the table is created as temporary. After the session, this table will be dropped.

If a table name is provided, it will be checked for existence. If found, it will be used with the specified key and fld. Otherwise it will be created with key and fld, but it will not be dropped at the end of the session.

If a table name is provided, AutoCommit will be "On" for persistence, unless you provide a true trh attribute.

key

Defines the name of the key field in the database table. The default is h_key.

ktp

Defines the type of the key field in the database table. The default is depending on the underlying database. Probably unwise to change.

If the database allows the type to be indexed, the key field is defined as primary key.

Note that if your data conflicts with internal (database)limits, like having a key that is longer than what the index on a primary key permits, you should probably want to create the table yourself with a different index or field type.

fld

Defines the name of the value field in the database table. The default is h_value.

vtp

Defines the type of the fld field in the database table. The default is depending on the underlying database and most likely some kind of BLOB.

str

Defines the required persistence module. Currently supports the use of Storable, Sereal, JSON, JSON::Syck, YAML, YAML::Syck and XML::Dumper.

The default is undefined.

Passing any other value will cause a croak.

If you want to preserve Encoding on the hash values, you should use this feature. (except where PV8 has a - in the table below)

Here is a table of supported data types given a data structure like this:

    my %deep = (
	UND => undef,
	IV  => 1,
	NV  => 3.14159265358979,
	PV  => "string",
	PV8 => "ab\ncd\x{20ac}\t",
	PVM => $!,
	RV  => \$DBD,
	AR  => [ 1..2 ],
	HR  => { key => "value" },
	OBJ => ( bless { auto_diag => 1 }, "Text::CSV_XS" ),
	RX  => qr{^re[gG]e?x},
	FMT => *{$::{STDOUT}}{FORMAT},
	CR  => sub { "code"; },
	GLB => *STDERR,
	IO  => *{$::{STDERR}}{IO},
	);

              UND  IV  NV  PV PV8 PVM  RV  AR  HR OBJ  RX FMT  CR GLB  IO
 No streamer   x   x   x   x   x   x   x   x   x   x   -   -   -   -   -
 Storable      x   x   x   x   x   x   x   x   x   x   -   -   -   -   -
 Sereal        x   x   x   x   x   x   x   x   x   x   x   x   -   -   -
 JSON          x   x   x   x   x   x   -   x   x   -   -   -   -   -   -
 JSON::Syck    x   x   x   x   x   -   -   x   x   x   -   x   -   -   -
 YAML          x   x   x   x   x   -   x   x   x   x   x   x   -   -   -
 YAML::Syck    x   x   x   x   x   -   x   x   x   x   -   x   -   -   -
 XML::Dumper   x   x   x   x   x   x   x   x   x   x   -   x   -   -   -
 FreezeThaw    x   x   x   x   -   x   x   x   x   x   -   x   -   x   -
 Bencode       -   x   x   x   -   x   -   x   x   -   -   -   -   x   -

So, Storable does not support persistence of types CODE, REGEXP, FORMAT, IO, and GLOB. Be sure to test if all of your data types are supported by the serializer you choose. YMMV.

"No streamer" might work inside the current process if reference values are stored, but it is highly unlikely they are persistent.

Also note that this module does not yet support dynamic deep structures. See Nesting and deep structures.

trh

Use transaction Handles. By default none of the operations is guarded by transaction handling for speed reasons. Set trh to a true value cause all actions to be surrounded by begin_work and commit. Note that this may have a big impact on speed.

Encoding

Tie::Hash::DBD stores keys and values as binary data. This means that all Encoding and magic is lost when the data is stored, and thus is also not available when the data is restored, hence all internal information about the data is also lost, which includes the UTF8 flag.

If you want to preserve the UTF8 flag you will need to store internal flags and use the streamer option:

tie my %hash, "Tie::Hash::DBD", { str => "Storable" };

If you do not want the performance impact of Storable just to be able to store and retrieve UTF-8 values, there are two ways to do so:

# Use utf-8 from database
tie my %hash, "Tie::Hash::DBD", "dbi:Pg:", { vtp => "text" };
$hash{foo} = "The teddybear costs \x{20ac} 45.95";

# use Encode
tie my %hash, "Tie::Hash::DBD", "dbi:Pg:";
$hash{foo} = encode "UTF-8", "The teddybear costs \x{20ac} 45.95";

Note that using Encode will allow other binary data too where using the database encoding does not:

$hash{foo} = pack "L>A*", time, encode "UTF-8", "Price: \x{20ac} 45.95";

Nesting and deep structures

Tie::Hash::DBD stores keys and values as binary data. This means that all structure is lost when the data is stored and not available when the data is restored. To maintain deep structures, use the streamer option:

tie my %hash, "Tie::Hash::DBD", { str => "Storable" };

Note that changes inside deep structures do not work. See "TODO".

METHODS

drop ()

If a table was used with persistence, the table will not be dropped when the untie is called. Dropping can be forced using the drop method at any moment while the hash is tied:

(tied %hash)->drop;

PREREQUISITES

The only real prerequisite is DBI but of course that uses the DBD driver of your choice. Some drivers are (very) actively maintained. Be sure to to use recent Modules. DBD::SQLite for example seems to require version 1.29 or up.

RESTRICTIONS and LIMITATIONS

As Oracle does not allow BLOB, CLOB or LONG to be indexed or selected on, the keys will be converted to ASCII for Oracle. The maximum length for a converted key in Oracle is 4000 characters. The fact that the key has to be converted to ASCII representation, also excludes undef as a valid key value.

DBD::Oracle limits the size of BLOB-reads to 4kb by default, which is too small for reasonable data structures. Tie::Hash::DBD locally raises this value to 4Mb, which is still an arbitrary limit.
Storable does not support persistence of perl types IO, REGEXP, CODE, FORMAT, and GLOB. Future extensions might implement some alternative streaming modules, like Data::Dump::Streamer or use mixin approaches that enable you to fit in your own.
Note that neither DBD::CSV nor DBD::Unify support AutoCommit.
For now, Firebird does not support TEXT (or CLOB) in DBD::Firebird at a level required by Tie::Hash::DBD. Neither does it support arbitrary length index on VARCHAR fields so it can neither be a primary key nor can it be the subject of a (unique) index hence large sets will be slow.

Firebird support is stalled.

TODO

Update on deep changes

Currently, nested structures do not get updated when it is an change in a deeper part.

tie my %hash, "Tie::Hash::DBD", $dbh, { str => "Storable" };

$hash{deep} = {
    int  => 1,
    str  => "foo",
    };

$hash{deep}{int}++; # No effect :(

Documentation

Better document what the implications are of storing data content in a database and restoring that. It will not be fool proof.

Mixins

Maybe: implement a feature that would enable plugins or mixins to do the streaming or preservation of other data attributes.

AUTHOR

H.Merijn Brand <h.m.brand@xs4all.nl>

COPYRIGHT AND LICENSE

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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