Tie::Hash::DBD, tie a plain hash to a database table
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, }; $hash{key} = $value; # INSERT $hash{key} = 3; # UPDATE delete $hash{key}; # DELETE $value = $hash{key}; # SELECT %hash = (); # CLEAR
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.
DB_File
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.
key
value
h_key
h_value
I think it would make sense to merge the functionality that this module provides into Tie::DBI.
Tie::DBI
The tie call accepts two arguments:
The first argument is the connection specifier. This is either and open database handle or a DBI_DSN string.
DBI_DSN
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::mysql, DBD::Oracle, and DBD::Unify.
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 CSV, as expected, mysql, and Unify. For Unify and mysql that is because these do not allow indexing on the key field so they cannot be set to be primary key.
The second argument is optional and should - if passed - be a hashref to options. The following options are recognized:
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.
t_tie_dbdh_42253_1
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.
fld
Defines the name of the key field in the database table. The default is h_key.
Defines the name of the value field in the database table. The default is h_value.
Defines the required persistence module. Currently only supports the use of Storable. The default is undefined.
Storable
Note that Storable does not support persistence of perl types CODE, REGEXP, IO, FORMAT, and GLOB.
CODE
REGEXP
IO
FORMAT
GLOB
If you want to preserve Encoding on the hash values, you should use this feature.
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.
Tie::Hash::DBD
UTF8
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" };
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:
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:
untie
drop
(tied %hash)->drop;
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.
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.
undef
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.
DBD::Oracle
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.
Data::Dump::Streamer
Better document what the implications are of storing data content in a database and restoring that. It will not be fool proof.
Maybe: implement a feature that would enable plugins or mixins to do the streaming or preservation of other data attributes.
H.Merijn Brand <h.m.brand@xs4all.nl>
Copyright (C) 2010-2010 H.Merijn Brand
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
DBI, Tie::DBI, Tie::Hash
To install Tie::Hash::DBD, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Tie::Hash::DBD
CPAN shell
perl -MCPAN -e shell install Tie::Hash::DBD
For more information on module installation, please visit the detailed CPAN module installation guide.