Provided by: libtie-dbi-perl_1.08-2_all 
NAME
Tie::DBI - Tie hashes to DBI relational databases
SYNOPSIS
use Tie::DBI;
tie %h,'Tie::DBI','mysql:test','test','id',{CLOBBER=>1};
tie %h,'Tie::DBI',{db => 'mysql:test',
table => 'test',
key => 'id',
user => 'nobody',
password => 'ghost',
CLOBBER => 1};
# fetching keys and values
@keys = keys %h;
@fields = keys %{$h{$keys[0]}};
print $h{'id1'}->{'field1'};
while (($key,$value) = each %h) {
print "Key = $key:\n";
foreach (sort keys %$value) {
print "\t$_ => $value->{$_}\n";
}
}
# changing data
$h{'id1'}->{'field1'} = 'new value';
$h{'id1'} = { field1 => 'newer value',
field2 => 'even newer value',
field3 => "so new it's squeaky clean" };
# other functions
tied(%h)->commit;
tied(%h)->rollback;
tied(%h)->select_where('price > 1.20');
@fieldnames = tied(%h)->fields;
$dbh = tied(%h)->dbh;
DESCRIPTION
This module allows you to tie Perl associative arrays (hashes) to SQL databases using the
DBI interface. The tied hash is associated with a table in a local or networked database.
One column becomes the hash key. Each row of the table becomes an associative array, from
which individual fields can be set or retrieved.
USING THE MODULE
To use this module, you must have the DBI interface and at least one DBD (database driver)
installed. Make sure that your database is up and running, and that you can connect to it
and execute queries using DBI.
Creating the tie
tie %var,'Tie::DBI',[database,table,keycolumn] [,\%options]
Tie a variable to a database by providing the variable name, the tie interface (always
"Tie::DBI"), the data source name, the table to tie to, and the column to use as the hash
key. You may also pass various flags to the interface in an associative array.
database
The database may either be a valid DBI-style data source string of the form
"dbi:driver:database_name[:other information]", or a database handle that has
previously been opened. See the documentation for DBI and your DBD driver for
details. Because the initial "dbi" is always present in the data source, Tie::DBI
will add it for you if necessary.
Note that some drivers (Oracle in particular) have an irritating habit of appending
blanks to the end of fixed-length fields. This will screw up Tie::DBI's routines for
getting key names. To avoid this you should create the database handle with a
ChopBlanks option of TRUE. You should also use a PrintError option of true to avoid
complaints during STORE and LISTFIELD calls.
table
The table in the database to bind to. The table must previously have been created
with a SQL CREATE statement. This module will not create tables for you or modify the
schema of the database.
key The column to use as the hash key. This column must prevoiusly have been defined when
the table was created. In order for this module to work correctly, the key column
must be declared unique and not nullable. For best performance, the column should be
also be declared a key. These three requirements are automatically satisfied for
primary keys.
It is possible to omit the database, table and keycolumn arguments, in which case the
module tries to retrieve the values from the options array. The options array contains a
set of option/value pairs. If not provided, defaults are assumed. The options are:
user
Account name to use for database authentication, if necessary. Default is an empty
string (no authentication necessary).
password
Password to use for database authentication, if necessary. Default is an empty string
(no authentication necessary).
db The database to bind to the hash, if not provided in the argument list. It may be a
DBI-style data source string, or a previously-opened database handle.
table
The name of the table to bind to the hash, if not provided in the argument list.
key The name of the column to use as the hash key, if not provided in the argument list.
CLOBBER (default 0)
This controls whether the database is writable via the bound hash. A zero value (the
default) makes the database essentially read only. An attempt to store to the hash
will result in a fatal error. A CLOBBER value of 1 will allow you to change
individual fields in the database, and to insert new records, but not to delete entire
records. A CLOBBER value of 2 allows you to delete records, but not to erase the
entire table. A CLOBBER value of 3 or higher will allow you to erase the entire
table.
Operation Clobber Comment
$i = $h{strawberries}->{price} 0 All read operations
$h{strawberries}->{price} += 5 1 Update fields
$h{bananas}={price=>23,quant=>3} 1 Add records
delete $h{strawberries} 2 Delete records
%h = () 3 Clear entire table
undef %h 3 Another clear operation
All database operations are contingent upon your access privileges. If your account
does not have write permission to the database, hash store operations will fail
despite the setting of CLOBBER.
AUTOCOMMIT (default 1)
If set to a true value, the "autocommit" option causes the database driver to commit
after every store statement. If set to a false value, this option will not commit to
the database until you explicitly call the Tie::DBI commit() method.
The autocommit option defaults to true.
DEBUG (default 0)
When the DEBUG option is set to a non-zero value the module will echo the contents of
SQL statements and other debugging information to standard error. Higher values of
DEBUG result in more verbose (and annoying) output.
WARN (default 1)
If set to a non-zero value, warns of illegal operations, such as attempting to delete
the value of the key column. If set to a zero value, these errors will be ignored
silently.
CASESENSITIV (default 0)
If set to a non-zero value, all Fieldnames are casesensitiv. Keep in mind, that your
database has to support casesensitiv Fields if you want to use it.
USING THE TIED ARRAY
The tied array represents the database table. Each entry in the hash is a record, keyed
on the column chosen in the tie() statement. Ordinarily this will be the table's primary
key, although any unique column will do.
Fetching an individual record returns a reference to a hash of field names and values.
This hash reference is itself a tied object, so that operations on it directly affect the
database.
Fetching information
In the following examples, we will assume a database table structured like this one:
-produce-
produce_id price quantity description
strawberries 1.20 8 Fresh Maine strawberries
apricots 0.85 2 Ripe Norwegian apricots
bananas 1.30 28 Sweet Alaskan bananas
kiwis 1.50 9 Juicy New York kiwi fruits
eggs 1.00 12 Farm-fresh Atlantic eggs
We tie the variable %produce to the table in this way:
tie %produce,'Tie::DBI',{db => 'mysql:stock',
table => 'produce',
key => 'produce_id',
CLOBBER => 2 # allow most updates
};
We can get the list of keys this way:
print join(",",keys %produce);
=> strawberries,apricots,bananas,kiwis
Or get the price of eggs thusly:
$price = $produce{eggs}->{price};
print "The price of eggs = $price";
=> The price of eggs = 1.2
String interpolation works as you would expect:
print "The price of eggs is still $produce{eggs}->{price}"
=> The price of eggs is still 1.2
Various types of syntactic sugar are allowed. For example, you can refer to
$produce{eggs}{price} rather than $produce{eggs}->{price}. Array slices are fully
supported as well:
($apricots,$kiwis) = @produce{apricots,kiwis};
print "Kiwis are $kiwis->{description};
=> Kiwis are Juicy New York kiwi fruits
($price,$description) = @{$produce{eggs}}{price,description};
=> (2.4,'Farm-fresh Atlantic eggs')
If you provide the tied hash with a comma-delimited set of record names, and you are not
requesting an array slice, then the module does something interesting. It generates a
single SQL statement that fetches the records from the database in a single pass (rather
than the multiple passes required for an array slice) and returns the result as a
reference to an array. For many records, this can be much faster. For example:
$result = $produce{apricots,bananas};
=> ARRAY(0x828a8ac)
($apricots,$bananas) = @$result;
print "The price of apricots is $apricots->{price}";
=> The price of apricots is 0.85
Field names work in much the same way:
($price,$quantity) = @{$produce{apricots}{price,quantity}};
print "There are $quantity apricots at $price each";
=> There are 2 apricots at 0.85 each";
Note that this takes advantage of a bit of Perl syntactic sugar which automagically treats
$h{'a','b','c'} as if the keys were packed together with the $; pack character. Be
careful not to fall into this trap:
$result = $h{join( ',', 'apricots', 'bananas' )};
=> undefined
What you really want is this:
$result = $h{join( $;, 'apricots', 'bananas' )};
=> ARRAY(0x828a8ac)
Updating information
If CLOBBER is set to a non-zero value (and the underlying database privileges allow it),
you can update the database with new values. You can operate on entire records at once or
on individual fields within a record.
To insert a new record or update an existing one, assign a hash reference to the record.
For example, you can create a new record in %produce with the key "avocados" in this
manner:
$produce{avocados} = { price => 2.00,
quantity => 8,
description => 'Choice Irish avocados' };
This will work with any type of hash reference, including records extracted from another
table or database.
Only keys that correspond to valid fields in the table will be accepted. You will be
warned if you attempt to set a field that doesn't exist, but the other fields will be
correctly set. Likewise, you will be warned if you attempt to set the key field. These
warnings can be turned off by setting the WARN option to a zero value. It is not
currently possible to add new columns to the table. You must do this manually with the
appropriate SQL commands.
The same syntax can be used to update an existing record. The fields given in the hash
reference replace those in the record. Fields that aren't explicitly listed in the hash
retain their previous values. In the following example, the price and quantity of the
"kiwis" record are updated, but the description remains the same:
$produce{kiwis} = { price=>1.25,quantity=>20 };
You may update existing records on a field-by-field manner in the natural way:
$produce{eggs}{price} = 1.30;
$produce{eggs}{price} *= 2;
print "The price of eggs is now $produce{eggs}{price}";
=> The price of eggs is now 2.6.
Obligingly enough, you can use this syntax to insert new records too, as in
$produce{mangoes}{description}="Sun-ripened Idaho mangoes". However, this type of update
is inefficient because a separate SQL statement is generated for each field. If you need
to update more than one field at a time, use the record-oriented syntax shown earlier.
It's much more efficient because it gets the work done with a single SQL command.
Insertions and updates may fail for any of a number of reasons, most commonly:
1. You do not have sufficient privileges to update the database
2. The update would violate an integrity constraint, such as making a non-nullable field
null, overflowing a numeric field, storing a string value in a numeric field, or violating
a uniqueness constraint.
The module dies with an error message when it encounters an error during an update. To
trap these erorrs and continue processing, wrap the update an eval().
Other functions
The tie object supports several useful methods. In order to call these methods, you must
either save the function result from the tie() call (which returns the object), or call
tied() on the tie variable to recover the object.
connect(), error(), errstr()
These are low-level class methods. Connect() is responsible for establishing the
connection with the DBI database. Errstr() and error() return $DBI::errstr and
$DBI::error respectively. You may may override these methods in subclasses if you
wish. For example, replace connect() with this code in order to use persistent
database connections in Apache modules:
use Apache::DBI; # somewhere in the declarations
sub connect {
my ($class,$dsn,$user,$password,$options) = @_;
return Apache::DBI->connect($dsn,$user,
$password,$options);
}
commit()
(tied %produce)->commit();
When using a database with the autocommit option turned off, values that are stored
into the hash will not become permanent until commit() is called. Otherwise they are
lost when the application terminates or the hash is untied.
Some SQL databases don't support transactions, in which case you will see a warning
message if you attempt to use this function.
rollback()
(tied %produce)->rollback();
When using a database with the autocommit option turned off, this function will roll
back changes to the database to the state they were in at the last commit(). This
function has no effect on database that don't support transactions.
select_where()
@keys=(tied %produce)->select_where('price > 1.00 and quantity < 10');
This executes a limited form of select statement on the tied table and returns a list
of records that satisfy the conditions. The argument you provide should be the
contents of a SQL WHERE clause, minus the keyword "WHERE" and everything that
ordinarily precedes it. Anything that is legal in the WHERE clause is allowed,
including function calls, ordering specifications, and sub-selects. The keys to those
records that meet the specified conditions are returned as an array, in the order in
which the select statement returned them.
Don't expect too much from this function. If you want to execute a complex query,
you're better off using the database handle (see below) to make the SQL query yourself
with the DBI interface.
dbh()
$dbh = (tied %produce)->dbh();
This returns the tied hash's underlying database handle. You can use this handle to
create and execute your own SQL queries.
CLOBBER, DEBUG, WARN
You can get and set the values of CLOBBER, DEBUG and WARN by directly accessing the
object's hash:
(tied %produce)->{DEBUG}++;
This lets you change the behavior of the tied hash on the fly, such as temporarily
granting your program write permission.
There are other variables there too, such as the name of the key column and database
table. Change them at your own risk!
PERFORMANCE
What is the performance hit when you use this module rather than the direct DBI interface?
It can be significant. To measure the overhead, I used a simple benchmark in which Perl
parsed a 6180 word text file into individual words and stored them into a database,
incrementing the word count with each store. The benchmark then read out the words and
their counts in an each() loop. The database driver was mySQL, running on a 133 MHz
Pentium laptop with Linux 2.0.30. I compared Tie::RDBM, to DB_File, and to the same task
using vanilla DBI SQL statements. The results are shown below:
UPDATE FETCH
Tie::DBI 70 s 6.1 s
Vanilla DBI 14 s 2.0 s
DB_File 3 s 1.06 s
There is about a five-fold penalty for updates, and a three-fold penalty for fetches when
using this interface. Some of the penalty is due to the overhead for creating sub-objects
to handle individual fields, and some of it is due to the inefficient way the store and
fetch operations are implemented. For example, using the tie interface, a statement like
$h{record}{field}++ requires as much as four trips to the database: one to verify that the
record exists, one to fetch the field, and one to store the incremented field back. If
the record doesn't already exist, an additional statement is required to perform the
insertion. I have experimented with cacheing schemes to reduce the number of trips to the
database, but the overhead of maintaining the cache is nearly equal to the performance
improvement, and cacheing raises a number of potential concurrency problems.
Clearly you would not want to use this interface for applications that require a large
number of updates to be processed rapidly.
BUGS
BUGS
The each() call produces a fatal error when used with the Sybase driver to access
Microsoft SQL server. This is because this server only allows one query to be active at a
given time. A workaround is to use keys() to fetch all the keys yourself. It is not
known whether real Sybase databases suffer from the same problem.
The delete() operator will not work correctly for setting field values to null with
DBD::CSV or with DBD::Pg. CSV files do not have a good conception of database nulls.
Instead you will set the field to an empty string. DBD::Pg just seems to be broken in
this regard.
AUTHOR
Lincoln Stein, lstein@cshl.org
COPYRIGHT
Copyright (c) 1998, Lincoln D. Stein
This library is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.
AVAILABILITY
The latest version can be obtained from:
http://www.genome.wi.mit.edu/~lstein/Tie-DBI/
SEE ALSO
perl(1), DBI(3), Tie::RDBM(3)