Tie::DBI(3) User Contributed Perl Documentation Tie::DBI(3)NAMETie::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::DBIcommit() 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.
BUGSBUGS
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 ALSOperl(1), DBI(3), Tie::RDBM(3)perl v5.14.1 2010-04-10 Tie::DBI(3)
