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DBI-FAQ-0.36
River stage zero No dependents
/ DBI::FAQ

NAME

DBI::FAQ -- The Frequently Asked Questions for the Perl5 Database Interface

DBI Frequently Asked Questions v.0.36
Last updated: October 15th, 1998

SYNOPSIS

perldoc DBI::FAQ

VERSION

This document is currently at version 0.36, as of October 15th, 1998.

DESCRIPTION

This document serves to answer the most frequently asked questions on both the DBI Mailing Lists and personally to members of the DBI development team.

Basic Information & Information Sources

1.1 What is DBI, DBperl, Oraperl and *perl?

To quote Tim Bunce, the architect and author of DBI:

``DBI is a database access Application Programming Interface (API)
  for the Perl Language. The DBI API Specification defines a set
  of functions, variables and conventions that provide a consistent
  database interface independant of the actual database being used.''

In simple language, the DBI interface allows users to access multiple database types transparently. So, if you connecting to an Oracle, Informix, mSQL, Sybase or whatever database, you don't need to know the underlying mechanics of the 3GL layer. The API defined by DBI will work on all these database types.

A similar benefit is gained by the ability to connect to two different databases of different vendor within the one perl script, ie, I want to read data from an Oracle database and insert it back into an Informix database all within one program. The DBI layer allows you to do this simply and powerfully.

Here's a diagram that demonstrates the principle:

[ DBI Architecture ]

DBperl is the old name for the interface specification. It's usually now used to denote perl4 modules on database interfacing, such as, oraperl, isqlperl, ingperl and so on. These interfaces didn't have a standard API and are generally not supported.

Here's a list of DBperl modules, their corresponding DBI counterparts and support information. Please note, the author's listed here generally do not maintain the DBI module for the same database. These email addresses are unverified and should only be used for queries concerning the perl4 modules listed below. DBI driver queries should be directed to the dbi-users mailing list.

Module Name	Database Required   Author          DBI
-----------	-----------------   ------          ---
Sybperl     Sybase              Michael Peppler DBD::Sybase
                                <mpeppler@itf.ch>
Oraperl     Oracle 6 & 7        Kevin Stock     DBD::Oracle
                                <dbi-users@fugue.com>
Ingperl     Ingres              Tim Bunce &     DBD::Ingres
                                Ted Lemon
                                <dbi-users@fugue.com>
Interperl   Interbase           Buzz Moschetti  DBD::Interbase
                                <buzz@bear.com>
Uniperl     Unify 5.0           Rick Wargo      None
                                <rickers@coe.drexel.edu>
Pgperl      Postgres            Igor Metz       DBD::Pg
                                <metz@iam.unibe.ch>
Btreeperl   NDBM                John Conover    SDBM?
                                <john@johncon.com>
Ctreeperl   C-Tree              John Conover    None
                                <john@johncon.com>
Cisamperl   Informix C-ISAM     Mathias Koerber None
                                <mathias@unicorn.swi.com.sg>
Duaperl     X.500 Directory     Eric Douglas    None
            User Agent

However, some DBI modules have DBperl emulation layers, so, DBD::Oracle comes with an Oraperl emulation layer, which allows you to run legacy oraperl scripts without modification. The emulation layer translates the oraperl API calls into DBI calls and executes them through the DBI switch.

Here's a table of emulation layer information:

Module		    Emulation Layer     Status
------          ---------------     ------
DBD::Oracle     Oraperl             Complete
DBD::Informix   Isqlperl            Under development
DBD::Sybase     Sybperl             Working? ( Needs verification )
DBD::mSQL       Msqlperl            Experimentally released with 
                                    DBD::mSQL-0.61

The Msqlperl emulation is a special case. Msqlperl is a perl5 driver for mSQL databases, but does not conform to the DBI Specification. It's use is being deprecated in favour of DBD::mSQL. Msqlperl may be downloaded from CPAN via:

http://www.perl.com/cgi-bin/cpan_mod?module=Msqlperl

1.2. Where can I get it from?

DBI is primarily distributed from:

ftp://ftp.demon.co.uk/pub/perl/db

The Comprehensive Perl Archive Network resources should be used for retrieving up-to-date versions of the drivers, since local mirror sites usually lag. CPAN may be accessed via Tom Christiansen's splendid CPAN multiplexer program located at:

http://www.perl.com/CPAN/

For more specific version information and exact URLs of drivers, please see the DBI drivers list and the DBI module pages which can be found on:

http://www.arcana.co.uk/technologia/perl/DBI

1.3. Where can I get more information?

There are a few information sources on DBI.

POD documentation

PODs are chunks of documentation usually embedded within perl programs that document the code ``in place'', providing a useful resource for programmers and users of modules. POD for DBI and drivers is beginning to become more commonplace, and documentation for these modules can be read with the following commands.

The DBI Specification

The POD for the DBI Specification can be read with the:

perldoc DBI

command.

Oraperl

Users of the Oraperl emulation layer bundled with DBD::Oracle, may read up on how to program with the Oraperl interface by typing:

perldoc Oraperl

This will produce an updated copy of the original oraperl man page written by Kevin Stock for perl4. The oraperl API is fully listed and described there.

DBD::mSQL

Users of the DBD::mSQL module may read about some of the private functions and quirks of that driver by typing:

perldoc DBD::mSQL
Frequently Asked Questions

This document, the Frequently Asked Questions is also available as POD documentation! You can read this on your own system by typing:

perldoc DBI::FAQ

This may be more convenient to persons not permanently, or conveniently, connected to the Internet.

POD in general

Information on writing POD, and on the philosophy of POD in general, can be read by typing:

perldoc perlpod

Users with the Tk module installed may be interested to learn there is a Tk-based POD reader available called tkpod, which formats POD in a convenient and readable way.

Rambles, Tidbits and Observations
http://www.arcana.co.uk/technologia/perl/DBI/tidbits

There are a series of occasional rambles from various people on the DBI mailing lists who, in an attempt to clear up a simple point, end up drafting fairly comprehensive documents. These are quite often varying in quality, but do provide some insights into the workings of the interfaces.

``DBI -- The perl5 Database Interface''

This is an article written by Alligator Descartes and Tim Bunce on the structure of DBI. It was published in issue 5 of ``The Perl Journal''. It's extremely good. Go buy the magazine. In fact, buy all of them! ``The Perl Journal''s WWW site is:

http://www.tpj.com

This article has now been placed online thanks to the gracious permission of Jon Orwant, editor of ``The Perl Journal''. It can be accessed via the WWW at

http://www.arcana.co.uk/technologia/perl/DBI/doc/tpj5/index.html

The article contains information that is now out of date with regards to the DBI 1.0 Specification since it was written approximately 2 years ago.

``DBperl''

This article, published in the November 1996 edition of ``Dr. Dobbs Journal'' concerned DBperl. The author of this edition apparently did not bother to contact any of the DBI development team members for verification of the information contained within his article. Several reviews of the article on the dbi-users mailing list were disparaging, to say the least. The fact the article was written about DBperl instead of DBI hints at the staleness of the information.

However, we include the reference for completeness' sake.

``The Perl5 Database Interface''

This item is a book being written by Alligator Descartes and Tim Bunce and will be published by O'Reilly and Associates early in 1999.

Here is the putative table of contents for the book.

* Introduction
     + Databases
     + CGI / WWW
     + perl
* Basic Database Concepts
     + Types of Database
          o Flat File
          o AnyDBM
          o RDBMS
     + Using Which Database For What...
* SQL
     + Why SQL?
     + Structuring Information In Databases
     + Retrieving Data From Databases
     + Manipulating Data and Data Structures
* DBI Architecture
* Programming with DBI
     + DBI Initialization
     + Handles
          o Driver Handles
          o Database Handles
          o Statement Handles
     + Connection and Disconnection
     + Handling Errors
     + Issuing Simple Queries
     + Executing Atomic Statements
     + Statement MetaData
     + More perl-ish Statements
     + Binding
     + Transaction Handling
     + Utility Methods
     + Handle Attributes and Dynamic Variables
* DBI and ODBC
* The Database Drivers
     + DBD::Oracle and oraperl
     + DBD::Informix and isqlperl
     + DBD::mSQL and Msqlperl
* Case Studies
     + DBI and the WWW
     + Data Migration and Warehousing
     + Administration Software
* Appendix: API Reference / Specification
* Appendix: Resources
README files

The README files included with each driver occasionally contains some useful information ( no, really! ) that may be pertinent to the user. Please read them. It makes our worthless existences more bearable. These can all be read from the main DBI WWW page at:

http://www.arcana.co.uk/technologia/perl/DBI
Mailing Lists

There are three mailing lists for DBI run by Ted Lemon. These can all be subscribed to and unsubscribed from via the World Wide Web at the URL of:

http://www.fugue.com/dbi

The lists that users may participate in are:

dbi-announce

This mailing list is for announcements only. Very low traffic. The announcements are usually posted on the main DBI WWW page.

If you cannot successfully use the form on the above WWW page, please subscribe to the list in the following manner:

Email: 'dbi-announce-request@fugue.com' with a message body of
'subscribe'
dbi-dev

This mailing list is intended for the use of developers discussing ideas and concepts for the DBI interface, API and driver mechanics. Only any use for developers, or interested parties. Low traffic.

If you cannot successfully use the form on the above WWW page, please subscribe to the list in the following manner:

Email: 'dbi-dev-request@fugue.com' with a message body of
'subscribe'
dbi-users

This mailing list is a general discussion list used for bug reporting, problem discussion and general enquiries. Medium traffic.

If you cannot successfully use the form on the above WWW page, please subscribe to the list in the following manner:

Email: 'dbi-users-request@fugue.com' with a message body of
'subscribe'
Mailing List Archives
US Mailing List Archives
http://outside.organic.com/mail-archives/dbi-users/

Searchable hypermail archives of the three mailing lists, and some of the much older traffic have been set up for users to browse.

European Mailing List Archives
http://www.rosat.mpe-garching.mpg.de/mailing-lists/PerlDB-Interest

As per the US archive above.

Compilation Problems

2.1. Compilation problems or "It fails the test!"

First off, consult the README for that driver in case there is useful information about the problem. It may be a known problem for your given architecture and operating system or database. You can check the README files for each driver in advance online at:

http://www.arcana.co.uk/technologia/perl/DBI

If it's a known problem, you'll probably have to wait till it gets fixed. If you're really needing it fixed, try the following:

Attempt to fix it yourself

This technique is generally not recommended to the faint-hearted. If you do think you have managed to fix it, then, send a patch file ( context diff ) to the author with an explanation of:

  • What the problem was, and test cases, if possible.

  • What you needed to do to fix it. Please make sure you mention everything.

  • Platform information, database version, perl version, module version and DBI version.

Email the author Do NOT whinge!

Please email the address listed in the WWW pages for whichever driver you are having problems with. Do not directly email the author at a known address unless it corresponds with the one listed.

We tend to have real jobs to do, and we do read the mailing lists for problems. Besides, we may not have access to <insert your favourite brain-damaged platform here> and couldn't be of any assistance anyway! Apologies for sounding harsh, but that's the way of it!

However, you might catch one of these creative genii at 3am when we're doing this sort of stuff anyway, and get a patch within 5 minutes. The atmosphere in the DBI circle is that we do appreciate the users' problems, since we work in similar environments.

If you are planning to email the author, please furnish as much information as possible, ie:

  • ALL the information asked for in the README file in the problematic module. And we mean ALL of it. We don't put lines like that in documentation for the good of our health, or to meet obscure README file standards of length.

  • If you have a core dump, try the Devel::CoreStack module for generating a stack trace from the core dump. Send us that too. Devel::CoreStack can be found on CPAN at:

    http://www.perl.com/cgi-bin/cpan_mod?module=Devel::CoreStack

  • Module versions, perl version, test cases, operating system versions and any other pertinent information.

Remember, the more information you send us, the quicker we can track problems down. If you send us no useful information, expect nothing back.

Email the dbi-users Mailing List

It's usually a fairly intelligent idea to cc the mailing list anyway with problems. The authors all read the lists, so you lose nothing by mailing there.

Platform and Driver Issues

3.1 What's the difference between ODBC and DBI?

Good question! To be filled in more detail! Meanwhile, see the notes at the end of the DBI README file.

3.2 Is DBI supported under Windows 95 / NT platforms?

Finally, yes! Jeff Urlwin has been working diligently on building DBI and DBD::Oracle under these platforms, and, with the advent of a stabler perl and a port of MakeMaker, the project has come on by great leaps and bounds.

The DBI and DBD::Oracle Win32 ports are now a standard part of DBI, so, downloading DBI of version higher than 0.81 should work fine as should using the most recent DBD::Oracle version.

3.3 Can I access Microsoft Access or SQL-Server databases with DBI?

Yes, use the DBD::ODBC driver.

3.4 Is the a DBD for <insert favourite database here>?

Is is listed on the DBI drivers page?

http://www.arcana.co.uk/technologia/perl/DBI

If not, no. A complete absence of a given database driver from that page means that no-one has announced any intention to work on it, not that such a driver is impossible to write.

A corollary of the above statement implies that if you see an announcement for a driver not on the above page, there's a good chance it's not actually a DBI driver, and may not conform to the specifications. Therefore, questions concerning problems with that code should not really be addressed to the DBI Mailing Lists.

3.5 What's DBM? And why should I use DBI instead?

Extracted from ``DBI - The Database Interface for Perl 5'':

``UNIX was originally blessed with simple file-based ``databases'', namely
the dbm system. dbm lets you store data in files, and retrieve
that data quickly. However, it also has serious drawbacks.

    File Locking

    The dbm systems did not allow particularly robust file locking
    capabilities, nor any capability for correcting problems arising through
    simultaneous writes [ to the database ].

    Arbitrary Data Structures

    The dbm systems only allows a single fixed data structure:
    key-value pairs. That value could be a complex object, such as a
    [ C ] struct, but the key had to be unique. This was a large
    limitation on the usefulness of dbm systems.

However, dbm systems still provide a useful function for users with
simple datasets and limited resources, since they are fast, robust and 
extremely well-tested. Perl modules to access dbm systems have now
been integrated into the core Perl distribution via the
AnyDBM_File module.''

To sum up, DBM is a perfectly satisfactory solution for essentially read-only databases, or small and simple datasets. However, for more powerful and scaleable datasets, not to mention robust transactional locking, users are recommended to use DBI.

3.6 What database do you recommend me using?

This is a particularly thorny area in which an objective answer is difficult to come by, since each dataset, proposed usage and system configuration differs from person to person.

From the current author's point of view, if the dataset is relatively small, being tables of less than 1 million rows, and less than 1000 tables in a given database, then mSQL is a perfectly acceptable solution to your problem. This database is extremely cheap, is wonderfully robust and has excellent support. More information is available on the Hughes Technology WWW site at:

http://www.hughes.com.au

If the dataset is larger than 1 million row tables or 1000 tables, or if you have either more money, or larger machines, I would recommend Oracle RDBMS. Oracle's WWW site is an excellent source of more information.

http://www.oracle.com

Informix is another high-end RDBMS that is worth considering. There are several differences between Oracle and Informix which are too complex for this document to detail. Information on Informix can be found on their WWW site at:

http://www.informix.com

In the case of WWW fronted applications, mSQL may be a better option due to slow connection times between a CGI script and the Oracle RDBMS and also the amount of resource each Oracle connection will consume. mSQL is lighter resource-wise and faster.

These views are not necessarily representative of anyone else's opinions, and do not reflect any corporate sponsorship or views. They are provided as-is.

3.7 Is <insert feature here> supported in DBI?

Given that we're making the assumption that the feature you have requested is a non-standard database-specific feature, then the answer will be no.

DBI reflects a generic API that will work for most databases, and has no database-specific functionality.

However, driver authors may, if they so desire, include hooks to database-specific functionality through the func() method defined in the DBI API. Script developers should note that use of functionality provided via the func() methods is unlikely to be portable across databases.

Programming Questions

4.1 Is DBI any use for CGI programming?

In a word, yes! DBI is hugely useful for CGI programming! In fact, I would tentatively say that CGI programming is one of two top uses for DBI.

DBI confers the ability to CGI programmers to power WWW-fronted databases to their users, which provides users with vast quantities of ordered data to play with. DBI also provides the possibility that, if a site is receiving far too much traffic than their database server can cope with, they can upgrade the database server behind the scenes with no alterations to the CGI scripts.

4.2 How do I get faster connection times with DBD::Oracle and CGI?

Contributed by John D. Groenveld

The Apache httpd maintains a pool of httpd children to service client requests.

Using the Apache mod_perl module by Doug MacEachern, the perl interpreter is embedded with the httpd children. The CGI, DBI, and your other favorite modules can be loaded at the startup of each child. These modules will not be reloaded unless changed on disk.

For more information on Apache, see the Apache Project's WWW site:

http://www.apache.org

The mod_perl module can be downloaded from CPAN via:

http://www.perl.com/cgi-bin/cpan_mod?module=Apache

4.3 How do I get persistent connections with DBI and CGI?

Contributed by John D. Groenveld

Using Edmund Mergl's Apache::DBI module, database logins are stored in a hash with each of these httpd child. If your application is based on a single database user, this connection can be started with each child. Currently, database connections cannot be shared between httpd children.

Apache::DBI can be downloaded from CPAN via:

http://www.perl.com/cgi-bin/cpan_mod?module=Apache::DBI

4.4 ``When I run a perl script from the command line, it works, but, when I run it under the httpd, it fails!'' Why?

Basically, a good chance this is occurring is due to the fact that the user that you ran it from the command line as has a correctly configured set of environment variables, in the case of DBD::Oracle, variables like $ORACLE_HOME, $ORACLE_SID or TWO_TASK.

The httpd process usually runs under the user id of nobody, which implies there is no configured environment. Any scripts attempting to execute in this situation will correctly fail.

To solve this problem, set the environment for your database in a BEGIN { } block at the top of your script. This will solve the problem.

Similarly, you should check your httpd error logfile for any clues, as well as the ``Idiot's Guide To Solving Perl / CGI Problems'' and ``Perl CGI Programming FAQ'' for further information. It is unlikely the problem is DBI-related.

The ``Idiot's Guide To Solving Perl / CGI Problems'' can be located at:

http://www.perl.com/perl/faq/index.html

as can the ``Perl CGI Programming FAQ''. Read BOTH these documents carefully!

4.5 How do I get the number of rows returned from a SELECT statement?

Contributed by Tim Bunce

Count them. Read the DBI docs for the rows() method.

Miscellaneous Questions

5.1 Can I do multi-threading with DBI?

As of the current date of this FAQ ( see top of page ), no. perl does not support multi-threading. However, multi-threading is expected to become part of the perl core distribution as of version 5.005, which implies that DBI may support multi-threading fairly soon afterwards.

As of version 1.02 of DBI, DBI is not multi-thread-aware and use of multi-threading with DBI may result in undefined behaviour.

For some OCI example code for Oracle that has multi-threaded SELECT statements, see:

http://www.arcana.co.uk/technologia/oracle/oci/orathreads.tar.gz

5.2 How do I handle BLOB data with DBI?

To be written.

5.3 How can I invoke stored procedures with DBI?

Assuming that you have created a stored procedure within the target database, eg, an Oracle database, you can use $dbh->do() to immediately execute the procedure. For example,

$dbh->do( "BEGIN someProcedure; END;" );

5.4 How can I get return values from stored procedures with DBI?

Contributed by Jeff Urlwin

$sth = $dbh->prepare( "BEGIN foo(:1, :2, :3); END;" );
$sth->bind_param(1, $a);
$sth->bind_param_inout(2, \$path, 2000);
$sth->bind_param_inout(3, \$success, 2000);
$sth->execute;

Remember to perform error checking, though!

5.5 How can I create or drop a database with DBI?

Database creation and deletion are concepts that are entirely too abstract to be adequately supported by DBI. For example, Oracle does not support the concept of dropping a database at all! Also, in Oracle, the database server essentially is the database, whereas in mSQL, the server process runs happily without any databases created in it. The problem is too disparate to attack.

Some drivers, therefore, support database creation and deletion through the private func() methods. You should check the documentation for the drivers you are using to see if they support this mechanism.

5.6 How can I commit or rollback a statement with DBI?

To be written. See the commit() and rollback() methods in the DBI README.

5.7 How are NULL values handled by DBI?

NULL values in DBI are specified to be treated as the value undef. NULLs can be inserted into databases as NULL, for example:

$rv = $dbh->do( "INSERT INTO table VALUES( NULL )" );

but when queried back, the NULLs should be tested against undef. This is standard across all drivers.

5.8 What are these func() methods all about?

The func() method is defined within DBI as being an entry point for database-specific functionality, eg, the ability to create or drop databases. Invoking these driver-specific methods is simple, for example, to invoke a createDatabase method that has one argument, we would write:

$rv =$dbh->func( 'argument', 'createDatabase' );

Software developers should note that the func() methods are non-portable between databases.

5.9 Is DBI Year 2000 Compliant?

DBI has no knowledge of understanding of what dates are. Therefore, DBI itself does not have a Year 2000 problem. Individual drivers may use date handling code internally and therefore be potentially susceptible to the Year 2000 problem, but this is unlikely.

You may also wish to read the ``Does Perl have a Year 2000 problem?'' section of the Perl FAQ at:

http://www.perl.com/CPAN/doc/FAQs/FAQ/PerlFAQ.html

Support and Training

The Perl5 Database Interface is FREE software. IT COMES WITHOUT WARRANTY OF ANY KIND. See the DBI README for more details.

However, some organizations are providing either technical support or training programs on DBI. The present author has no knowledge as to the quality of these services. The links are included for reference purposes only.

Commercial Support

The Perl Clinic

The Perl Clinic can arrange commercial support contracts for Perl, DBI, DBD::Oracle and Oraperl. Support is provided by the company with whom Tim Bunce, author of DBI and DBD::Oracle, works. For more information on their services, please see:

http://www.perlclinic.com

for more details.

Training

No training programs are known at this time.

Other References

In this section, we present some miscellaneous WWW links that may be of some interest to DBI users. These are not verified and may result in unknown sites or missing documents.

http://www-ccs.cs.umass.edu/db.html
http://www.odmg.org/odmg93/updates_dbarry.html
http://www.jcc.com/sql_stnd.html

AUTHOR

Alligator Descartes <descarte@arcana.co.uk>. Portions are Copyright their original stated authors.

COPYRIGHT

This document is Copyright (c)1994-1998 Alligator Descartes, with portions Copyright (c)1994-1998 their original authors. This module is released under the 'Artistic' license which you can find in the perl distribution.

This document is Copyright (c)1997-1998 Alligator Descartes. All rights reserved. Permission to distribute this document, in full or in part, via email, Usenet, ftp archives or http is granted providing that no charges are involved, reasonable attempt is made to use the most current version and all credits and copyright notices are retained ( the AUTHOR and COPYRIGHT sections ). Requests for other distribution rights, including incorporation into commercial products, such as books, magazine articles or CD-ROMs should be made to Alligator Descartes <descarte@arcana.co.uk>.