1DBI::FAQ(3) User Contributed Perl Documentation DBI::FAQ(3)
2
6 DBI::FAQ -- The Frequently Asked Questions for the Perl5 Database
7 Interface
8
10 perldoc DBI::FAQ
11
13 This document is currently at version 0.38, as of February 8th, 2000.
14 That's very old. A newer FAQ can be found at
16 <http://faq.dbi-support.com/>
17
19 This document serves to answer the most frequently asked questions on
20 both the DBI Mailing Lists and personally to members of the DBI
21 development team.
22
24 1.1 What is DBI, DBperl, Oraperl and *perl?
25 To quote Tim Bunce, the architect and author of DBI:
26 ``DBI is a database access Application Programming Interface (API)
28 for the Perl Language. The DBI API Specification defines a set
29 of functions, variables and conventions that provide a consistent
30 database interface independant of the actual database being used.''
31 In simple language, the DBI interface allows users to access multiple
33 database types transparently. So, if you connecting to an Oracle,
34 Informix, mSQL, Sybase or whatever database, you don't need to know the
35 underlying mechanics of the 3GL layer. The API defined by DBI will work
36 on all these database types.
37 A similar benefit is gained by the ability to connect to two different
39 databases of different vendor within the one perl script, ie, I want to
40 read data from an Oracle database and insert it back into an Informix
41 database all within one program. The DBI layer allows you to do this
42 simply and powerfully.
43 DBperl is the old name for the interface specification. It's usually
45 now used to denote perl4 modules on database interfacing, such as,
46 oraperl, isqlperl, ingperl and so on. These interfaces didn't have a
47 standard API and are generally not supported.
48 Here's a list of DBperl modules, their corresponding DBI counterparts
50 and support information. Please note, the author's listed here
51 generally do not maintain the DBI module for the same database. These
52 email addresses are unverified and should only be used for queries
53 concerning the perl4 modules listed below. DBI driver queries should be
54 directed to the dbi-users mailing list.
55 Module Name Database Required Author DBI
57 ----------- ----------------- ------ ---
58 Sybperl Sybase Michael Peppler DBD::Sybase
59 <mpeppler@itf.ch>
60 Oraperl Oracle 6 & 7 Kevin Stock DBD::Oracle
61 <dbi-users@perl.org>
62 Ingperl Ingres Tim Bunce & DBD::Ingres
63 Ted Lemon
64 <dbi-users@perl.org>
65 Interperl Interbase Buzz Moschetti DBD::Interbase
66 <buzz@bear.com>
67 Uniperl Unify 5.0 Rick Wargo None
68 <rickers@coe.drexel.edu>
69 Pgperl Postgres Igor Metz DBD::Pg
70 <metz@iam.unibe.ch>
71 Btreeperl NDBM John Conover SDBM?
72 <john@johncon.com>
73 Ctreeperl C-Tree John Conover None
74 <john@johncon.com>
75 Cisamperl Informix C-ISAM Mathias Koerber None
76 <mathias@unicorn.swi.com.sg>
77 Duaperl X.500 Directory Eric Douglas None
78 User Agent
79 However, some DBI modules have DBperl emulation layers, so, DBD::Oracle
81 comes with an Oraperl emulation layer, which allows you to run legacy
82 oraperl scripts without modification. The emulation layer translates
83 the oraperl API calls into DBI calls and executes them through the DBI
84 switch.
85 Here's a table of emulation layer information:
87 Module Emulation Layer Status
89 ------ --------------- ------
90 DBD::Oracle Oraperl Complete
91 DBD::Informix Isqlperl Under development
92 DBD::Ingres Ingperl Complete?
93 DBD::Sybase Sybperl Working? ( Needs verification )
94 DBD::mSQL Msqlperl Experimentally released with
95 DBD::mSQL-0.61
96 The Msqlperl emulation is a special case. Msqlperl is a perl5 driver
98 for mSQL databases, but does not conform to the DBI Specification. It's
99 use is being deprecated in favour of DBD::mSQL. Msqlperl may be
100 downloaded from CPAN via:
101 http://www.perl.com/cgi-bin/cpan_mod?module=Msqlperl
103 1.2. Where can I get it from?
105 The Comprehensive Perl Archive Network resources should be used for
106 retrieving up-to-date versions of the DBI and drivers. CPAN may be
107 accessed via Tom Christiansen's splendid CPAN multiplexer program
108 located at:
109 http://www.perl.com/CPAN/
111 For more specific version information and exact URLs of drivers, please
113 see the DBI drivers list and the DBI module pages which can be found
114 on:
115 http://dbi.perl.org/
117 http://www.symbolstone.org/technology/perl/DBI
118 This list is automatically generated on a nightly basis from CPAN and
120 should be up-to-date.
121 1.3. Where can I get more information?
123 There are a few information sources on DBI.
124 "Programming the Perl DBI"
126 "Programming the Perl DBI" is the official book on the DBI written
127 by Alligator Descartes and Tim Bunce and published by O'Reilly &
128 Associates. The book was released on February 9th, 2000.
129 The table of contents is:
131 Preface
133 1. Introduction
134 From Mainframes to Workstations
135 Perl
136 DBI in the Real World
137 A Historical Interlude and Standing Stones
138 2. Basic Non-DBI Databases
139 Storage Managers and Layers
140 Query Languages and Data Functions
141 Standing Stones and the Sample Database
142 Flat-File Databases
143 Putting Complex Data into Flat Files
144 Concurrent Database Access and Locking
145 DBM Files and the Berkeley Database Manager
146 The MLDBM Module
147 Summary
148 3. SQL and Relational Databases
149 The Relational Database Methodology
150 Datatypes and NULL Values
151 Querying Data
152 Modifying Data Within Tables
153 Creating and Destroying Tables
154 4. Programming with the DBI
155 DBI Architecture
156 Handles
157 Data Source Names
158 Connection and Disconnection
159 Error Handling
160 Utility Methods and Functions
161 5. Interacting with the Database
162 Issuing Simple Queries
163 Executing Non-SELECT Statements
164 Binding Parameters to Statements
165 Binding Output Columns
166 do() Versus prepare()
167 Atomic and Batch Fetching
168 6. Advanced DBI
169 Handle Attributes and Metadata
170 Handling LONG/LOB Data
171 Transactions, Locking, and Isolation
172 7. ODBC and the DBI
173 ODBC -- Embraced and Extended
174 DBI -- Thrashed and Mutated
175 The Nuts and Bolts of ODBC
176 ODBC from Perl
177 The Marriage of DBI and ODBC
178 Questions and Choices
179 Moving Between Win32::ODBC and the DBI
180 And What About ADO?
181 8. DBI Shell and Database Proxying
182 dbish -- The DBI Shell
183 Database Proxying
184 A. DBI Specification
185 B. Driver and Database Characteristics
186 C. ASLaN Sacred Site Charter
187 Index
188 The book should be available from all good bookshops and can be
190 ordered online either <I>via</I> O'Reilly & Associates
191 http://www.oreilly.com/catalog/perldbi
193 or Amazon
195 http://www.amazon.com/exec/obidos/ASIN/1565926994/dbi
197 POD documentation
199 PODs are chunks of documentation usually embedded within perl
200 programs that document the code ``in place'', providing a useful
201 resource for programmers and users of modules. POD for DBI and
202 drivers is beginning to become more commonplace, and documentation
203 for these modules can be read with the "perldoc" program included
204 with Perl.
205 The DBI Specification
207 The POD for the DBI Specification can be read with the:
208 perldoc DBI
210 command. The Specification also forms Appendix A of
212 "Programming the Perl DBI".
213 Oraperl
215 Users of the Oraperl emulation layer bundled with DBD::Oracle,
216 may read up on how to program with the Oraperl interface by
217 typing:
218 perldoc Oraperl
220 This will produce an updated copy of the original oraperl man
222 page written by Kevin Stock for perl4. The oraperl API is fully
223 listed and described there.
224 Drivers
226 Users of the DBD modules may read about some of the private
227 functions and quirks of that driver by typing:
228 perldoc <driver>
230 For example, the DBD::mSQL driver is bundled with driver-
232 specific documentation that can be accessed by typing
233 perldoc DBD::mSQL
235 Frequently Asked Questions
237 This document, the Frequently Asked Questions is also available
238 as POD documentation! You can read this on your own system by
239 typing:
240 perldoc DBI::FAQ
242 This may be more convenient to persons not permanently, or
244 conveniently, connected to the Internet. The DBI::FAQ module
245 should be downloaded and installed for the more up-to-date
246 version.
247 The version of DBI::FAQ shipped with the "DBI" module may be
249 slightly out of date.
250 POD in general
252 Information on writing POD, and on the philosophy of POD in
253 general, can be read by typing:
254 perldoc perlpod
256 Users with the Tk module installed may be interested to learn
258 there is a Tk-based POD reader available called "tkpod", which
259 formats POD in a convenient and readable way. This is available
260 via CPAN as the module called Tk::POD and is highly
261 recommended.
262 Driver and Database Characteristics
264 The driver summaries that were produced for Appendix B of
265 "Programming the Perl DBI" are available online at:
266 http://dbi.perl.org/
268 http://www.symbolstone.org/technology/perl/DBI
269 in the driver information table. These summaries contain
271 standardised information on each driver and database which should
272 aid you in selecting a database to use. It will also inform you
273 quickly of any issues within drivers or whether a driver is not
274 fully compliant with the DBI Specification.
275 Rambles, Tidbits and Observations
277 http://dbi.perl.org/tidbits
278 http://www.symbolstone.org/technology/perl/DBI/tidbits
279 There are a series of occasional rambles from various people on the
281 DBI mailing lists who, in an attempt to clear up a simple point,
282 end up drafting fairly comprehensive documents. These are quite
283 often varying in quality, but do provide some insights into the
284 workings of the interfaces.
285 Articles
287 A list of articles discussing the DBI can be found on the DBI WWW
288 page at:
289 http://dbi.perl.org/
291 http://www.symbolstone.org/technology/perl/DBI
292 These articles are of varying quality and age, from the original
294 Perl Journal article written by Alligator and Tim, to more recent
295 debacles published online from about.com.
296 README files
298 The README files included with each driver occasionally contains
299 some useful information ( no, really! ) that may be pertinent to
300 the user. Please read them. It makes our worthless existences more
301 bearable. These can all be read from the main DBI WWW page at:
302 http://dbi.perl.org/
304 http://www.symbolstone.org/technology/perl/DBI
305 Mailing Lists
307 There are three mailing lists for DBI:
308 dbi-announce@perl.org -- for announcements, very low traffic
310 dbi-users@perl.org -- general user support
311 dbi-dev@perl.org -- for driver developers (no user support)
312 For information on how to subscribe, set digest mode etc, and
314 unsubscribe, send an email message (the content will be ignored)
315 to:
316 dbi-announce-help@perl.org
318 dbi-users-help@perl.org
319 dbi-dev-help@perl.org
320 Mailing List Archives
322 US Mailing List Archives
323 http://outside.organic.com/mail-archives/dbi-users/
324 Searchable hypermail archives of the three mailing lists, and
326 some of the much older traffic have been set up for users to
327 browse.
328 European Mailing List Archives
330 http://www.rosat.mpe-garching.mpg.de/mailing-lists/PerlDB-Interest
331 As per the US archive above.
333
335 2.1. Compilation problems or "It fails the test!"
336 First off, consult the README for that driver in case there is useful
337 information about the problem. It may be a known problem for your given
338 architecture and operating system or database. You can check the README
339 files for each driver in advance online at:
340 http://dbi.perl.org/
342 http://www.symbolstone.org/technology/perl/DBI
343 If it's a known problem, you'll probably have to wait till it gets
345 fixed. If you're really needing it fixed, try the following:
346 Attempt to fix it yourself
348 This technique is generally not recommended to the faint-hearted.
349 If you do think you have managed to fix it, then, send a patch file
350 ( context diff ) to the author with an explanation of:
351 · What the problem was, and test cases, if possible.
353 · What you needed to do to fix it. Please make sure you mention
355 everything.
356 · Platform information, database version, perl version, module
358 version and DBI version.
359 Email the author Do NOT whinge!
361 Please email the address listed in the WWW pages for whichever
362 driver you are having problems with. Do not directly email the
363 author at a known address unless it corresponds with the one
364 listed.
365 We tend to have real jobs to do, and we do read the mailing lists
367 for problems. Besides, we may not have access to <insert your
368 favourite brain-damaged platform here> and couldn't be of any
369 assistance anyway! Apologies for sounding harsh, but that's the way
370 of it!
371 However, you might catch one of these creative genii at 3am when
373 we're doing this sort of stuff anyway, and get a patch within 5
374 minutes. The atmosphere in the DBI circle is that we do appreciate
375 the users' problems, since we work in similar environments.
376 If you are planning to email the author, please furnish as much
378 information as possible, ie:
379 · ALL the information asked for in the README file in the
381 problematic module. And we mean ALL of it. We don't put lines
382 like that in documentation for the good of our health, or to
383 meet obscure README file standards of length.
384 · If you have a core dump, try the Devel::CoreStack module for
386 generating a stack trace from the core dump. Send us that too.
387 Devel::CoreStack can be found on CPAN at:
388 http://www.perl.com/cgi-bin/cpan_mod?module=Devel::CoreStack
390 · Module versions, perl version, test cases, operating system
392 versions and any other pertinent information.
393 Remember, the more information you send us, the quicker we can
395 track problems down. If you send us no useful information, expect
396 nothing back.
397 Finally, please be aware that some authors, including Tim Bunce,
399 specifically request that you do not mail them directly. Please
400 respect their wishes and use the email addresses listed in the
401 appropriate module "README" file.
402 Email the dbi-users Mailing List
404 It's usually a fairly intelligent idea to cc the mailing list
405 anyway with problems. The authors all read the lists, so you lose
406 nothing by mailing there.
407
409 3.1 What's the difference between ODBC and DBI?
410 In terms of architecture - not much: Both define programming
411 interfaces. Both allow multiple drivers to be loaded to do the actual
412 work.
413 In terms of ease of use - much: The DBI is a 'high level' interface
415 that, like Perl itself, strives to make the simple things easy while
416 still making the hard things possible. The ODBC is a 'low level'
417 interface. All nuts-bolts-knobs-and-dials.
418 Now there's an ODBC driver for the DBI (DBD::ODBC) the "What's the
420 difference" question is more usefully rephrased as:
421 Chapter 7 of "Programming the Perl DBI" covers this topic in far more
423 detail and should be consulted.
424 3.2 What's the difference between Win32::ODBC and DBD::ODBC?
426 The DBI, and thus DBD::ODBC, has a different philosophy from the
427 Win32::ODBC module:
428 The Win32::ODBC module is a 'thin' layer over the low-level ODBC API.
430 The DBI defines a simpler 'higher level' interface.
431 The Win32::ODBC module gives you access to more of the ODBC API. The
433 DBI and DBD::ODBC give you access to only the essentials. (But, unlike
434 Win32::ODBC, the DBI and DBD::ODBC do support parameter binding and
435 multiple prepared statements which reduces the load on the database
436 server and can dramatically increase performance.)
437 The Win32::ODBC module only works on Win32 systems. The DBI and
439 DBD::ODBC are very portable and work on Win32 and Unix.
440 The DBI and DBD::ODBC modules are supplied as a standard part of the
442 Perl 5.004 binary distribution for Win32 (they don't work with the
443 older, non-standard, ActiveState port).
444 Scripts written with the DBI and DBD::ODBC are faster than Win32::ODBC
446 on Win32 and are trivially portable to other supported database types.
447 The DBI offers optional automatic printing or die()ing on errors which
449 makes applications simpler and more robust.
450 The current DBD::ODBC driver version 0.16 is new and not yet fully
452 stable. A new release is due soon [relative to the date of the next
453 TPJ issue :-] and will be much improved and offer more ODBC
454 functionality.
455 To summarise: The Win32::ODBC module is your best choice if you need
457 access to more of the ODBC API than the DBI gives you. Otherwise, the
458 DBI and DBD::ODBC combination may be your best bet.
459 Chapter 7 of "Programming the Perl DBI" covers this topic in far more
461 detail and should be consulted.
462 3.3 Is DBI supported under Windows 95 / NT platforms?
464 Finally, yes! Jeff Urlwin has been working diligently on building DBI
465 and DBD::ODBC under these platforms, and, with the advent of a stabler
466 perl and a port of MakeMaker, the project has come on by great leaps
467 and bounds.
468 The DBI and DBD::Oracle Win32 ports are now a standard part of DBI, so,
470 downloading DBI of version higher than 0.81 should work fine as should
471 using the most recent DBD::Oracle version.
472 3.4 Can I access Microsoft Access or SQL-Server databases with DBI?
474 Yes, use the DBD::ODBC driver.
475 3.5 Is the a DBD for <insert favourite database here>?
477 Is is listed on the DBI drivers page?
478 http://dbi.perl.org/
480 http://www.symbolstone.org/technology/perl/DBI
481 If not, no. A complete absence of a given database driver from that
483 page means that no-one has announced any intention to work on it, not
484 that such a driver is impossible to write.
485 A corollary of the above statement implies that if you see an
487 announcement for a driver not on the above page, there's a good chance
488 it's not actually a DBI driver, and may not conform to the
489 specifications. Therefore, questions concerning problems with that code
490 should not really be addressed to the DBI Mailing Lists.
491 3.6 What's DBM? And why should I use DBI instead?
493 Extracted from ``DBI - The Database Interface for Perl 5'':
494 ``UNIX was originally blessed with simple file-based ``databases'', namely
496 the dbm system. dbm lets you store data in files, and retrieve
497 that data quickly. However, it also has serious drawbacks.
498 File Locking
500 The dbm systems did not allow particularly robust file locking
502 capabilities, nor any capability for correcting problems arising through
503 simultaneous writes [ to the database ].
504 Arbitrary Data Structures
506 The dbm systems only allows a single fixed data structure:
508 key-value pairs. That value could be a complex object, such as a
509 [ C ] struct, but the key had to be unique. This was a large
510 limitation on the usefulness of dbm systems.
511 However, dbm systems still provide a useful function for users with
513 simple datasets and limited resources, since they are fast, robust and
514 extremely well-tested. Perl modules to access dbm systems have now
515 been integrated into the core Perl distribution via the
516 AnyDBM_File module.''
517 To sum up, DBM is a perfectly satisfactory solution for essentially
519 read-only databases, or small and simple datasets. However, for more
520 scaleable dataset handling, not to mention robust transactional
521 locking, users are recommended to use a more powerful database engine
522 via DBI.
523 Chapter 2 of "Programming the Perl DBI" discusses DBM files in detail.
525 3.7 What database do you recommend me using?
527 This is a particularly thorny area in which an objective answer is
528 difficult to come by, since each dataset, proposed usage and system
529 configuration differs from person to person.
530 From the current author's point of view, if the dataset is relatively
532 small, being tables of less than 1 million rows, and less than 1000
533 tables in a given database, then mSQL is a perfectly acceptable
534 solution to your problem. This database is extremely cheap, is
535 wonderfully robust and has excellent support. More information is
536 available on the Hughes Technology WWW site at:
537 http://www.hughes.com.au
539 You may also wish to look at MySQL which is a more powerful database
541 engine that has a similar feel to mSQL.
542 http://www.tcx.se
544 If the dataset is larger than 1 million row tables or 1000 tables, or
546 if you have either more money, or larger machines, I would recommend
547 Oracle RDBMS. Oracle's WWW site is an excellent source of more
548 information.
549 http://www.oracle.com
551 Informix is another high-end RDBMS that is worth considering. There are
553 several differences between Oracle and Informix which are too complex
554 for this document to detail. Information on Informix can be found on
555 their WWW site at:
556 http://www.informix.com
558 In the case of WWW fronted applications, mSQL may be a better option
560 due to slow connection times between a CGI script and the Oracle RDBMS
561 and also the amount of resource each Oracle connection will consume.
562 mSQL is lighter resource-wise and faster.
563 These views are not necessarily representative of anyone else's
565 opinions, and do not reflect any corporate sponsorship or views. They
566 are provided as-is.
567 3.8 Is <insert feature here> supported in DBI?
569 Given that we're making the assumption that the feature you have
570 requested is a non-standard database-specific feature, then the answer
571 will be no.
572 DBI reflects a generic API that will work for most databases, and has
574 no database-specific functionality.
575 However, driver authors may, if they so desire, include hooks to
577 database-specific functionality through the "func()" method defined in
578 the DBI API. Script developers should note that use of functionality
579 provided via the "func()" methods is very unlikely to be portable
580 across databases.
581
583 4.1 Is DBI any use for CGI programming?
584 In a word, yes! DBI is hugely useful for CGI programming! In fact, I
585 would tentatively say that CGI programming is one of two top uses for
586 DBI.
587 DBI confers the ability to CGI programmers to power WWW-fronted
589 databases to their users, which provides users with vast quantities of
590 ordered data to play with. DBI also provides the possibility that, if a
591 site is receiving far too much traffic than their database server can
592 cope with, they can upgrade the database server behind the scenes with
593 no alterations to the CGI scripts.
594 4.2 How do I get faster connection times with DBD::Oracle and CGI?
596 Contributed by John D. Groenveld
597 The Apache "httpd" maintains a pool of "httpd" children to service
599 client requests.
600 Using the Apache mod_perl module by Doug MacEachern, the perl
602 interpreter is embedded with the "httpd" children. The CGI, DBI, and
603 your other favorite modules can be loaded at the startup of each child.
604 These modules will not be reloaded unless changed on disk.
605 For more information on Apache, see the Apache Project's WWW site:
607 http://www.apache.org
609 The mod_perl module can be downloaded from CPAN via:
611 http://www.perl.com/cgi-bin/cpan_mod?module=Apache
613 4.3 How do I get persistent connections with DBI and CGI?
615 Contributed by John D. Groenveld
616 Using Edmund Mergl's Apache::DBI module, database logins are stored in
618 a hash with each of these "httpd" child. If your application is based
619 on a single database user, this connection can be started with each
620 child. Currently, database connections cannot be shared between
621 "httpd" children.
622 Apache::DBI can be downloaded from CPAN via:
624 http://www.perl.com/cgi-bin/cpan_mod?module=Apache::DBI
626 4.4 ``When I run a perl script from the command line, it works, but, when I
628 run it under the "httpd", it fails!'' Why?
629 Basically, a good chance this is occurring is due to the fact that the
630 user that you ran it from the command line as has a correctly
631 configured set of environment variables, in the case of DBD::Oracle,
632 variables like "ORACLE_HOME", "ORACLE_SID" or "TWO_TASK".
633 The "httpd" process usually runs under the user id of "nobody", which
635 implies there is no configured environment. Any scripts attempting to
636 execute in this situation will correctly fail.
637 One way to solve this problem is to set the environment for your
639 database in a "BEGIN { }" block at the top of your script. Another
640 technique is to configure your WWW server to pass-through certain
641 environment variables to your CGI scripts.
642 Similarly, you should check your "httpd" error logfile for any clues,
644 as well as the ``Idiot's Guide To Solving Perl / CGI Problems'' and
645 ``Perl CGI Programming FAQ'' for further information. It is unlikely
646 the problem is DBI-related.
647 The ``Idiot's Guide To Solving Perl / CGI Problems'' can be located at:
649 http://www.perl.com/perl/faq/index.html
651 as can the ``Perl CGI Programming FAQ''. Read BOTH these documents
653 carefully!
654 4.5 How do I get the number of rows returned from a "SELECT" statement?
656 Count them. Read the DBI docs for the "rows()" method.
657
659 5.1 Can I do multi-threading with DBI?
660 Perl version 5.005 and later can be built to support multi-threading.
661 The DBI, as of version 1.02, does not yet support multi-threading so it
662 would be unsafe to let more than one thread enter the DBI at the same
663 time.
664 It is expected that some future version of the DBI will at least be
666 thread-safe (but not thread-hot) by automatically blocking threads
667 intering the DBI while it's already in use.
668 For some OCI example code for Oracle that has multi-threaded "SELECT"
670 statements, see:
671 http://www.symbolstone.org/technology/oracle/oci/orathreads.tar.gz
673 5.2 How do I handle BLOB data with DBI?
675 Handling BLOB data with the DBI is very straight-forward. BLOB columns
676 are specified in a SELECT statement as per normal columns. However, you
677 also need to specify a maximum BLOB size that the <I>database
678 handle</I> can fetch using the "LongReadLen" attribute.
679 For example:
681 ### $dbh is a connected database handle
683 $sth = $dbh->prepare( "SELECT blob_column FROM blobby_table" );
684 $sth->execute;
685 would fail.
687 ### $dbh is a connected database handle
689 ### Set the maximum BLOB size...
690 $dbh->{LongReadLen} = 16384; ### 16Kb...Not much of a BLOB!
691 $sth = $dbh->prepare( "..." );
693 would succeed <I>provided no column values were larger than the
695 specified value</I>.
696 If the BLOB data is longer than the value of "LongReadLen", then an
698 error will occur. However, the DBI provides an additional piece of
699 functionality that will automatically truncate the fetched BLOB to the
700 size of "LongReadLen" if it is longer. This does not cause an error to
701 occur, but may make your fetched BLOB data useless.
702 This behaviour is regulated by the "LongTruncOk" attribute which is
704 defaultly set to a false value ( thus making overlong BLOB fetches fail
705 ).
706 ### Set BLOB handling such that it's 16Kb and can be truncated
708 $dbh->{LongReadLen} = 16384;
709 $dbh->{LongTruncOk} = 1;
710 Truncation of BLOB data may not be a big deal in cases where the BLOB
712 contains run-length encoded data, but data containing checksums at the
713 end, for example, a ZIP file, would be rendered useless.
714 5.3 How can I invoke stored procedures with DBI?
716 The DBI does not define a database-independent way of calling stored
717 procedures.
718 However, most database that support them also provide a way to call
720 them from SQL statements - and the DBI certainly supports that.
721 So, assuming that you have created a stored procedure within the target
723 database, eg, an Oracle database, you can use $dbh->"do()" to
724 immediately execute the procedure. For example,
725 $dbh->do( "BEGIN someProcedure; END;" ); # Oracle-specific
727 You should also be able to "prepare" and "execute", which is the
729 recommended way if you'll be calling the procedure often.
730 5.4 How can I get return values from stored procedures with DBI?
732 Contributed by Jeff Urlwin
733 $sth = $dbh->prepare( "BEGIN foo(:1, :2, :3); END;" );
735 $sth->bind_param(1, $a);
736 $sth->bind_param_inout(2, \$path, 2000);
737 $sth->bind_param_inout(3, \$success, 2000);
738 $sth->execute;
739 Remember to perform error checking, though! ( Or use the "RaiseError"
741 attribute ).
742 5.5 How can I create or drop a database with DBI?
744 Database creation and deletion are concepts that are entirely too
745 abstract to be adequately supported by DBI. For example, Oracle does
746 not support the concept of dropping a database at all! Also, in Oracle,
747 the database server essentially is the database, whereas in mSQL, the
748 server process runs happily without any databases created in it. The
749 problem is too disparate to attack in a worthwhile way.
750 Some drivers, therefore, support database creation and deletion through
752 the private "func()" methods. You should check the documentation for
753 the drivers you are using to see if they support this mechanism.
754 5.6 How can I "commit" or "rollback" a statement with DBI?
756 See the "commit()" and "rollback()" methods in the DBI Specification.
757 Chapter 6 of "Programming the Perl DBI" discusses transaction handling
759 within the context of DBI in more detail.
760 5.7 How are "NULL" values handled by DBI?
762 "NULL" values in DBI are specified to be treated as the value "undef".
763 "NULL"s can be inserted into databases as "NULL", for example:
764 $rv = $dbh->do( "INSERT INTO table VALUES( NULL )" );
766 but when queried back, the "NULL"s should be tested against "undef".
768 This is standard across all drivers.
769 5.8 What are these "func()" methods all about?
771 The "func()" method is defined within DBI as being an entry point for
772 database-specific functionality, eg, the ability to create or drop
773 databases. Invoking these driver-specific methods is simple, for
774 example, to invoke a "createDatabase" method that has one argument, we
775 would write:
776 $rv =$dbh->func( 'argument', 'createDatabase' );
778 Software developers should note that the "func()" methods are non-
780 portable between databases.
781 5.9 Is DBI Year 2000 Compliant?
783 DBI has no knowledge of understanding of what dates are. Therefore, DBI
784 itself does not have a Year 2000 problem. Individual drivers may use
785 date handling code internally and therefore be potentially susceptible
786 to the Year 2000 problem, but this is unlikely.
787 You may also wish to read the ``Does Perl have a Year 2000 problem?''
789 section of the Perl FAQ at:
790 http://www.perl.com/CPAN/doc/FAQs/FAQ/PerlFAQ.html
792
794 The Perl5 Database Interface is FREE software. IT COMES WITHOUT
795 WARRANTY OF ANY KIND. See the DBI README for more details.
796 However, some organizations are providing either technical support or
798 training programs on DBI. The present author has no knowledge as to the
799 quality of these services. The links are included for reference
800 purposes only and should not be regarded as recommendations in any way.
801 Caveat emptor.
802 Commercial Support
804 The Perl Clinic
805 The Perl Clinic provides commercial support for Perl and Perl
806 related problems, including the DBI and its drivers. Support is
807 provided by the company with whom Tim Bunce, author of DBI and
808 DBD::Oracle, works and ActiveState. For more information on their
809 services, please see:
810 http://www.perlclinic.com
812 Training
814 Westlake Solutions
815 A hands-on class for experienced Perl CGI developers that teaches
816 how to write database-connected CGI scripts using Perl and DBI.pm.
817 This course, along with four other courses on CGI scripting with
818 Perl, is taught in Washington, DC; Arlington, Virginia; and on-site
819 worldwide upon request.
820 See:
822 http://www.westlake.com/training
824 for more details.
826
828 In this section, we present some miscellaneous WWW links that may be of
829 some interest to DBI users. These are not verified and may result in
830 unknown sites or missing documents.
831 http://www-ccs.cs.umass.edu/db.html
833 http://www.odmg.org/odmg93/updates_dbarry.html
834 http://www.jcc.com/sql_stnd.html
835
837 Alligator Descartes <http://www.symbolstone.org/descarte/contact.html>.
838 Portions are Copyright their original stated authors.
839
841 This document is Copyright (c)1994-2000 Alligator Descartes, with
842 portions Copyright (c)1994-2000 their original authors. This module is
843 released under the 'Artistic' license which you can find in the perl
844 distribution.
845 This document is Copyright (c)1997-2000 Alligator Descartes. All rights
847 reserved. Permission to distribute this document, in full or in part,
848 via email, Usenet, ftp archives or http is granted providing that no
849 charges are involved, reasonable attempt is made to use the most
850 current version and all credits and copyright notices are retained (
851 the AUTHOR and COPYRIGHT sections ). Requests for other distribution
852 rights, including incorporation into commercial products, such as
853 books, magazine articles or CD-ROMs should be made to Alligator
854 Descartes <http://www.symbolstone.org/descarte/contact.html>.
855perl v5.10.1 2007-11-15 DBI::FAQ(3)