XBase::Index(3) User Contributed Perl Documentation XBase::Index(3)NAMEXBase::Index - base class for the index files for dbf
SYNOPSIS
use XBase;
my $table = new XBase "data.dbf";
my $cur = $table->prepare_select_with_index("id.ndx",
"ID", "NAME);
$cur->find_eq(1097);
while (my @data = $cur->fetch()) {
last if $data[0] != 1097;
print "@data\n";
}
This is a snippet of code to print ID and NAME fields from dbf data.dbf
where ID equals 1097. Provided you have index on ID in file id.ndx. You
can use the same code for ntx and idx index files. For the cdx and
mdx, the prepare_select call would be
prepare_select_with_index(['rooms.cdx', 'ROOMNAME'])
so instead of plain filename you specify an arrayref with filename and
an index tag in that file. The reason is that cdx and mdx can contain
multiple indexes in one file and you have to distinguish, which you
want to use.
DESCRIPTION
The module XBase::Index is a collection of packages to provide index
support for XBase-like dbf database files.
An index file is generaly a file that holds values of certain database
field or expression in sorted order, together with the record number
that the record occupies in the dbf file. So when you search for a
record with some value, you first search in this sorted list and once
you have the record number in the dbf, you directly fetch the record
from dbf.
What indexes do
To make the searching in this ordered list fast, it's generally
organized as a tree -- it starts with a root page with records that
point to pages at lower level, etc., until leaf pages where the pointer
is no longer a pointer to the index but to the dbf. When you search for
a record in the index file, you fetch the root page and scan it
(lineary) until you find key value that is equal or grater than that
you are looking for. That way you've avoided reading all pages
describing the values that are lower. Here you descend one level, fetch
the page and again search the list of keys in that page. And you repeat
this process until you get to the leaf (lowest) level and here you
finaly find a pointer to the dbf. XBase::Index does this for you.
Some of the formats also support multiple indexes in one file --
usually there is one top level index that for different field values
points to different root pages in the index file (so called tags).
XBase::Index supports (or aims to support) the following index formats:
ndx, ntx, mdx, cdx and idx. They differ in a way they store the keys
and pointers but the idea is always the same: make a tree of pages,
where the page contains keys and pointer either to pages at lower
levels, or to dbf (or both). XBase::Index only supports read only
access to the index fields at the moment (and if you need writing them
as well, follow reading because we need to have the reading support
stable before I get to work on updating the indexes).
Testing your index file (and XBase::Index)
You can test your index using the indexdump script in the main
directory of the DBD::XBase distribution (I mean test XBase::Index on
correct index data, not testing corrupted index file, of course ;-)
Just run
./indexdump ~/path/index.ndx
./indexdump ~/path/index.cdx tag_name
or
perl -Ilib ./indexdump ~/path/index.cdx tag_name
if you haven't installed this version of XBase.pm/DBD::XBase yet. You
should get the content of the index file. On each row, there is the key
value and a record number of the record in the dbf file. Let me know if
you get results different from those you expect. I'd probably ask you
to send me the index file (and possibly the dbf file as well), so that
I can debug the problem.
The index file is (as already noted) a complement to a dbf file. Index
file without a dbf doesn't make much sense because the only thing that
you can get from it is the record number in the dbf file, not the
actual data. But it makes sense to test -- dump the content of the
index to see if the sequence is OK.
The index formats usually distinguish between numeric and character
data. Some of the file formats include the information about the type
in the index file, other depend on the dbf file. Since with indexdump
we only look at the index file, you may need to specify the -type
option to indexdump if it complains that it doesn't know the data type
of the values (this is the case with cdx at least). The possible values
are num, char and date and the call would be like
./indexdump -type=num ~/path/index.cdx tag_name
(this -type option may not work with all index formats at the moment --
will be fixed and patches always welcome).
You can use "-ddebug" option to indexdump to see how pages are fetched
and decoded, or run debugger to see the calls and parsing.
Using the index files to speed up searches in dbf
The syntax for using the index files to access data in the dbf file is
generally
my $table = new XBase "tablename";
# or any other arguments to get the XBase object
# see XBase(3)
my $cur = $table->prepare_select_with_index("indexfile",
"list", "of", "fields", "to", "return");
or
my $cur = $table->prepare_select_with_index(
[ "indexfile_with_tags", "tag_name" ],
"list", "of", "fields", "to", "return");
where we specify the tag in the index file (this is necessary with cdx
and mdx). After we have the cursor, we can search to given record and
start fetching the data:
$cur->find_eq('jezek');
while (my @data = $cur->fetch) { # do something
Supported index formats
The following table summarizes which formats are supproted by
XBase::Index. If the field says something else that Yes, I welcome
testers and offers of example index files.
Reading of index files -- types supported by XBase::Index
type string numeric date
----------------------------------------------------------
ndx Yes Yes Yes (you need to
convert to Julian)
ntx Yes Yes Untested
idx Untested Untested Untested
(but should be pretty usable)
mdx Untested Untested Untested
cdx Yes Yes Untested
Writing of index files -- not supported untill the reading
is stable enough.
So if you have access to an index file that is untested or unsupported
and you care about support of these formats, contact me. If you are
able to actually generate those files on request, the better because I
may need specific file size or type to check something. If the file
format you work with is supported, I still appreciate a report that it
really works for you.
Please note that there is very little documentation about the file
formats and the work on XBase::Index is heavilly based on making
assumption based on real life data. Also, the documentation is often
wrong or only describing some format variations but not the others. I
personally do not need the index support but am more than happy to make
it a reality for you. So I need your help -- contact me if it doesn't
work for you and offer me your files for testing. Mentioning word XBase
somewhere in the Subject line will get you (hopefully ;-) fast
response. Mentioning work Help or similar stupidity will probably make
my filters to consider your email as spam. Help yourself by making my
life easier in helping you.
Programmer's notes
Programmers might find the following information usefull when trying to
debug XBase::Index from their files:
The XBase::Index module contains the basic XBase::Index package and
also packages XBase::ndx, XBase::ntx, XBase::idx, XBase::mdx and
XBase::cdx, and for each of these also a package
XBase::index_type::Page. Reading the file goes like this: you create as
object calling either new XBase::Index or new XBase::ndx (or whatever
the index type is). This can also be done behind the scenes, for
example XBase::prepare_select_with_index calls new XBase::Index. The
index file is opened using the XBase::Base::new/open and then the
XBase::index_type::read_header is called. This function fills the basic
data fields of the object from the header of the file. The new method
returns the object corresponding to the index type.
Then you probably want to do $index->prepare_select or
$index->prepare_select_eq, that would possition you just before record
equal or greater than the parameter (record in the index file, that
is). Then you do a series of fetch'es that return next pair of (key,
pointer_to_dbf). Behind the scenes, prepare_select_eq or fetch call
XBase::Index::get_record which in turn calls
XBase::index_type::Page::new. From the index file perspective, the
atomic item in the file is one index page (or block, or whatever you
call it). The XBase::index_type::Page::new reads the block of data from
the file and parses the information in the page -- pages have more or
less complex structures. Page::new fills the structure, so that the
fetch calls can easily check what values are in the page.
For some examples, please see eg/use_index in the distribution
directory.
VERSION
1.02
AVAILABLE FROM
http://www.adelton.com/perl/DBD-XBase/
AUTHOR
(c) 1998--2011 Jan Pazdziora.
SEE ALSOXBase(3), XBase::FAQ(3)perl v5.14.1 2011-03-03 XBase::Index(3)
