1XBase::Index(3)       User Contributed Perl Documentation      XBase::Index(3)
2
3
4

NAME

6       XBase::Index - base class for the index files for dbf
7

SYNOPSIS

9               use XBase;
10               my $table = new XBase "data.dbf";
11               my $cur = $table->prepare_select_with_index("id.ndx",
12                       "ID", "NAME);
13               $cur->find_eq(1097);
14
15               while (my @data = $cur->fetch()) {
16                       last if $data[0] != 1097;
17                       print "@data\n";
18               }
19
20       This is a snippet of code to print ID and NAME fields from dbf data.dbf
21       where ID equals 1097. Provided you have index on ID in file id.ndx. You
22       can use the same code for ntx and idx index files.  For the cdx and
23       mdx, the prepare_select call would be
24
25               prepare_select_with_index(['rooms.cdx', 'ROOMNAME'])
26
27       so instead of plain filename you specify an arrayref with filename and
28       an index tag in that file. The reason is that cdx and mdx can contain
29       multiple indexes in one file and you have to distinguish, which you
30       want to use.
31

DESCRIPTION

33       The module XBase::Index is a collection of packages to provide index
34       support for XBase-like dbf database files.
35
36       An index file is generaly a file that holds values of certain database
37       field or expression in sorted order, together with the record number
38       that the record occupies in the dbf file. So when you search for a
39       record with some value, you first search in this sorted list and once
40       you have the record number in the dbf, you directly fetch the record
41       from dbf.
42
43   What indexes do
44       To make the searching in this ordered list fast, it's generally
45       organized as a tree -- it starts with a root page with records that
46       point to pages at lower level, etc., until leaf pages where the pointer
47       is no longer a pointer to the index but to the dbf. When you search for
48       a record in the index file, you fetch the root page and scan it
49       (lineary) until you find key value that is equal or grater than that
50       you are looking for. That way you've avoided reading all pages
51       describing the values that are lower. Here you descend one level, fetch
52       the page and again search the list of keys in that page. And you repeat
53       this process until you get to the leaf (lowest) level and here you
54       finaly find a pointer to the dbf. XBase::Index does this for you.
55
56       Some of the formats also support multiple indexes in one file --
57       usually there is one top level index that for different field values
58       points to different root pages in the index file (so called tags).
59
60       XBase::Index supports (or aims to support) the following index formats:
61       ndx, ntx, mdx, cdx and idx. They differ in a way they store the keys
62       and pointers but the idea is always the same: make a tree of pages,
63       where the page contains keys and pointer either to pages at lower
64       levels, or to dbf (or both). XBase::Index only supports read only
65       access to the index fields at the moment (and if you need writing them
66       as well, follow reading because we need to have the reading support
67       stable before I get to work on updating the indexes).
68
69   Testing your index file (and XBase::Index)
70       You can test your index using the indexdump script in the main
71       directory of the DBD::XBase distribution (I mean test XBase::Index on
72       correct index data, not testing corrupted index file, of course ;-)
73       Just run
74
75               ./indexdump ~/path/index.ndx
76               ./indexdump ~/path/index.cdx tag_name
77
78       or
79
80               perl -Ilib ./indexdump ~/path/index.cdx tag_name
81
82       if you haven't installed this version of XBase.pm/DBD::XBase yet. You
83       should get the content of the index file. On each row, there is the key
84       value and a record number of the record in the dbf file. Let me know if
85       you get results different from those you expect. I'd probably ask you
86       to send me the index file (and possibly the dbf file as well), so that
87       I can debug the problem.
88
89       The index file is (as already noted) a complement to a dbf file. Index
90       file without a dbf doesn't make much sense because the only thing that
91       you can get from it is the record number in the dbf file, not the
92       actual data. But it makes sense to test -- dump the content of the
93       index to see if the sequence is OK.
94
95       The index formats usually distinguish between numeric and character
96       data. Some of the file formats include the information about the type
97       in the index file, other depend on the dbf file. Since with indexdump
98       we only look at the index file, you may need to specify the -type
99       option to indexdump if it complains that it doesn't know the data type
100       of the values (this is the case with cdx at least). The possible values
101       are num, char and date and the call would be like
102
103               ./indexdump -type=num ~/path/index.cdx tag_name
104
105       (this -type option may not work with all index formats at the moment --
106       will be fixed and patches always welcome).
107
108       You can use "-ddebug" option to indexdump to see how pages are fetched
109       and decoded, or run debugger to see the calls and parsing.
110
111   Using the index files to speed up searches in dbf
112       The syntax for using the index files to access data in the dbf file is
113       generally
114
115               my $table = new XBase "tablename";
116                       # or any other arguments to get the XBase object
117                       # see XBase(3)
118               my $cur = $table->prepare_select_with_index("indexfile",
119                       "list", "of", "fields", "to", "return");
120
121       or
122
123               my $cur = $table->prepare_select_with_index(
124                       [ "indexfile_with_tags", "tag_name" ],
125                       "list", "of", "fields", "to", "return");
126
127       where we specify the tag in the index file (this is necessary with cdx
128       and mdx). After we have the cursor, we can search to given record and
129       start fetching the data:
130
131               $cur->find_eq('jezek');
132               while (my @data = $cur->fetch) { # do something
133
134   Supported index formats
135       The following table summarizes which formats are supproted by
136       XBase::Index. If the field says something else that Yes, I welcome
137       testers and offers of example index files.
138
139         Reading of index files -- types supported by XBase::Index
140
141         type  string          numeric         date
142         ----------------------------------------------------------
143         ndx   Yes             Yes             Yes (you need to
144                                               convert to Julian)
145
146         ntx   Yes             Yes             Untested
147
148         idx   Untested        Untested        Untested
149               (but should be pretty usable)
150
151         mdx   Untested        Untested        Untested
152
153         cdx   Yes             Yes             Untested
154
155
156         Writing of index files -- not supported untill the reading
157         is stable enough.
158
159       So if you have access to an index file that is untested or unsupported
160       and you care about support of these formats, contact me. If you are
161       able to actually generate those files on request, the better because I
162       may need specific file size or type to check something. If the file
163       format you work with is supported, I still appreciate a report that it
164       really works for you.
165
166       Please note that there is very little documentation about the file
167       formats and the work on XBase::Index is heavilly based on making
168       assumption based on real life data. Also, the documentation is often
169       wrong or only describing some format variations but not the others.  I
170       personally do not need the index support but am more than happy to make
171       it a reality for you. So I need your help -- contact me if it doesn't
172       work for you and offer me your files for testing. Mentioning word XBase
173       somewhere in the Subject line will get you (hopefully ;-) fast
174       response. Mentioning work Help or similar stupidity will probably make
175       my filters to consider your email as spam. Help yourself by making my
176       life easier in helping you.
177
178   Programmer's notes
179       Programmers might find the following information usefull when trying to
180       debug XBase::Index from their files:
181
182       The XBase::Index module contains the basic XBase::Index package and
183       also packages XBase::ndx, XBase::ntx, XBase::idx, XBase::mdx and
184       XBase::cdx, and for each of these also a package
185       XBase::index_type::Page. Reading the file goes like this: you create as
186       object calling either new XBase::Index or new XBase::ndx (or whatever
187       the index type is). This can also be done behind the scenes, for
188       example XBase::prepare_select_with_index calls new XBase::Index.  The
189       index file is opened using the XBase::Base::new/open and then the
190       XBase::index_type::read_header is called. This function fills the basic
191       data fields of the object from the header of the file. The new method
192       returns the object corresponding to the index type.
193
194       Then you probably want to do $index->prepare_select or
195       $index->prepare_select_eq, that would possition you just before record
196       equal or greater than the parameter (record in the index file, that
197       is). Then you do a series of fetch'es that return next pair of (key,
198       pointer_to_dbf). Behind the scenes, prepare_select_eq or fetch call
199       XBase::Index::get_record which in turn calls
200       XBase::index_type::Page::new. From the index file perspective, the
201       atomic item in the file is one index page (or block, or whatever you
202       call it). The XBase::index_type::Page::new reads the block of data from
203       the file and parses the information in the page -- pages have more or
204       less complex structures. Page::new fills the structure, so that the
205       fetch calls can easily check what values are in the page.
206
207       For some examples, please see eg/use_index in the distribution
208       directory.
209

VERSION

211       1.05
212

AVAILABLE FROM

214       http://www.adelton.com/perl/DBD-XBase/
215

AUTHOR

217       (c) 1998--2013 Jan Pazdziora.
218

SEE ALSO

220       XBase(3), XBase::FAQ(3)
221
222
223
224perl v5.30.1                      2020-01-29                   XBase::Index(3)
Impressum