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.TH MGINTRO 1 \*(Dt CITRI
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.SH NAME
mgintro \- introduction to the MG system
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.SH DESCRIPTION
The MG (Managing Gigabytes) system is a collection of
programs which comprise a full-text retrieval system.
A full-text retrieval system allows one to create a
database out of some given documents and then do queries
upon it to retrieve any relevant documents.
It is "full-text" in the sense that every word in the
text is indexed and the query operates only on this index 
to do the searching.
.PP
For example, one could have a database on the book,
"Alice in Wonderland." A document could be represented by
each paragraph in the book.
Having built up the "Alice" database, one could do queries
such as "cat alice grin" and retrieve any paragraphs which
match the query. The matching could either be boolean, that
is the retrieved paragraphs contain a boolean expression of
the query terms e.g. "cat alice grin"; or the matching
could be ranked i.e. the most relevant documents to the query
in relevance order, using some standard heuristic measure.
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.SS Motivation
If one wants to find some particular information which
is stored in a computer text file then one has a few alternative
courses of action. One can operate directly on the text files
with utilities such as grep or can process the text files into
some form of database. Grep is generally limited to identifying
lines by matching on regular expressions. If the collection 
of files which grep operates on becomes large, then continual
passes over the entire text on each query becomes expensive.
However, its usage is simple as no auxiliary files must be created.
.PP
A database consists of some data and indexes into that data. By having
indexes one can query a large database quickly. Standard databases
divide the data up into records of fields. This means that the granularity
of search is a field. In a full-text system, such as \*r,
there are no fields 
(or there is an arbitrary sized list of word fields per document) 
and instead every word is indexed. 
Using this method, we can except free-form information and yet be fast on searches.
The next question is what is the overhead of this database.
In \*r most files which are produced are in a compressed form. The
two notable compressed files being the given data and the index, called
an "inverted file". By compressing the files it is possible to have the
size of the database smaller than the size of the source data.
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.SS Typical Usage
The most common use for \*r
has been as a search database on unix mail files.
However, any set of text data can be used, one just needs to determine
what constitutes a document (see 
.BR mgintro++ (1)
).
\*r has also been used on large collections such
as Comact (Commonwealth Acts of Australia) which is around 132 megabytes 
and also on sizes up to around 2 gigabytes for TREC 
(a mixture of collections such as the Wall Street Journal 
and Associated Press).
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.SS Getting Started with \*r
The first thing to do is install the package; 
please follow the INSTALL instructions.
Having done this, it is necessary to set a couple of environment variables.
MGDATA should be set to a directory which is to hold subdirectories for
each database that you build. For example: 
.IP
.B mkdir ~/mgdata; setenv MGDATA ~/mgdata.
.LP
If you want to try out building some sample databases then there is
some sample data such as the "Alice In Wonderland" book. To make sure
this is accessible you should set the environment variable MGSAMPLE.
For example: 
.IP
.B setenv MGSAMPLE ~/mg/SampleData
.LP
Here, "~/mg/SampleData" should contain alice.z .
.PP
To build the Alice database (to be contained in $MGDATA/alice subdirectory), 
type the command 
.IP
.B mgbuild alice 
.LP
Assuming all went well and some status messages 
were printed indicating the build was completed, then type 
.IP
.B mgquery alice 
.LP
to query the database. 
You can type a few words at the prompt, hit return and
some relevant documents, Alice paragraphs, should be retrieved.
Type ".set query ranked" to do ranking queries. Please refer to the 
.BR mgquery (1)
man-page for more information on the commands and options of 
.BR mgquery (1).
.PP
The next thing to do is to use \*r
on a more personal database. If you have
your mail stored in subdirectories of ~/Mail, such as is done if you use the
typical set up of 
.BR elm (1), 
then type 
.IP
.B mgbuild allfiles 
.LP
If, however, you keep all your mail in ~/mbox or ~/sentmail, then type 
.IP
.B mgbuild mailfiles
.LP
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.SH AVAILABILITY
The \*r software for SunOS 4, Solaris, HPUX, and MIPS,
can be ftped from: munnari.oz.au [128.250.1.21] in the directory
/pub/mg. 
.\"--------------------------------------------------------------
.SH SEE ALSO
.na
.BR mgintro++ (1),
.BR mgbuild (1),
.BR mgquery (1)
.br
"Guide To The \*r System", in Appendix A of the book:
.PP
.RS
.nf
Ian H. Witten, Alistair Moffat, and Timothy C. Bell
.I "Managing Gigabytes: Compressing and Indexing Documents and Images"
Van Nostrand Reinhold
1994
xiv + 429 pages
US$54.95
ISBN 0-442-01863-0
Library of Congress catalog number TA1637 .W58 1994.
.fi
.RE