MyConText - Indexing documents with MySQL as storage


    use MyConText;
    use DBI;
    # connect to database (regular DBI)
    my $dbh = DBI->connect('dbi:mysql:database', 'user', 'passwd');
    # create a new index
    my $ctx = MyConText->create($dbh, 'ctx_web_1',
                'frontend' => 'string', 'backend' => 'blob');
    # or open existing one
    # my $ctx = MyConText->open($dbh, 'ctx_web_1');

    # index documents
    $ctx->index_document('krtek', 'krtek leze pod zemi');
    $ctx->index_document('jezek', 'Jezek ma ostre bodliny.');

    # search for matches
    my @documents = $ctx->contains('krtek');
    my @docs = $ctx->econtains('+krtek', '-Jezek');


MyConText is a pure man's solution for indexing contents of documents. It uses the MySQL database to store the information about words and documents and provides Perl interface for indexing new documents, making changes and searching for matches. For MyConText, a document is nearly anything -- Perl scalar, file, Web document, database field.

The basic style of interface is shown above. What you need is a MySQL database and a DBI with DBD::mysql. Then you create a MyConText index -- a set of tables that maintain all necessary information. Once created it can be accessed many times, either for updating the index (adding documents) or searching.

MyConText uses one basic table to store parameters of the index. Second table is used to store the actual information about documents and words, and depending on the type of the index (specified during index creation) there may be more tables to store additional information (like conversion from external string names (eg. URL's) to internal numeric form). For a user, these internal thingies and internal behaviour of the index are not important. The important part is the API, the methods to index document and ask questions about words in documents. However, certain understanding of how it all works may be usefull when you are deciding if this module is for you and what type of index will best suit your needs.


From the user, application point of view, the MyConText index stores documents that are named in a certain way, allows adding new documents, and provides methods to ask: "give me list of names of documents that contain this list of words". The MyConText index doesn't store the documents itself. Instead, it stores information about words in the documents in such a structured way that it makes easy and fast to look up what documents contain certain words and return names of the documents.

MyConText provides a couple of predefined frontend classes that specify various types of documents (and the way they relate to their names).


By default, user specifies the integer number of the document and the content (body) of the document. The code would for example read

        $ctx->index_document(53, 'zastavujeme vyplaty vkladu');

and MyConText will remember that the document 53 contains three words. When looking for all documents containing word (string) vklad, a call

        my @docs = $ctx->contains('vklad%');

would return numbers of all documents containing words starting with 'vklad', 53 among them.

So here it's user's responsibility to maintain a relation between the document numbers and their content, to know that a document 53 is about vklady. Perhaps the documents are already stored somewhere and have inique numeric id.


Frontend string allows the user to specify the names of the documents as strings, instead of numbers. Still the user has to specify both the name of the document and the content:

                        'Odstrante z dosadu deti!');

After that,


will return 'upozorneni' as one of the names of documents with word 'deti' in it.


To index files, use the frontend file. Here the content of the document is clearly the content of the file specified by the filename, so in a call to index_document, only the name is needed -- the content of the file is read by the MyConText transparently:

        my @files = $ctx->contains('penguin');

Web document can be indexed by the frontend url. MyConText uses LWP to get the document and then parses it normally:


You can have a MyConText index that indexes char or blob fields in MySQL table. Since MySQL doesn't support triggers, you have to call the index_document method of MyConText any time something changes in the table. So the sequence probably will be

        $dbh->do('insert into the_table (id, data, other_fields)
                values (?, ?, ?)', {}, $name, $data, $date_or_something);

When calling contains, the id (name) of the record will be returned. If the id in the_table is numeric, it's directly used as the internal numeric id, otherwise a string's way of converting the id to numeric form is used.

The structure of MyConText is very flexible and adding new frontend (what will be indexed) is very easy.


While frontend specifies what is indexed and how the user sees the collection of documents, backend is about low level database way of actually storing the information in the tables. Three types are available:


For each word, a blob holding list of all documents containing that word is stored in the table, with the count (number of occurencies) associated with each document number. That makes it for very compact storage. Since the document names (for example URL) are internally converted to numbers, storing and fetching the data is fast. However, updating the information is very slow, since information concerning one document is spread across all table, without any direct database access. Updating a document (or merely reindexing it) requires update of all blobs, which is slow.

The list of documents is stored sorted by document name so that fetching an information about a document for one word is relatively easy, still a need to update (or at least scan) all records in the table makes this storage unsuitable for collections of documents that often change.


The column backend stores a word/document pair in database fields, indexing both, thus allowing both fast retrieval and updates -- it's easy to delete all records describing one document and insert new ones. However, the database indexes that have to be maintained are large.

Both blob and column backends only store a count -- number of occurencies of the word in the document (and even this can be switched off, yielding just a yes/no information about the word's presence). This allows questions like

        all documents containing words 'voda' or 'Mattoni'
                but not a word 'kyselka'

but you cannot ask whether a document contains a phrase 'kyselka Mattoni' because such information is not maintained by these types of backends.


To allow phrase matching, a phrase backend is available. For each word and document number it stores a blob of lists of positions of the word in the document. A query

        $ctx->contains('kyselk%', 'Mattoni');

then only returns those documents (document names/numbers) where word kyselka (or kyselky, or so) is just before word Mattoni.

Mixing frontends and backends

Any frontend can be used with any backend in one MyConText index. You can index Web documents with url frontend and phrase backend to be able to find phrases in the documents. And you can use the default, number based document scheme with blob backend to use the disk space as efficiently as possible -- this is usefull for example for mailing-list archives, where we need to index huge number of documents that do not change at all.

Finding optimal combination is very important and may require some analysis of the document collection and manipulation, as well as the speed and storage requirements. Benchmarking on actual target platform is very usefull during the design phase.


The following methods are available on the user side as MyConText API.

        my $ctx = MyConText->create($dbh, $index_name, %opts);

The class method create creates index of given name (the name of the index is the name of its basic parameter table) and all necessary tables, returns an object -- newly created index. The options that may be specified after the index name define the frontend and backend types, storage parameters (how many bits for what values), etc. See below for list of create options and discussion of their use.

        my $ctx = MyConText->open($dbh, $index_name);

Opens and returns object, accessing specifies MyConText index. Since all the index parameters and information are stored in the $index_name table (including names of all other needed tables), the database handler and the name of the parameter table are the only needed arguments.

        $ctx->index_document(45, 'Sleva pri nakupu stribra.');

For the default and string frontends, two arguments are expected -- the name (number or string) of the document and its content. For file and url frontends only the name of the document is needed. The method returns number of words indexed (subject to wild change).


Removes information about document from the index. Note that for blob backend this is very time consuming process.

        my @docs = $ctx->contains('sleva', 'strib%');

Returns list of names (numbers or strings, depending on the frontend) of documents that contain some of specified words.

        my @docs = $ctx->contains('sleva', '+strib%', '-zlato');

Econtains stands for extended contains and allows words to be prefixed by plus or minus signs to specify that the word must or mustn't be present in the document for it to match.

contains_hashref, econtains_hashref

Similar to contains and econtains, only instead of list of document names, there methods return a hash reference to a hash where keys are the document names and values are the number of occurencies of the words.


Removes all tables associated with the index, including the base parameter table. Effectivelly destroying the index form the database.


Here we list the options that may be passed to MyConText->create call. These allow to specify the style and storage parameters in great detail.


The backend type, default blob, possible values blob, column and phrase (see above for explanation).


The frontend type. The default frontend requires the user to specify numeric id of the document together with the content of the document, other possible values are string, file and url (see above for more info).


Maximum length of words that may be indexed, default 30.


Name of the table where the actual data about word/document relation is stored. By default, the name of the index (of the base table) with _data suffix is used.


Any frontend that uses strings as names of documents needs to maintain a conversion table from these names to internal integer ids. This value specifies maximum length of these string names (URLs, file names, ...).


Only for blob backend. When looking for information about specific document in the list stored in the blob, the blob backend uses division of interval to find the correct place in the blob. When the interval gets equal or shorter that this value, all values are fetched from the database and the final search is done in Perl code sequentially.


With column or phase backends, MyConText maintains a numeric id for each word to optimize the space requirements. The word_id_bits parameter specifies the number of bits to reserve for this conversion and thus effectively limits number of distinct words that may be indexed. The default is 16 bits and possible values are 8, 16, 24 or 32 bits.


Name of the table that holds conversion from words to their numeric id (for column and phrase backends). By default is the name of the index with _words suffix.


A number of bits to hold a numeric id of the document (that is either provided by the user (with default frontend) or generated by the module to accomplish the conversion from the string name of the document). This value limits the maximum number of documents to hold. The default is 16 bits and possible values are 8, 16 and 32 bits for blob backend and 8, 16, 24 and 32 bits for column and phrase backends.


Name of the table that holds conversion from string names of documents to their numeric id, by default the name of the index with _docid suffix.


Number of bits reserved for storing number of occurencies of each word in the document. The default is 8 and possible values are the same as with doc_id_bits.


With phrase backend, MyConText stores positions of each word of the documents. This value specifies how much space should be reserved for this purpose. The default is 32 bits and possible values are 8, 16 or 32 bits. This value limits the maximum number of words of each document that can be stored.


MyConText allows the user to provide any Perl code that will be used to split the content of the document to words. The code will be evalled inside of the MyConText code. The default is

        $data =~ /(\w{2,$word_length})/g

and shows that the input is stored in the variable $data and the code may access any other variable available in the perl_and_index_data_* methods (see source), especially $word_length to get the maximum length of words and $backend to get the backend object.

The default value also shows that by default, the minimum length of words indexed is 2.


The output words of splitter (and also any parameter of (e)contains* methods) are send to filter that may do further processing. Filter is again a Perl code, the default is

        map { lc $_ }

showing that the filter operates on input list and by default does conversion to lowercase (yielding case insensitive index).


Because user defined splitter or filter may depend on other things that it is reasonable to set before the actual procession of words, you can use yet another Perl hook to set things up. The default is

        use locale

For table frontend; this is the name of the table that will be indexed.


For table frontend; this is the name of the column in the table_name that contains the documents -- data to be indexed. It can also have a form table.column that will be used if the table_name option is not specified.


For table frontend; this is the name of the field in table_name that holds names (ids) of the records. If not specified, a field that has primary key on it is used. If this field is numeric, it's values are directly used as identifiers, otherwise a conversion to numeric values is made.


The create and open methods return the MyConText object on success, upon failure they return undef and set error message in $MyConText::errstr variable.

All other methods return reasonable (documented above) value on success, failure is signalized by unreasonable (typically undef or null) return value; the error message may then be retrieved by $ctx->errstr method call.


This documentation describes MyConText module version 0.49.


Error handling needs more polishing.

We do not check if the stored values are larger that specified by the *_bits parameters.

No CGI administration tool at the moment.

Econtains doesn't work with phrase backend.

No scoring algorithm implemented.

No support for stop words at the moment.


(c) 1999 Jan Pazdziora,, at Faculty of Informatics, Masaryk University in Brno, Czech Republic

All rights reserved. This package is free software; you can redistribute it and/or modify it under the same terms as Perl itself.


DBI(3), mycontextadmin(1).

OTHER PRODUCTS and why I've written this module

I'm aware of DBIx::TextIndex module and about UdmSearch utility, and about htdig and glimpse on the non-database side of the world.

To me, using a database gives reasonable maintenance benefits. With products that use their own files to store the information (even if the storage algorithms are efficient and well thought of), you always struggle with permissions on files and directories for various users, with files that somebody accidently deleted or mungled, and making the index available remotely is not trivial.

That's why I've wanted a module that will use a database as a storage backend. With MySQL, you get remote access and access control for free, and on many web servers MySQL is part of the standard equipment. So using it for text indexes seemed natural.

However, existing DBIx::TextIndex and UdmSearch are too narrow-aimed to me. The first only supports indexing of data that is stored in the database, but you may not always want or need to store the documents in the database as well. The UdmSearch on the other hand is only for web documents, making it unsuitable for indexing mailing-list archives or local data.

I believe that MyConText is reasonably flexible and still very efficient. It doesn't enforce its own idea of what is good for you -- the number of options is big and you can always extend the module with your own backend of frontend if you feel that those provided are not sufficient. Or you can extend existing by adding one or two parameters that will add new features. Of course, patches are always welcome. MyConText is a tool that can be deployed in many projects. It's not a complete environment since different people have different needs. On the other hand, the methods that it provides make it easy to build a complete solution on top of this in very short course of time.

I was primarily inspired by the ConText cartrige of Oracle server. Since MySQL doesn't support triggers, it showed up that Perl interface will be needed. Of course, porting this module to (for example) PostgreSQL should be easy, so different name is probably needed. On the other hand, the code is sometimes very MySQL specific to make the module work efficiently, so I didn't want a name that would suggest that it's a generic tool that will work with any SQL database.