Dimensional modeling and metadata
 


Dimensional modeling (DM) is the name of a set of techniques and concepts used in data warehouse design. It is considered to be different from entity-relationship modeling (ER). Dimensional Modeling does not necessarily involve a relational database. The same modeling approach, at the logical level, can be used for any physical form, such as multidimensional database or even flat files. According to data warehousing consultant Ralph Kimball, DM is a design technique for databases intended to support end-user queries in a data warehouse. It is oriented around understandability and performance. According to him, although transaction-oriented ER is very useful for the transaction capture, it should be avoided for end-user delivery.

Dimensional modeling always uses the concepts of facts (measures), and dimensions (context). Facts are typically (but not always) numeric values that can be aggregated, and dimensions are groups of hierarchies and descriptors that define the facts. For example, sales amount is a fact; timestamp, product, register#, store#, etc. are elements of dimensions. Dimensional models are built by business process area, e.g. store sales, inventory, claims, etc. Because the different business process areas share some but not all dimensions, efficiency in design, operation, and consistency, is achieved using conformed dimensions, i.e. using one copy of the shared dimension across subject areas. The term "conformed dimensions" was originated by Ralph Kimball.

Dimensional modeling process:

The dimensional model is built on a star-like schema, with dimensions surrounding the fact table. To build the schema, the following design model is used:

  1. Choose the business process
  2. Declare the grain
  3. Identify the dimensions
  4. Identify the fact
Choose the business process

The process of dimensional modeling builds on a 4-step design method that helps to ensure the usability of the dimensional model and the use of the data warehouse. The basics in the design build on the actual business process which the data warehouse should cover. Therefore the first step in the model is to describe the business process which the model builds on. This could for instance be a sales situation in a retail store. To describe the business process, one can choose to do this in plain text or use basic Business Process Modeling Notation (BPMN) or other design guides like the Unified Modeling Language (UML).

Declare the grain

After describing the Business Process, the next step in the design is to declare the grain of the model. The grain of the model is the exact description of what the dimensional model should be focusing on. This could for instance be “An individual line item on a customer slip from a retail store”. To clarify what the grain means, you should pick the central process and describe it with one sentence. Furthermore the grain (sentence) is what you are going to build your dimensions and fact table from. You might find it necessary to go back to this step to alter the grain due to new information gained on what your model is supposed to be able to deliver.

Identify the dimensions

The third step in the design process is to define the dimensions of the model. The dimensions must be defined within the grain from the second step of the 4-step process. Dimensions are the foundation of the fact table, and is where the data for the fact table is collected. Typically dimensions are nouns like date, store, inventory etc. These dimensions are where all the data is stored. For example, the date dimension could contain data such as year, month and weekday.

Identify the facts

After defining the dimensions, the next step in the process is to make keys for the fact table. This step is to identify the numeric facts that will populate each fact table row. This step is closely related to the business users of the system, since this is where they get access to data stored in the data warehouse. Therefore most of the fact table rows are numerical, additive figures such as quantity or cost per unit, etc.

Dimension Normalization

Dimensional normalization or snowflaking removes redundant attributes, which are known in the normal flatten de-normalized dimensions. Dimensions are strictly joined together in sub dimensions.

Snowflaking has an influence on the data structure that differs from many philosophies of data warehouses. Single data (fact) table surrounded by multiple descriptive (dimension) tables

Developers often don't normalize dimensions due to several facts:

  1. Normalization makes the data structure more complex
  2. Performance can be slower, due to the many joins between tables
  3. The space savings are minimal
  4. The use of bitmap indexes can't be done
  5. Query Performance, 3NF databases suffer from performance problems when aggregating or retrieving many dimensional values that may require analysis. If you are only going to do operational reports then you may be able to get by with 3NF because your operational user will be looking for very fine grain data.

There are some arguments on why normalization can be useful. It can be an advantage when part of hierarchy is common to more than one dimension. For example, a geographic dimension may be reusable because both the customer and supplier dimensions use it.

Benefits of dimensional modeling

Benefits of the dimensional modeling are following:

  • Understandability - Compared to the normalized model, the dimensional model is easier to understand and more intuitive. In dimensional models, information is grouped into coherent business categories or dimensions, making it easier to read and interpret. Simplicity also allows software to navigate databases efficiently. In normalized models, data is divided into many discrete entities and even a simple business process might result in dozens of tables joined together in a complex way.
  • Query performance - Dimensional models are more denormalized and optimized for data querying, while normalized models seek to eliminate data redundancies and are optimized for transaction loading and updating. The predictable framework of a dimensional model allows the database to make strong assumptions about the data that aid in performance. Each dimension is an equivalent entry point into the fact table, and this symmetrical structure allows effective handling of complex queries. Query optimization for star join databases is simple, predictable, and controllable.
  • Extensibility - Dimensional models are extensible and easily accommodate unexpected new data. Existing tables can be changed in place either by simply adding new data rows into the table or executing SQL alter table commands. No queries or other applications that sit on top of the Warehouse need to be reprogrammed to accommodate changes. Old queries and applications continue to run without yielding different results. But in normalized models each modification should be considered carefully, because of the complex dependencies between database tables.

The term metadata is ambiguous, as it is used for two fundamentally different concepts (types). Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at design time the application contains no data. In this case the correct description would be "data about the containers of data". Descriptive metadata, on the other hand, is about individual instances of application data, the data content. In this case, a useful description (resulting in a disambiguating neologism) would be "data about data content" or "content about content" thus metacontent. Descriptive, Guide and the National Information Standards Organization concept of administrative metadata are all subtypes of metacontent.

Metadata (metacontent) is traditionally found in the card catalogs of libraries. As information has become increasingly digital, metadata is also used to describe digital data using metadata standards specific to a particular discipline. By describing the contents and context of data files, the quality of the original data/files is greatly increased. For example, a webpage may include metadata specifying what language it is written in, what tools were used to create it, and where to go for more on the subject, allowing browsers to automatically improve the experience of users.

 

Metadata (metacontent) is defined as data providing information about one or more aspects of the data, such as:

  • Means of creation of the data
  • Purpose of the data
  • Time and date of creation
  • Creator or author of data
  • Location on a computer network where the data was created
  • Standards used

For example, a digital image may include metadata that describes how large the picture is, the color depth, the image resolution, when the image was created, and other data. A text document's metadata may contain information about how long the document is, who the author is, when the document was written, and a short summary of the document.

Metadata is data. As such, metadata can be stored and managed in a database, often called a Metadata registry or Metadata repository. However, without context and a point of reference, it can be impossible to identify metadata just by looking at it. For example: by itself, a database containing several numbers, all 13 digits long could be the results of calculations or a list of numbers to plug into an equation - without any other context, the numbers themselves can be perceived as the data. But if given the context that this database is a log of a book collection, those 13-digit numbers may now be ISBNs - information that refers to the book, but is not itself the information within the book.

The term "metadata" was coined in 1968 by Philip Bagley, in his book "Extension of programming language concepts" where it is clear that he uses the term in the ISO 11179 "traditional" sense, which is "structural metadata" i.e. "data about the containers of data"; rather than the alternate sense "content about individual instances of data content" or metacontent, the type of data usually found in library catalogues. Since then the fields of information management, information science, information technology, librarianship and GIS have widely adopted the term. In these fields the word metadata is defined as "data about data". While this is the generally accepted definition, various disciplines have adopted their own more specific explanation and uses of the term.

Libraries

Metadata has been used in various forms as a means of cataloging archived information. The Dewey Decimal System employed by libraries for the classification of library materials is an early example of metadata usage. Library catalogues used 3x5 inch cards to display a book's title, author, subject matter, and a brief plot synopsis along with an abbreviated alpha-numeric identification system which indicated the physical location of the book within the library's shelves. Such data helps classify, aggregate, identify, and locate a particular book. Another form of older metadata collection is the use by US Census Bureau of what is known as the "Long Form." The Long Form asks questions that are used to create demographic data to create patterns and to find patterns of distribution. For the purposes of this article, an "object" refers to any of the following:

  • A physical item such as a book, CD, DVD, map, chair, table, flower pot, etc.
  • An electronic file such as a digital image, digital photo, document, program file, database table, etc.

Photographs

Metadata may be written into a digital photo file that will identify who owns it, copyright & contact information, what camera created the file, along with exposure information and descriptive information such as keywords about the photo, making the file searchable on the computer and/or the Internet. Some metadata is written by the camera and some is input by the photographer and/or software after downloading to a computer. However, not all digital cameras enable you to edit metadata; this functionality has been available on most Nikon DSLRs since the Nikon D3 and on most new Canon cameras since the Canon EOS 7D.

Photographic Metadata Standards are governed by organizations that develop the following standards. They include, but are not limited to:

  • IPTC Information Interchange Model IIM (International Press Telecommunications Council),
  • IPTC Core Schema for XMP
  • XMP – Extensible Metadata Platform (an ISO standard)
  • Exif – Exchangeable image file format, Maintained by CIPA (Camera & Imaging Products Association) and published by JEITA (Japan Electronics and Information Technology Industries Association)
  • Dublin Core (Dublin Core Metadata Initiative – DCMI)
  • PLUS (Picture Licensing Universal System).

Video

Metadata is particularly useful in video, where information about its contents (such as transcripts of conversations and text descriptions of its scenes) are not directly understandable by a computer, but where efficient search is desirable.

Web pages

Web pages often include metadata in the form of meta tags. Description and keywords meta tags are commonly used to describe the Web page's content. Most search engines use this data when adding pages to their search index.

Creation of metadata

Metadata can be created either by automated information processing or by manual work. Elementary metadata captured by computers can include information about when a file was created, who created it, when it was last updated, file size and file extension.

Metadata types

The metadata application is manyfold covering a large variety of fields of application there are nothing but specialised and well accepted models to specify types of metadata. Bretheron & Singley (1994) distinguish between two distinct classes: structural/control metadata and guide metadata. Structural metadata is used to describe the structure of computer systems such as tables, columns and indexes. Guide metadata is used to help humans find specific items and is usually expressed as a set of keywords in a natural language. According to Ralph Kimball metadata can be divided into 2 similar categories—Technical metadata and Business metadata. Technical metadata correspond to internal metadata, business metadata to external metadata. Kimball adds a third category named Process metadata. On the other hand, NISO distinguishes between three types of metadata: descriptive, structural and administrative. Descriptive metadata is the information used to search and locate an object such as title, author, subjects, keywords, publisher; structural metadata gives a description of how the components of the object are organised; and administrative metadata refers to the technical information including file type. Two sub-types of administrative metadata are rights management metadata and preservation metadata.


Metadata is one of the important keys to the success of the data warehousing and business intelligence effort.  Metadata management answers these questions:

  • What is Metadata?
  • How can Metadata be Managed?
  • Extracting Metadata from Legacy Systems

What is Metadata?

 

Metadata is your control panel to the data warehouse.  It is data that describes the data warehousing and business intelligence system:

  • Reports
  • Cubes
  • Tables (Records, Segments, Entities, etc.)
  • Columns (Fields, Attributes, Data Elements, etc.)
  • Keys
  • Indexes

Metadata is often used to control the handling of data and describes:

  • Rules
  • Transformations
  • Aggregations
  • Mappings

The power of metadata is that enables data warehousing personnel to develop and control the system without writing code in languages such as: Java, C# or Visual Basic.  This saves time and money both in the initial set up and on going management.

Data Warehouse Metadata

Data warehousing has specific metadata requirements.  Metadata that describes tables typically includes:

  • Physical Name
  • Logical Name
  • Type: Fact, Dimension, Bridge
  • Role: Legacy, OLTP, Stage,
  • DBMS: DB2, Informix, MS SQL Server, Oracle, Sybase
  • Location
  • Definition
  • Notes

Metadata describes columns within tables:

  • Physical Name
  • Logical Name
  • Order in Table
  • Datatype
  • Length
  • Decimal Positions
  • Nullable/Required
  • Default Value
  • Edit Rules
  • Definition
  • Notes

How can Data Warehousing Metadata be Managed?

Data warehousing and business intelligence metadata is best managed through a combination of people, process and tools.

The people side requires that people be trained in the importance and use of metadata.  They need to understand how and when to use tools as well as the benefits to be gained through metadata.

The process side incorporates metadata management into the data warehousing and business intelligence life cycle.  As the life cycle progresses metadata is entered into the appropriate tool and stored in a metadata repository for further use.

Metadata can be managed through individual tools:

  • Metadata manager / repository
  • Metadata extract tools
  • Data modeling
  • ETL
  • BI Reporting

Metadata Manager / Repository

Metadata can be managed through a shared repository that combines information from multiple sources.

The metadata manager can be purchased as a software package or built as "home grown" system.  Many organizations start with a spreadsheet containing data definitions and then grow to a more sophisticated approach.

Extracting Metadata from Input Sources

Metadata can be obtained through a manual process of keying in metadata or through automated processes. Scanners can extract metadata from text such as SQL DDL or COBOL programs. Other tools can directly access metadata through SQL catalogs and other metadata sources.

Picking the appropriate metadata extract tools is a key part of metadata management.

Many data modeling tools include a metadata extract capability - otherwise known as "reverse engineering".  Through this tool, database information about tables and columns can be extracted.  The information can then be exported from the data modeling tool to the metadata manager.