Dimension (data warehouse)

A slowly changing dimension is a set of data attributes that change slowly over a period of time rather than changing regularly e.g. address or name. These attributes can change over a period of time and that will get combined as a slowly changing dimension. These dimension can be classified in types:[3]

• Type 0 (Retain original): Attributes never change. No history.
• Type 1 (Overwrite): Old values are overwritten with new values for attribute. No history.
• Type 2 (Add new row): A new row is created with either a start date / end date or a version for a new value. This creates history.
• Type 3 (Add new attribute): A new column is created for a new value. History is limited to the number of columns designated for storing historical data.
• Type 4 (Add history table): One table keeps the current value, while the history is saved in a second table.
• Type 5 (Combined Approach 1 + 4): Combination of type 1 and type 4. History is created through a second history table.
• Type 6 (Combined Approach 1 + 2 + 3): Combination of type 1, type 2 and type 3. History is created through separate row and attributes.
• Type 7 (Hybrid Approach): Both surrogate and natural key are used.[4]

A conformed dimension is a set of data attributes that have been physically referenced in multiple database tables using the same key value to refer to the same structure, attributes, domain values, definitions and concepts. A conformed dimension cuts across many facts.

Dimensions are conformed when they are either exactly the same (including keys) or one is a proper subset of the other. Most important, the row headers produced in two different answer sets from the same conformed dimension(s) must be able to match perfectly.'

Conformed dimensions are either identical or strict mathematical subsets of the most granular, detailed dimension. Dimension tables are not conformed if the attributes are labeled differently or contain different values. Conformed dimensions come in several different flavors. At the most basic level, conformed dimensions mean exactly the same thing with every possible fact table to which they are joined. The date dimension table connected to the sales facts is identical to the date dimension connected to the inventory facts.[5]

A junk dimension is a convenient grouping of typically low-cardinality flags and indicators. By creating an abstract dimension, these flags and indicators are removed from the fact table while placing them into a useful dimensional framework.[6] A junk dimension is a dimension table consisting of attributes that do not belong in the fact table or in any of the existing dimension tables. The nature of these attributes is usually text or various flags, e.g. non-generic comments or just simple yes/no or true/false indicators. These kinds of attributes are typically remaining when all the obvious dimensions in the business process have been identified and thus the designer is faced with the challenge of where to put these attributes that do not belong in the other dimensions.

One solution is to create a new dimension for each of the remaining attributes, but due to their nature, it could be necessary to create a vast number of new dimensions resulting in a fact table with a very large number of foreign keys. The designer could also decide to leave the remaining attributes in the fact table but this could make the row length of the table unnecessarily large if, for example, the attribute is a long text string.

The solution to this challenge is to identify all the attributes and then put them into one or several junk dimensions. One junk dimension can hold several true/false or yes/no indicators that have no correlation with each other, so it would be convenient to convert the indicators into a more describing attribute. An example would be an indicator about whether a package had arrived: instead of indicating this as “yes” or “no”, it would be converted into "arrived" or "pending" in the junk dimension. The designer can choose to build the dimension table so it ends up holding all the indicators occurring with every other indicator so that all combinations are covered. This sets up a fixed size for the table itself which would be 2^x rows, where x is the number of indicators. This solution is appropriate in situations where the designer would expect to encounter a lot of different combinations and where the possible combinations are limited to an acceptable level. In a situation where the number of indicators are large, thus creating a very big table or where the designer only expects to encounter a few of the possible combinations, it would be more appropriate to build each row in the junk dimension as new combinations are encountered. To limit the size of the tables, multiple junk dimensions might be appropriate in other situations depending on the correlation between various indicators.

Junk dimensions are also appropriate for placing attributes like non-generic comments from the fact table. Such attributes might consist of data from an optional comment field when a customer places an order and as a result will probably be blank in many cases. Therefore, the junk dimension should contain a single row representing the blanks as a surrogate key that will be used in the fact table for every row returned with a blank comment field.[7]

A degenerate dimension is a key, such as a transaction number, invoice number, ticket number, or bill-of-lading number, that has no attributes and hence does not join to an actual dimension table. Degenerate dimensions are very common when the grain of a fact table represents a single transaction item or line item because the degenerate dimension represents the unique identifier of the parent. Degenerate dimensions often play an integral role in the fact table's primary key.[8]

Dimensions are often recycled for multiple applications within the same database. For instance, a "Date" dimension can be used for "Date of Sale", as well as "Date of Delivery", or "Date of Hire". This is often referred to as a "role-playing dimension". This can be implemented using a view over the same dimension table.

Usually dimension tables do not reference other dimensions via foreign keys. When this happens, the referenced dimension is called an outrigger dimension. Outrigger dimensions should be considered a data warehouse anti-pattern: it is considered a better practice to use some fact tables that relate the two dimensions. [9]

A conformed dimensions is said to be a shrunken dimension when it includes a subset of the rows and/or columns of the original dimension.[10]

A special type of dimension can be used to represent dates with a granularity of a day. Dates would be referenced in a fact table as foreign keys to a date dimension. The date dimension primary key could be a surrogate key or a number using the format YYYYMMDD.

The date dimension can include other attributes like the week of the year, or flags representing work days, holidays, etc. It could also include special rows representing: not known dates, or yet to be defined dates. The date dimension should be initialized with all the required dates, say the next 10 years of dates, or more if required, or past dates if events in the past are handled.

Time instead is usually best represented as a timestamp in the fact table. [11]

Since many fact tables in a data warehouse are time series of observations, one or more date dimensions are often needed. One of the reasons to have date dimensions is to place calendar knowledge in the data warehouse instead of hard-coded in an application. While a simple SQL date/timestamp is useful for providing accurate information about the time a fact was recorded, it can not give information about holidays, fiscal periods, etc. An SQL date/timestamp can still be useful to store in the fact table, as it allows for precise calculations.

Having both the date and time of day in the same dimension, may easily result in a huge dimension with millions of rows. If a high amount of detail is needed it is usually a good idea to split date and time into two or more separate dimensions. A time dimension with a grain of seconds in a day will only have 86400 rows. A more or less detailed grain for date/time dimensions can be chosen depending on needs. As examples, date dimensions can be accurate to year, quarter, month or day and time dimensions can be accurate to hours, minutes or seconds.

As a rule of thumb, time of day dimension should only be created if hierarchical groupings are needed or if there are meaningful textual descriptions for periods of time within the day (ex. “evening rush” or “first shift”).

If the rows in a fact table are coming from several time zones, it might be useful to store date and time in both local time and a standard time. This can be done by having two dimensions for each date/time dimension needed – one for local time, and one for standard time. Storing date/time in both local and standard time, will allow for analysis on when facts are created in a local setting and in a global setting as well. The standard time chosen can be a global standard time (ex. UTC), it can be the local time of the business’ headquarters (ex. CET), or any other time zone that would make sense to use.