VDM Public

Every TY (this year's) day, week, period or quarter, has a corresponding LY (last year) day, week, period or quarter. This implicit mapping is straightforward for 52-week years. The acid test, however, is in handling 53-week years.

There are two common approaches for defining the TY-LY relationship:

1. The TY and LY have the same day number (1-371) and week number (1-53.) Higher levels, i.e. periods and quarters, are made up of the same days and weeks following a 4-4-5 pattern, so any TY day belongs to the same period as its corresponding LY day. The only oddity is that we have to skip the 53d week in year-to-year comparisons.

The following configuration is based on these general guidelines. It covers data partition nodes, coordinator/administration node, application server and spare. We propose initially to configure all nodes as production data servers for flexibility and expandability. QA and Test nodes can be built with higher density and less I/O bandwidth per core. The following diagram captures the data path components, unit rates and relationships driving their configuration selections and sizing.

When it comes to time versioning of objects in a dimension, VDM enforces the following two basic rules:

• Object identity (ID) is separate from version identifier (VID). A version or an object is identified by the combination {ID+VID}
• An object cannot have more than one effective version at a given point in time

There are three methods by which the versioning intervals are assigned:

Dense versioning

Each object of the dimensions has exactly one version effective for any point in time from the beginning-of-time to the end-of-time.

Semi-dense versioning

The original article was published in the October/November 1992 issue of the Relational Journal, and proposed a way to effectively make ancestor-descendent searches in hierarchies working around relational database limitations, i.e. the lack of recursion capability. Since then variants on the technique appeared at trade journals and books. An excellent explanation is included in the "SQL for Smarties" by J. Celco as well as his more recent "Trees and Hierarchies in SQL fr Smarties".