2012-04-30 by Stefan Urbanek

Star Browser, Part 1: Mappings

Star Browser is new aggregation browser in for the Cubes – lightweight Python OLAP Framework. I am going to talk briefly about current state and why new browser is needed. Then I will describe in more details the new browser: how mappings work, how tables are joined. At the end I will mention what will be added soon and what is planned in the future.

Originally I wanted to write one blog post about this, but it was too long, so I am going to split it into three:

Why new browser?

Current denormalized browser is good, but not good enough. Firstly, it has grown into a spaghetti-like structure inside and adding new features is quite difficult. Secondly, it is not immediately clear what is going on inside and not only new users are getting into troubles. For example the mapping of logical to physical is not obvious; denormalization is forced to be used, which is good at the end, but is making OLAP newbies puzzled.

The new browser, called StarBrowser. is half-ready and will fix many of the old decisions with better ones.

Mapping

Cubes provides an analyst's view of dimensions and their attributes by hiding the physical representation of data. One of the most important parts of proper OLAP on top of the relational database is the mapping of physical attributes to logical.

First thing that was implemented in the new browser is proper mapping of logical attributes to physical table columns. For example, take a reference to an attribute name in a dimension product. What is the column of what table in which schema that contains the value of this dimension attribute?

There are two ways how the mapping is being done: implicit and explicit. The simplest, straightforward and most customizable is the explicit way, where the actual column reference is provided in the model description:

"mappings": {
    "product.name": "dm_products.product_name"
}

If it is in different schema or any part of the reference contains a dot:

"mappings": {
    "product.name": {
            "schema": "sales",
            "table": "dm_products",
            "column": "product_name"
        }
}

Disadvantage of the explicit way is it's verbosity and the fact that developer has to write more metadata, obviously.

Both, explicit and implicit mappings have ability to specify default database schema (if you are using Oracle, PostgreSQL or any other DB which supports schemas).

The mapping process process is like this:

Implicit Mapping

With implicit mapping one can match a database schema with logical model and does not have to specify additional mapping metadata. Expected structure is star schema with one table per (denormalized) dimension.

Basic rules:

fact table should have same name as represented cube
dimension table should have same name as the represented dimension, for example: product (singular)
references without dimension name in them are expected to be in the fact table, for example: amount, discount (see note below for simple flat dimensions)
column name should have same name as dimension attribute: name, code, description
if attribute is localized, then there should be one column per localization and should have locale suffix: description_en, description_sk, description_fr (see below for more information)

This means, that by default product.name is mapped to the table product and column name. Measure amount is mapped to the table sales and column amount

What about dimensions that have only one attribute, like one would not have a full date but just a year? In this case it is kept in the fact table without need of separate dimension table. The attribute is treated in by the same rule as measure and is referenced by simple year. This is applied to all dimensions that have only one attribute (representing key as well). This dimension is referred to as flat and without details.

Note for advanced users: this behavior can be disabled by setting simplify_dimension_references to False in the mapper. In that case you will have to have separate table for the dimension attribute and you will have to reference the attribute by full name. This might be useful when you know that your dimension will be more detailed.

Localization

Despite localization taking place first in the mapping process, we talk about it at the end, as it might be not so commonly used feature. From physical point of view, the data localization is very trivial and requires language denormalization - that means that each language has to have its own column for each attribute.

In the logical model, some of the attributes may contain list of locales that are provided for the attribute. For example product category can be in English, Slovak or German. It is specified in the model like this:

attributes = [{
    "name" = "category",
    "locales" = [en, sk, de],
}]

During the mapping process, localized logical reference is created first:

In short: if attribute is localizable and locale is requested, then locale suffix is added. If no such localization exists then default locale is used. Nothing happens to non-localizable attributes.

For such attribute, three columns should exist in the physical model. There are two ways how the columns should be named. They should have attribute name with locale suffix such as category_sk and category_en (underscore because it is more common in table column names), if implicit mapping is used. You can name the columns as you like, but you have to provide explicit mapping in the mapping dictionary. The key for the localized logical attribute should have .locale suffix, such as product.category.sk for Slovak version of category attribute of dimension product. Here the dot is used because dots separate logical reference parts.

Customization of the Implicit

The implicit mapping process has a little bit of customization as well:

dimension table prefix: you can specify what prefix will be used for all dimension tables. For example if the prefix is dim_ and attribute is product.name then the table is going to be dim_product.
fact table prefix: used for constructing fact table name from cube name. Example: having prefix ft_ all fact attributes of cube sales are going to be looked up in table ft_sales
fact table name: one can explicitly specify fact table name for each cube separately

The Big Picture

Here is the whole mapping schema, after localization: