Hierarchies & Snowflakes

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Questions and Answers

What does the attribute hierarchy provide in the context of data warehousing?

  • A complex algorithm for data compression.
  • A shorthand vocabulary to define the grain of each cube. (correct)
  • An encryption key for securing sensitive data.
  • A detailed map of the physical storage structure.

In data warehousing, what does it mean to 'line up all the hierarchies for a given star on a single diagram'?

  • To prepare the data for encryption.
  • To create a backup of the data warehouse.
  • To physically rearrange the data for faster access.
  • To document the attribute hierarchies and aggregation level represented by a cube. (correct)

Which schema consists of a fact table and a group of dimension tables, with each dimension table joined directly to the fact table?

  • Galaxy schema
  • Constellation schema
  • Star schema (correct)
  • Snowflake schema

What is the result called when the relationships between dimension attributes are made explicit in a dimensional design?

<p>Snowflake (A)</p> Signup and view all the answers

How does snowflaking a dimension relate to normalization in database design?

<p>Snowflaking is similar to normalization, guiding the design of operational systems. (C)</p> Signup and view all the answers

In the context of data warehousing, what is the primary drawback of using normalization techniques like snowflaking?

<p>It can detract from usability and negatively impact query performance. (D)</p> Signup and view all the answers

What is a key characteristic of dimensional models in relation to relational databases?

<p>Dimensional models are not always implemented in relational databases. (D)</p> Signup and view all the answers

What is the purpose of precomputing the various combinations of dimension values and fact values in a cube?

<p>To enable the data to be studied interactively (B)</p> Signup and view all the answers

What is a primary advantage of using a multidimensional database compared to a relational database for data warehousing?

<p>Increased speed of data access and analysis (D)</p> Signup and view all the answers

Which term describes the process of interactively changing the perspective on data, such as adding or removing attributes, and receiving immediate feedback?

<p>OLAP (Online Analytical Processing) (A)</p> Signup and view all the answers

In the context of OLAP, what does it mean to 'slice and dice' data?

<p>To interactively analyze data by viewing it from different dimensions. (C)</p> Signup and view all the answers

How does interacting with a star schema typically differ from interacting with a cube in a multidimensional database?

<p>Interacting with a star schema involves a query-and-response paradigm, while cubes offer highly responsive OLAP interaction. (B)</p> Signup and view all the answers

Which of the following describes the primary function of ETL in data warehousing?

<p>To extract, transform, and load data from source systems into the data warehouse. (A)</p> Signup and view all the answers

In a star schema loading process, what is the significance of loading dimension tables before fact tables?

<p>It is crucial because fact table rows contain foreign key references to dimension table rows. (B)</p> Signup and view all the answers

What is the first step in loading dimension tables during the ETL process?

<p>Extract source data from source(s). (C)</p> Signup and view all the answers

What is the purpose of surrogate keys in dimension tables?

<p>To serve as unique identifiers independent of natural keys. (C)</p> Signup and view all the answers

What is the primary goal of processing fact tables during the ETL process?

<p>To load transactional data and link it to dimensions through foreign keys. (D)</p> Signup and view all the answers

What step is essential before loading records into the fact table?

<p>Obtaining surrogate keys for each of the dimensions. (A)</p> Signup and view all the answers

Which operation is performed on facts to ensure they match the grain of the fact table?

<p>Aggregation (A)</p> Signup and view all the answers

Which of the following is a key characteristic of OLAP interaction with a cube?

<p>It provides instantaneous feedback as you slice and dice, drill up and drill down. (D)</p> Signup and view all the answers

What is the significance of 'normalization' in database design?

<p>It helps in data redundancy and improves storage efficiency. (C)</p> Signup and view all the answers

Which of the following steps is a standard practice as a project is loading dimension tables?

<p>Managing surrogate keys. (C)</p> Signup and view all the answers

In the context of data warehousing, what does 'Drilling' generally refer to?

<p>Analyze data at different levels of detail. (C)</p> Signup and view all the answers

Which statement best explains why normalization is typically avoided in analytical databases?

<p>It is exclusively for operational databases. (B)</p> Signup and view all the answers

What is a significant advantage to dimensional models being deployed on Multi-dimensional Databases?

<p>Faster to perform analytical operations. (A)</p> Signup and view all the answers

If you're 'slicing and dicing' a data cube, what are you trying to accomplish?

<p>Explore different views of your data. (D)</p> Signup and view all the answers

What is a core function of the 'Fact' table in a star schema?

<p>Storing core business metrics. (C)</p> Signup and view all the answers

In a star schema, what is the primary role of the dimension tables?

<p>To store descriptive information. (C)</p> Signup and view all the answers

During ETL, why is it useful to 'extract source data from source(s)'?

<p>To transfer it into a data warehouse for effective analysis. (D)</p> Signup and view all the answers

During dimensional modelling processes, what are 'surrogate keys' used for?

<p>Identifying records uniquely. (B)</p> Signup and view all the answers

What could possibly cause an ETL process to fail during the Fact Table Processing?

<p>Unavailability to the relevant primary keys. (A)</p> Signup and view all the answers

Given that you are utilizing a star schema in your data warehouse, which action would be the MOST critical during the data loading process?

<p>Load smaller dimension tables first, and then the primary fact table. (A)</p> Signup and view all the answers

Suppose you need to represent 'order dollars' in your cube, but your source data tracks orders at the product and day level. What do you need to consider?

<p>The potential need to roll up values to the 'quarter' level using aggregation. (D)</p> Signup and view all the answers

You've implemented a data warehouse with a star schema but notice query performance is degrading. Which design change would MOST quickly help?

<p>Denormalizing dimension tables to reduce the number of joins required. (A)</p> Signup and view all the answers

You're tasked with designing an ETL process for a data warehouse. Your source system contains historical snapshots of customer data which are slowly changing, but you're only tracking the very latest version. Which approach is MOST suitable for loading dimension tables?

<p>Implementing a full refresh strategy, overwriting the dimension table with each run. (B)</p> Signup and view all the answers

Which of the following represents the correct order of operations in a typical ETL process?

<p>Extract -&gt; Transform -&gt; Load. (B)</p> Signup and view all the answers

What does the acronym MDB stand for in the context of data warehousing?

<p>Multidimensional Database (C)</p> Signup and view all the answers

In data warehousing, what is meant by a 'cube'?

<p>A three-dimensional representation of data that allows for quick data analysis. (C)</p> Signup and view all the answers

Flashcards

Dimensional Design

A design approach for data warehouses focused on organizing data into dimensions and facts for analysis.

Hierarchies & Drilling

Navigating through levels of detail in a data hierarchy to gain insights.

Attribute hierarchy

Provides a shorthand way to define the level of detail stored in a data cube.

Star Schema

A simple data warehouse structure with a central fact table and surrounding dimension tables.

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Snowflake

A data warehouse design where dimension tables are further normalized into multiple related tables.

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Operational System

A database suitable for operational systems supporting transactions that modify data.

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Normalization

The process of organizing data to minimize redundancy and improve integrity.

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Cube

A model that stores data in a format optimized for fast analysis, often pre-aggregated.

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Multidimensional Database (MDB)

A database designed for storing and querying data organized in multiple dimensions.

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Precompute

Calculating and storing data aggregations in advance for faster query performance.

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Online Analytical Processing (OLAP)

Analyzing data in real-time by interactively changing perspectives and attributes.

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OLAP Interaction

A highly responsive interaction with a cube, providing instantaneous feedback for data analysis.

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Dimensional models

Data models are not always implemented in relational databases.

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Slice and Dice

Slicing is selecting a dimension from a data cube and dicing consists of picking smaller portions of the cube based on dimension values.

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ETL

Extracting, transforming, and loading data from various sources into a data warehouse.

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Foreign key references

Each fact table row contains foreign key references to dimension table rows.

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Extract source data

Extracting source data from one or more locations.

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Idenitfy changes and records

Identifying new and changed dimension records.

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Order of operation for fact tables

Acquire Source Data, Organize source data, calculate facts, look up dimension keys and finally save fact table.

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Study Notes

Hierarchies & Drilling

  • The attribute hierarchy provides a shorthand way to define the grain of each cube.
  • A cube can hold order dollars at the brand (product) and quarter (day) levels.
  • Aligning hierarchies on a single diagram and drawing a horizontal line defines the aggregation level of a cube.

Snowflakes

  • A star schema is composed of a fact table and a group of dimension tables, each directly connected to the fact table.
  • A snowflake schema results when dimension attributes are explicitly detailed in a dimensional design.
  • Snowflaking is like normalization, which guides the design of operational systems.
  • Normalization is not ideal for analytic settings because it is meant for operational systems supporting varied, simultaneous transactions that facilitate data modifications.
  • Analytic databases do not share the same usage pattern as operational systems, making normalization unnecessary.
  • Unnecessary complexity from normalization detracts from usability, complicates ETL, and can negatively affect performance and does not guarantee data integrity.

Cubes

  • Dimensional models are not always in relational databases.
  • A multidimensional database (MDB) uses a cube to store dimensional information.
  • The main concept of a cube is to precompute combinations of dimension and fact values for interactive study.

Multidimensional vs Relational Storage

  • Multidimensional databases have the advantage of speed.
  • Cubes let users modify their data perspective by adding or removing attributes, with immediate feedback.
  • This process is known as Online Analytical Processing (OLAP).
  • OLAP interaction with a cube enables slicing, dicing, drilling up, and drilling down with instantaneous feedback.
  • Star schemas use a require a query-and-response paradigm, so each change requires a new query.

Star Schema

  • A star schema includes a fact table and dimension tables, directly connected.

Loading a Star Schema

  • Each row in the fact table has foreign key references to rows in dimension tables.
  • Dimension rows must be ready before a fact table row can be inserted.
  • Dimension processing happens first, then fact table processing.

Loading Dimension Tables

  • Loading dimension tables involves extracting source data, assembling dimensional attributes, identifying new/changed dimension records, managing surrogate keys, processing new records, and processing type 1 and 2 SCD changes.

Loading Fact Tables

  • Loading fact tables includes extracting data from source systems, computing facts, aggregating facts to match grain, obtaining surrogate keys, and loading fact table records.

ETL Tools

  • Pentaho
  • Alteryx
  • Talend Data Preparation
  • ClearStory Data
  • Dataiku
  • Datameer
  • Datawatch
  • Informatica REV
  • Paxata
  • Platfora
  • Sagent DataFlow
  • Trifacta
  • SAP Agile Data Preparation

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