Understanding Sort Keys in Data Warehousing Tables Quiz

Explore essential concepts of sort keys in data warehousing systems, including their purpose, usage, and impact on query performance. This quiz helps reinforce foundational knowledge for optimizing large-scale database tables using effective sort key strategies.

1. Purpose of Sort Keys

   What is the primary purpose of defining a sort key when creating a large table in a columnar database system?

   1. To enforce referential integrity between tables
   2. To increase the maximum table size limit
   3. To restrict which columns can be indexed
   4. To improve query performance by organizing data storage order

   Explanation: Sort keys determine the order in which the data is physically stored, making certain queries faster, especially those that filter or sort on the sort key columns. Increasing table size or restricting column indexing is not related to sort keys. Likewise, sort keys do not enforce referential integrity; that function belongs to foreign keys.

2. Choosing the Right Sort Key

   When designing a table for time-series data that is commonly queried by date, which column should typically be included in the sort key?

   1. The date or timestamp column
   2. A random identifier column
   3. A text description column
   4. A boolean status column

   Explanation: Including the date or timestamp column in the sort key benefits queries filtering data by date, enabling more efficient data reads. A random identifier or boolean status lack natural ordering for typical range queries. Text description columns are usually not ideal for sorting in time-based queries.

3. Types of Sort Keys

   Which of the following best describes a compound sort key in data warehousing?

   1. A sort key that randomly rearranges rows after every load
   2. A sort key composed of multiple columns in a specific left-to-right order
   3. A sort key used only for enforcing uniqueness
   4. A single column defined as the sort key

   Explanation: A compound sort key involves a sequence of columns, and data is physically ordered by the first key, then the second, etc. A single column is just a simple sort key. Compound sort keys do not randomly rearrange rows, and they are not related to enforcing uniqueness, which is handled by constraints.

4. Impact of Unused Sort Keys

   What could happen if queries rarely use the columns defined in a table’s sort key for filtering or sorting?

   1. Indexing on the table becomes disabled
   2. The table will automatically drop the unused sort key
   3. All insert operations will fail
   4. Query performance gain from the sort key is minimal or none

   Explanation: If queries do not use the sort key columns, the expected performance benefits do not materialize, as the data organization is not leveraged. The table does not drop the sort key automatically nor does it disable indexing; insert operations continue to work as usual. Only query performance is affected.

5. Sort Key Storage Behavior

   How does the use of a sort key affect the physical storage of data within a large analytic table?

   1. Sort keys encrypt the data for security
   2. All columns are stored in random order regardless of sort key
   3. Rows with similar sort key values are grouped together on disk
   4. Sort keys add an extra duplicate column to the table

   Explanation: The sort key organizes the table so that rows with similar sort key values are placed together, improving scan efficiency for relevant queries. Columns are not stored randomly if a sort key is defined, and sort keys do not duplicate data or provide encryption. Encryption is a separate data management feature.