Overview

In a distributed framework, data is distributed on DNs. Data on one or more DNs is stored on a physical storage device. To properly define a table, you must:

1. Evenly distribute data on each DN to avoid the available capacity decrease of a cluster caused by insufficient storage space of the storage device associated with a DN. Specifically, select a proper distribution key to avoid data skew.
2. Evenly assign table scanning tasks on each DN to avoid that a single DN is overloaded by the table scanning tasks. Specifically, do not select columns in the equivalent filter of a base table as the distribution key.
3. Reduce the data volume scanned by using the partition pruning mechanism.
4. Minimize random I/Os by using clustering.
5. Avoid data shuffle and reduce the network pressure by selecting the join-condition or group by column as the distribution key.

The distribution key is the core for defining a table. Figure 1 shows the procedure of defining a table. The table definition is created during the database design and is reviewed and modified during the SQL statement optimization.