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Lu, Huayi
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Reviewed-by: Hasko, Vladimir <vladimir.hasko@t-systems.com> Co-authored-by: Lu, Huayi <luhuayi@huawei.com> Co-committed-by: Lu, Huayi <luhuayi@huawei.com>
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2.9 KiB
Reviewing and Modifying a Table Definition
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:
- 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.
- Evenly assign table scanning tasks on each DN to avoid that a 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.
- Reduce the data volume scanned by using the partition pruning mechanism.
- Avoid the use of random I/O by using clustering or partial clustering.
- Avoid data shuffle to reduce the network pressure by selecting the join-condition column or group by column as the distribution column.
The distribution column is the core for defining a table. The following figure 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.
Figure 1 Procedure of defining a table
Parent topic: Reviewing and Modifying a Table Definition