Query Plan Evaluation for Redshift Cluster Tables
Question
Your team has setup tables in a Redshift cluster.
They are now evaluating the performance of queries to ensure that it is up to the mark when users start using the tables.
They are evaluating the query plan.
When evaluating the query plan, which of the following results would require the team to re-check on the distribution styles used for the underlying tables.
Choose 2 answers from the options given below.
Answers
Explanations
Click on the arrows to vote for the correct answer
A. B. C. D.Answer - B and C.
The AWS Documentation mentions the following.
DS_DIST_NONE and DS_DIST_ALL_NONE are good.
They indicate that no distribution was required for that step because all of the joins are collocated.
DS_DIST_INNER means that the step will probably have a relatively high cost because the inner table is being redistributed to the nodes.
DS_DIST_ALL_INNER is not good.
It means the entire inner table is redistributed to a single slice because the outer table uses DISTSTYLE ALL, so that a copy of the entire outer table is located on each node.
Since this is clearly mentioned in the AWS Documentation, all other options are invalid.
For more information on data distribution in Redshift, please refer to the below URL.
https://docs.aws.amazon.com/redshift/latest/dg/c_data_redistribution.htmlIn Amazon Redshift, tables can be distributed in two ways: distribution style (DS) key and DS all. DS key distributes the data based on a specific column, whereas DS all replicates the data to every node in the cluster. When queries are run, the query planner determines the optimal query execution plan, which may involve redistributing the data among the nodes. The distribution style chosen for the table can affect query performance significantly.
Answer A: DS_DIST_NONE means that the table is not distributed. Instead, the table is replicated to every node in the cluster. This distribution style is usually used for small tables that can fit in memory on every node. It would not require the team to re-check the distribution styles used for the underlying tables.
Answer B: DS_DIST_INNER means that the table is distributed based on a column that is frequently used in join clauses. If the query plan shows a large amount of data movement, it may indicate that the distribution style used for the table is not optimal. However, it does not necessarily require the team to re-check the distribution styles used for the underlying tables.
Answer C: DS_DIST_ALL_INNER means that all tables involved in the query are distributed using the same key. If the query plan shows a large amount of data movement, it may indicate that the distribution style used for one or more tables is not optimal, and the team may need to re-check the distribution styles used for the underlying tables.
Answer D: DS_DIST_ALL_NONE means that all tables involved in the query are replicated to every node in the cluster. This distribution style is usually used for small tables that can fit in memory on every node. If the query plan shows a large amount of data movement, it may indicate that the distribution style used for the table is not optimal. However, it does not necessarily require the team to re-check the distribution styles used for the underlying tables.
Therefore, the two answers that would require the team to re-check on the distribution styles used for the underlying tables are C. DS_DIST_ALL_INNER and D. DS_DIST_ALL_NONE.
