Case: Selecting an Appropriate Distribution Column

Distribution columns are used to distribute data to different nodes. A proper distribution key can avoid data skew.

When performing join query, you are advised to select the join condition in the query as the distribution key. When a join condition is used as a distribution key, related data is distributed locally on DNs, reducing the cost of data flow between DNs and improving the query speed.

Before optimization

Use a as the distribution column of t1 and t2. The table definition is as follows:

    
         CREATE TABLE t1 (a int, b int) DISTRIBUTE BY HASH (a);
CREATE TABLE t2 (a int, b int) DISTRIBUTE BY HASH (a);

The following query is executed:

    
         SELECT * FROM t1, t2 WHERE t1.a = t2.b;

In this case, the execution plan contains Streaming(type: REDISTRIBUTE), that is, the DN redistributes data to all DNs based on the selected column. This will cause a large amount of data to be transmitted between DNs, as shown in Figure 1.

Figure 1 Selecting an appropriate distribution column (1)
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After optimization

Use the join condition in the query as the distribution key and run the following statement to changethe distribution key of t2 as b:

    
         ALTER TABLE t2 DISTRIBUTE BY HASH (b);

After the distribution column of table t2 is changed to column b, the execution plan does not contain Streaming(type: REDISTRIBUTE). This reduces the amount of communication data between DNs and reduces the execution time from 8.7 ms to 2.7 ms, improving query performance, as shown in Figure 2.

Figure 2 Selecting an appropriate distribution column (2)
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