ClickHouse EXPLAIN: Determine JOIN Order in Query Execution Plan

Introduction

In ClickHouse, the join order in an execution plan is determined by the query optimizer, which analyzes the query and generates an optimal plan for executing the query. The optimizer uses statistics about the tables and indexes involved in the query to determine the most efficient join order.

Using EXPLAIN command

To determine the join order in an execution plan in ClickHouse, you can use the EXPLAIN command. The EXPLAIN command shows the query execution plan and provides details about how the query will be executed, including the join order.

Here is an example of using the EXPLAIN command to determine the join order in ClickHouse:

EXPLAIN SELECT *
FROM table1
JOIN table2 ON table1.key = table2.key
JOIN table3 ON table2.key = table3.key

The output of the EXPLAIN command will show the execution plan for the query, including the join order. Here is an example output:

Query id: 123456789
+------+-------------+-------+---------------+----------------------+---------------+---------+-------+--------+-------------+
| step |     name    |   r   |      w        | input partitions info | output format | is_done | source|processes|used_memory, B|
+------+-------------+-------+---------------+----------------------+---------------+---------+-------+--------+-------------+
|    1 | join        | 10000 | 0             |                      | Arrow         |       0 |       | 1      |    10485760 |
|    2 |   join      | 10000 | 0             |                      | Arrow         |       0 |       | 1      |    10485760 |
|    3 |     source  | 10000 | 0             |                      | Arrow         |       0 |       | 1      |     2621440 |
|    4 |     source  | 10000 | 0             |                      | Arrow         |       0 |       | 1      |     2621440 |
|    5 |     source  | 10000 | 0             |                      | Arrow         |       0 |       | 1      |     2621440 |
+------+-------------+-------+---------------+----------------------+---------------+---------+-------+--------+-------------+

In the example output, you can see that the join order is determined by the order of the JOIN clauses in the SQL statement. The optimizer has chosen to join table1 and table2 first, and then join the result with table3. The execution plan also shows the estimated number of rows that will be processed for each step, as well as the amount of memory that will be used.

Conclusion

By using the EXPLAIN command, you can analyze the execution plan and determine the join order used by ClickHouse for your query. This can help you optimize your queries and improve query performance.

To read more about ClickHouse EXPLAIN, do consider reading the below articles

About Shiv Iyer 227 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.