Wrong join order
Adjusting query join order or join type can improve Impala performance.
By: Manish Maheshwari, Data Architect and Data Scientist at Cloudera, Inc.
Description and cause:
A slow query can also be caused by using incorrect join order or incorrect join types.
Solution:
- To identify the join type and join ordering errors, compare the actual number of rows (#Rows) to the estimated number of rows (Est. #Rows) in the query profile. If the actual versus estimated numbers of rows vary significantly, the join order and the selected join type might be incorrect.
- Ensure that you have statistics collected on all tables and columns so the correct join order and type can be automatically selected.
