Suppose the MERGE statement is in the form of:
MERGE target AS TARGETUSING source AS SOURCEON conditionWHEN MATCHED THEN UPDATEWHEN NOT MATCHED BY TARGET THEN INSERTWHEN NOT MATCHED BY SOURCE THEN DELETE;
Ways to improve the performance:
Create indexes: Ensure that the columns referenced in the
condition are properly indexed.
Separate filtering from matching: Ensure that the
condition only compares columns across the two tables (e.g.,
target.user_id=source.u_id), not a column with a constant (e.g.,
source.account_status='ACTIVE'). For comparisons between columns and constants, use theWHEN` clause.
Use query hints: For certain SQL engines, specify query hints may help. For example, when we are confident some index
idx1 will have few hits compared to some other index, we can hint the engine to
IGNORE INDEX (idx1).
Read the Query Plan: We may find out more ways to enhance the performance by reading the Query Plan. For example, the join order of tables or type of loop may not be ideal for the use case. They can be tweaked by adding other query hints to our statement.