Possible Incompatibility with Serial Results

The parallel execution result may be incompatible with the serial execution result in the following scenarios:

• The number of errors or alarms may increase.

  For queries that encounter errors or alarms during serial execution, if they are executed in parallel, each worker thread may report errors or alarms. As a result, the total number of errors or alarms increases.

• The precision is incorrect.

  During parallel execution, intermediate results may be generated. If the intermediate results are of the floating-point type, the floating-point data may have precision deviation. As a result, the final result may be slightly different.

• Sequences of the result sets are different.

  When multiple worker threads execute the query, the returned result set may have a different sequence from the serial execution sequence. If the GROUP BY statement exists, the sequence in the group after grouping may be different from the serial execution sequence. If the LIMIT statement exists, it is more likely that the parallel execution result is different from the serial execution result.