Final answer:
The $sum group accumulator in databases like MongoDB sums only numeric values, ignoring non-numeric ones, which are treated as if they were zero or simply excluded from the operation.
Step-by-step explanation:
The $sum group accumulator in the context of databases, particularly MongoDB, is used to calculate the sum of numeric values. When the $sum accumulator encounters non-numeric values, these are generally ignored, and the accumulator will simply continue summing the numeric values only. It does not perform any type of summing with non-numeric values. If the field does not exist in a document, it is treated as if the value were zero. If the field is an array, $sum treats the values as if they were separate documents, summing each separately.