Accumulators are probably the most widely used operators, allowing us to sum, average, get standard deviation statistics, and perform other operations in each member of our group.

• $sum is the sum of numerical values, ignores non-numerical values
• $avg is the average of numerical values, ignores non-numerical values
• $first / $last is the first and last value that passes through the pipeline stage, available in the group stage only
• $max / $min gets the maximum and minimum value that passes through the pipeline stage
• $push, will add a new element at the end of an input array, available in the group stage only
• $addToSet, will add an element (only if it does not exist) to an array treating it effectively as a set, available in the group stage only
• $stdDevPop / $stdDevSamp to get the population / sample standard deviation in the $project or $match stages

The aforementioned mentioned accumulators are available in the group or project pipeline phases except otherwise noted.