Optimize Memory Usage for MergeGlobalStatus's TopN

[hawkingrei]

Enhancement

During the MergeGlobalStatus process, the current method of waiting for all TopN values to be collected before merging them can lead to high memory consumption. This is especially problematic when dealing with large datasets or when the number of unique values is high.

The proposed enhancement involves modifying the mergeglobalstatus process to collect and process the TopN values concurrently. Rather than waiting for all TopN values to be collected, the process should start merging and processing the available TopN values as soon as they are collected. This approach would help optimize memory usage by reducing the need to hold all TopN values in memory before processing.