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If a lot of chunks are involved then the current pl/pgsql function to compute the size of each chunk via a nested loop is pretty slow. Additionally, the current functionality makes a system call to get the file size on disk for each chunk everytime this function is called. That again slows things down. We now have an approximate function which is implemented in C to avoid the issues in the pl/pgsql function. Additionally, this function also uses per backend caching using the smgr layer to compute the approximate size cheaply. The PG cache invalidation clears off the cached size for a chunk when DML happens into it. That size cache is thus able to get the latest size in a matter of minutes. Also, due to the backend caching, any long running session will only fetch latest data for new or modified chunks and can use the cached data (which is calculated afresh the first time around) effectively for older chunks.