Is the parameter maintenance_work_mem is per database or per instance??
And while exploring the configuration parameters I'm assuming that the sum of values(default values) for the parameters maintenance_work_mem(64MB), work_mem(4MB),temp_buffers(8MB), wal_buffers(4MB) will be equal to the value of shared buffers(128MB).
But the total corresponds to 80MB where is the remaining part of shared buffers(48MB) used?? I have read like clog buffers don't have any explicit parameter to be set, it is tuned automatically by database engine
So will that remaining will be used by the client requests??
I highly recommend reading over the Tuning section of the PG wiki for a pretty good, concise description of how the memory settings work and are related.
As a quick answer, shared_buffers is completely independent from all the other memory settings you mentioned. shared_buffers is allocated to PG when the database starts and that memory is always reserved for PG while it is running. The rest of the ones you mentioned (work_mem, maintenance_work_mem, etc) are all memory allocations that can be used in addition to shared_buffers and are allocated on an as-needed basis, some multiple times each.
I've also written some other blog posts on investigating more detail in how PG is actually using shared_buffers if you're interested