Postgresql update op is very very slow

I've a table with about 34601755 rows ,when I execute 'update msg_table set
type=0;' is very very slow, cost several hours, but still not complete?

Why postgresql is so slowly? Is the PG MVCC problem?

But I try it on Mysql, the same table and rows, it only cost about 340
seconds.

Any idea for the problem?


My machine config:
    Memory 8G, 8 piece 15K disk , 2CPU(Quad-Core) AMD
    OS: Red Hat AS4

My postgres.conf main parameter is following:


shared_buffers = 5GB                    # min 128kB or max_connections*16kB
                                        # (change requires restart)
temp_buffers = 512MB                    # min 800kB
work_mem = 400MB                                # min 64kB
maintenance_work_mem = 600MB            # min 1MB
max_fsm_pages = 262144 # 2G min max_fsm_relations*16, 6 bytes each
max_fsm_relations = 2000                # min 100, ~70 bytes each

bgwriter_delay = 20ms                   # 10-10000ms between rounds
bgwriter_lru_maxpages = 500             # 0-1000 max buffers written/round
bgwriter_lru_multiplier = 2.0           # 0-10.0 multipler on buffers