Is it a good starting point to the basic reason of doing vacuum?
from the manual,
"PostgreSQL's VACUUM command must be run on a regular basis for several
reasons:
To recover disk space occupied by updated or deleted rows.
To update data statistics used by the PostgreSQL query planner.
To protect against loss of very old data due to transaction ID wraparound.
"
It seems that only transaction ID wraparound can cause system failure in
loading-in new data.
For the dead rows problem, it just a matter of space; the data statistics is
just relate to the query performance.
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