hello Shridhar, hello Tom,
long story short:
Neither adding ANALYZE nor reducing the rows from 2000 at one time to
500 changes anything: the UnUsed value keeps growing, as does the
overall amount of pages. The VACUUM ANALYZE takes the same time or even
more.
Following your advice, Tom, I increased the rows to 200,000. The update
now takes about 140 secs, the VACUUM ANALYZE more than 200 secs. The
amount of pages increases a lot, as does UnUsed. Moreover, the
transaction log files get recycled a lot - which did not happen with
2000 rows at one time. But nevertheless - with 200000 rows at one time
there are only few cycles untill all 2 million rows are updated, so in
the end this should be the faster way.
thank you for your help!
Chantal