Gregory Stark wrote:
There is documentation
http://www.postgresql.org/docs/8.3/static/explicit-locking.html
However I found it very confusing when I was first learning. It's not really
the documentation's fault either, there are just a lot of different lock
levels with a lot of different combinations possible.
All DML, even selects, take a table-level shared lock on the tables involved
which blocks the tables from being dropped or truncated while the query is
running.
DELETE and UPDATE (and SELECT FOR UPDATE) take exclusive row-level locks. A
SELECT can read the old version of the record but another UPDATE will block
until your transaction finishes so it can update the most recent version. But
an update which doesn't need to look at that record won't be affected at all.
TRUNCATE and DROP take exclusive table-level locks which blocks anyone else
from even selecting from the table. It also means they can't proceed until all
queries which have already started reading the table finish.
DROP is still a lot heavier than TRUNCATE because it also has to drop (or
search for and throw an error) anything else dependent on the table. triggers,
views, etc.
Thanks for that - it's very useful. As you say I believe the
documentation is pretty good, it's just that we're not dealing in simple
issues here.
I definitely think I should do a delete rather than a truncate (or drop)
in my case.
Regards
Rob
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