On Thu, Sep 15, 2016 at 1:51 PM, Heikki Linnakangas <hlinn...@iki.fi> wrote: > BTW, does a 1-way merge make any sense? I was surprised to see this in the > log, even without this patch: > > LOG: finished 1-way merge step: CPU 0.62s/7.22u sec elapsed 8.43 sec > STATEMENT: SELECT COUNT(*) FROM (SELECT * FROM medium.random_ints ORDER BY > i) t; > LOG: finished 1-way merge step: CPU 0.62s/7.22u sec elapsed 8.43 sec > STATEMENT: SELECT COUNT(*) FROM (SELECT * FROM medium.random_ints ORDER BY > i) t; > LOG: finished 1-way merge step: CPU 0.62s/7.22u sec elapsed 8.43 sec > STATEMENT: SELECT COUNT(*) FROM (SELECT * FROM medium.random_ints ORDER BY > i) t; > LOG: finished 1-way merge step: CPU 0.62s/7.22u sec elapsed 8.43 sec > STATEMENT: SELECT COUNT(*) FROM (SELECT * FROM medium.random_ints ORDER BY > i) t; > LOG: finished 3-way merge step: CPU 0.62s/7.23u sec elapsed 8.44 sec > STATEMENT: SELECT COUNT(*) FROM (SELECT * FROM medium.random_ints ORDER BY > i) t; > LOG: finished 6-way merge step: CPU 0.62s/7.24u sec elapsed 8.44 sec > STATEMENT: SELECT COUNT(*) FROM (SELECT * FROM medium.random_ints ORDER BY > i) t; > LOG: finished 6-way merge step: CPU 0.62s/7.24u sec elapsed 8.45 sec > STATEMENT: SELECT COUNT(*) FROM (SELECT * FROM medium.random_ints ORDER BY > i) t;
Another thing that I think it worth pointing out here is that the number of merge passes shown is excessive, practically speaking. I suggested that we have something like checkpoint_warning for this last year, which Greg Stark eventually got behind, but Robert Haas didn't seem to like. Maybe this idea should be revisited. What do you think? There is no neat explanation for why it's considered excessive to checkpoint every 10 seconds, but not every 2 minutes. But, we warn about the former case by default, and not the latter. It's hard to know exactly where to draw the line, but that isn't a great reason to not do it (maybe one extra merge pass is a good threshold -- that's what I suggested once). I think that other database systems similarly surface multiple merge passes. It's just inefficient to ever do multiple merge passes, even if you're very frugal with memory. Certainly, it's almost impossible to defend doing 3+ passes these days. -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
