On 15 June 2013 16:39, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
>> On 15 June 2013 00:01, Josh Berkus <josh@agliodbs.com> wrote:
>>> If we're going to start adding reloptions for specific table behavior,
>>> I'd rather think of all of the optimizations we might have for a
>>> prospective "append-only table" and bundle those, rather than tying it
>>> to whether a certain index exists or not.
>
>> I agree that the FSM behaviour shouldn't be linked to index existence.
>> IMHO that should be a separate table parameter, WITH (fsm_mode = append)
>> Index only scans would also benefit from that.
>
> -1 ... I cannot believe that such a parameter would ever get turned on
> in production by anyone.
It's an extremely common use case, epecially with larger databases.
> If your table has a significant update rate,
> the resulting table bloat would make such behavior completely
> infeasible. If you have few enough updates to make such a behavior
> practical, then you can live with the expensive index updates instead.
Depends. What I had in mind was that "append" mode would allow updates
normally within the last 1 GB of a table, but relocate updates should
they occur in earlier segments. That would cover the majority of use
cases I see.
> I also find the analogy to index-only scans to be bogus, because those
> didn't require any user tuning.
I don't think anybody is suggesting there is a requirement for user
tuning on this type of index.
But I think having one parameter would make it the same as say, GIN
indexes which also have a single tuning parameter.
I think the idea that index only scans "just work" is itself bogus. A
mild random update pattern will reduce the benefit of IOS scans
considerably, which is hard to understand or control, especially
without any way of measuring it. We could eliminate that problem if we
chose, but its not even documented as a potential issue for some
reason so its hard to discuss. Not being able to see or control a
known problem is not the same thing as "doesn't require user tuning".
> There's a nearby thread complaining bitterly about our willingness to
> create hard-to-use, hard-to-tune "features". In its current form,
> this will be another one of those.
Not based upon anything mentioned here so far, or that I'm aware of.
I think it would be interesting to see some anonymous voting on
feature complaints, so we can see what people really think without
needing to risk offending the main authors of each of the various
parts of our software.
--Simon Riggs http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services