Re: [rfc] overhauling pgstat.stat

Sent from my iPad

On 08-Sep-2013, at 4:27, Tomas Vondra <tv(at)fuzzy(dot)cz> wrote:

> On 5.9.2013 09:36, Atri Sharma wrote:
>> On Thu, Sep 5, 2013 at 10:59 AM, Alvaro Herrera
>> <alvherre(at)2ndquadrant(dot)com> wrote:
>>> Satoshi Nagayasu wrote:
>>>
>>>> But, for now, I think we should have a real index for the
>>>> statistics data because we already have several index storages,
>>>> and it will allow us to minimize read/write operations.
>>>>
>>>> BTW, what kind of index would be preferred for this purpose?
>>>> btree or hash?
>>>
>>> I find it hard to get excited about using the AM interface for
>>> this purpose. To me it makes a lot more sense to have separate,
>>> much simpler code. We don't need any transactionality, user
>>> defined types, user defined operators, or anything like that.
>>
>> +1.
>>
>> But, would not rewriting a lot of existing functionalities
>> potentially lead to points of contention and/or much more effort?
>
> Don't forget the stats are written only by the postmaster, all the
> regular backends only read it (and eventually send updates back). But
> yes, contention might be a problem, because there will have to be some
> kind of locking that is not needed now when the postmaster is writing
> fresh copy into a new file.
>
> But I think we need to implement something and then measure this.
> Because it might even with the contention the overall performance might
> actually improve.
>
> I'd vote to try a simple approach first - adding some simple array
> 'index' allowing fast access to particular records etc. At least that
> was my plan. But feel free to implement something more advanced (e.g. a
> BTree storage) and we can compare the results.
>
>
+1 on the simple implementation and measure approach.

My focus here is to identify what kind of queries we expect to be serving from the stats.I think someone mentioned range queries upthread. I feel we should be looking at range trees as secondary index, if not the primary storage for pgstat.

Regards,

Atri