Re: [rfc] overhauling pgstat.stat

On 5.9.2013 09:36, Atri Sharma wrote:
> On Thu, Sep 5, 2013 at 10:59 AM, Alvaro Herrera 
> <alvherre@2ndquadrant.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.

Tomas