Re: Logical decoding on standby

On 29 March 2017 at 23:13, Simon Riggs <simon.riggs@2ndquadrant.com> wrote:
> On 29 March 2017 at 10:17, Craig Ringer <craig@2ndquadrant.com> wrote:
>> On 29 March 2017 at 16:44, Craig Ringer <craig@2ndquadrant.com> wrote:
>>
>>> * Split oldestCatalogXmin tracking into separate patch
>>
>> Regarding this, Simon raised concerns about xlog volume here.
>>
>> It's pretty negligible.
>>
>> We only write a new record when a vacuum runs after catalog_xmin
>> advances on the slot with the currently-lowest catalog_xmin (or, if
>> vacuum doesn't run reasonably soon, when the bgworker next looks).
>
> I'd prefer to slow things down a little, not be so eager.
>
> If we hold back update of the catalog_xmin until when we run
> GetRunningTransactionData() we wouldn't need to produce any WAL
> records at all AND we wouldn't need to have VACUUM do
> UpdateOldestCatalogXmin(). Bgwriter wouldn't need to perform an extra
> task.
>
> That would also make this patch about half the length it is.
>
> Let me know what you think.

Good idea.

We can always add a heuristic later to make xl_running_xacts get
emitted more often at high transaction rates if it's necessary.

Patch coming soon.

-- Craig Ringer                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services