Re: pg_stat_database.xact_commit + pg_stat_database.xact_rollback asa reliable metric for xid consumption ?

On 1/6/20 6:58 μ.μ., Rui DeSousa wrote:
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>> On Jun 1, 2020, at 7:03 AM, Julien Rouhaud <rjuju123@gmail.com> wrote:
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>> On Mon, Jun 1, 2020 at 12:48 PM Achilleas Mantzios
>> <achill@matrix.gatewaynet.com> wrote:
>>> On 1/6/20 1:36 μ.μ., Julien Rouhaud wrote:
>>>> On Mon, Jun 1, 2020 at 12:02 PM Achilleas Mantzios
>>>> <achill@matrix.gatewaynet.com> wrote:
>>>> Because read only transaction usually don't consume an xid.  So yes
>>>> pg_stat_database gives a more reasonable approximation of the real
>>>> number of transactions happening on the server.
>>> Thank you, so what would be the official way to monitor txid consumption (from a MVCC / Freezing admin POV)?
>> I'm not sure what you're really looking for.  If it's the txid
>> consumption rate, you can use txid_current() at regular interval to
>> compute it (note that this function will consume a txid).  For object
>> freeeze, you can use age(datfrozenxid) from pg_database, or
>> age(relfrozenxid) from pg_class.
>>
>>
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> I also record the stats tables every minute by either inserting into a table hist_ table or exporting the data out to
beinserted into another table.  When exporting the pg_stat_database table I added the following to record the
informationwithout the need to call txid_current():
 
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> coalesce(txid_current_if_assigned(), txid_snapshot_xmax(txid_current_snapshot())) as xact_txid, …
if this is in the same system, and this _hist table only gets inserts, 
selects/delets, why not just use xmin ? Also since you are inserting how 
can you have null  txid_current_if_assigned()?
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