Re: Timing of pgstats updates

On 11/18/2004 11:43 AM, Tom Lane wrote:
> "David Parker" <dparker@tazznetworks.com> writes:
>> What I think is happening with the missing pg_statistic entries:
>> The install of our application involves a lot of data importing (via
>> JDBC) in one large transaction, which can take up to 30 minutes. (I
>> realize I left out this key piece of info in my original post...)
> 
>> The pg_autovacuum logic is relying on data from pg_stat_all_tables to
>> make the decision about running analyze. As far as I can tell, the data
>> in this view gets updated outside of the transaction, because I saw the
>> numbers growing while I was importing. I saw pg_autovacuum log messages
>> for running analyze on several tables, but no statistics data showed up
>> for these, I assume because the actual data in the table wasn't yet
>> visible to pg_autovacuum because the import transaction had not finished
>> yet.
> 
>> When the import finished, not all of the tables affected by the import
>> were re-visited because they had not bumped up over the threshold again,
>> even though the analyze run for those tables had not generated any stats
>> because of the still-open transaction.
> 
> Bingo.  The per-table activity stats are sent to the collector whenever
> the backend waits for a client command.  Given a moderately long
> transaction block doing updates, it's not hard at all to imagine that
> autovacuum would kick off vacuum and/or analyze while the updating
> transaction is still in progress.  The resulting operation is of course
> a waste of time.
> 
> It'd be trivial to adjust postgres.c so that per-table stats are
> only transmitted when we exit the transaction (basically move the
> pgstat_report_tabstat call down a couple lines so it's not called if
> IsTransactionOrTransactionBlock).
> 
> This seems like a good change to me.  Does anyone not like it?
> 
>             regards, tom lane

Sounds reasonable here.


Jan

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