Re: Why is vacuum_freeze_min_age 100m?

On Tue, Aug 11, 2009 at 5:23 PM, Josh Berkus<josh(at)agliodbs(dot)com> wrote:
> On 8/11/09 2:14 PM, Josh Berkus wrote:
>> All,
>>
>> I've just been tweaking some autovac settings for a large database, and
>> came to wonder: why does vacuum_max_freeze_age default to such a high
>> number?  What's the logic behind that?
>>
>> AFAIK, you want max_freeze_age to be the largest possible interval of
>> XIDs where an existing transaction might still be in scope, but no
>> larger.  Yes?
>>
>> If that's the case, I'd assert that users who do actually go through
>> 100M XIDs within a transaction window are probably doing some
>> hand-tuning.  And we could lower the default for most users
>> considerably, such as to 1 million.
>
> (replying to myself) actually, we don't want to set FrozenXID until the
> row is not likely to be modified again.  However, for most small-scale
> installations (ones where the user has not done any tuning) that's still
> likely to be less than 100m transactions.

I don't think that's the name of the parameter, since a Google search
gives zero hits. There are so many fiddly parameters for this thing
that I don't want to speculate about which one you meant.

...Robert