On 23.07.24 11:13, Aleksander Alekseev wrote: >> Here is the fix. It can be tested like this: >> [...] > > PFA the rebased patchset.
I'm wondering about the 64-bit GUCs.
At first, it makes sense that if there are settings that are counted in terms of transactions, and transaction numbers are 64-bit integers, then those settings should accept 64-bit integers.
But what is the purpose and effect of setting those parameters to such huge numbers? For example, what is the usability of being able to set
vacuum_failsafe_age = 500000000000
Also in the rebased patch set I cannot find the above, so I cannot evaluate what it does.
In the past I have pushed for some mechanism to produce warnings like we currently have approaching xid wraparound when a certain threshold is met. Is this that mechanism?
I think in the world of 32-bit transaction IDs, you can intuitively interpret most of these "transaction age" settings as "percent toward disaster". For example,
vacuum_freeze_table_age = 150000000
is 7% toward disaster, and
vacuum_failsafe_age = 1600000000
is 75% toward disaster.
However, if there is no more disaster threshold at 2^31, what is the guidance for setting these? Or more radically, why even run transaction-count-based vacuum at all?
Conversely, if there is still some threshold (not disaster, but efficiency or something else), would it still be useful to keep these settings well below 2^31? In which case, we might not need 64-bit GUCs.
Your 0004 patch adds support for 64-bit GUCs but doesn't actually convert any existing GUCs to use that. (Unlike the reloptions, which your patch coverts.) And so there is no documentation about these questions.
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Best Wishes,
Chris Travers
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