Re: Excessive cost of OpClassCache flushes in CLOBBER_CACHE_ALWAYS mode

Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
> On 7/4/21 3:57 PM, Tom Lane wrote:
>> I'm now a little dubious about my claim that this would have helped find
>> any bugs. Invalidating a finished OpClassCache entry does not model any
>> real-world scenario, because as noted elsewhere in LookupOpclassInfo,
>> once such a cache entry is populated it is kept for the rest of the
>> session. Also, the claim in the comment that we need this to test
>> a cache flush during load seems like nonsense: if we have
>> debug_invalidate_system_caches_always turned on, then we'll test
>> the effects of such flushes throughout the initial population of
>> a cache entry. Doing it over and over again adds nothing.
>>
>> So I'm now fairly strongly tempted to remove this code outright
>> (effectively reverting 03ffc4d6d). Another possibility now that
>> we have debug_invalidate_system_caches_always is to increase the
>> threshold at which this happens, making it more like
>> CLOBBER_CACHE_RECURSIVE.

> If we don't think it's adding anything useful just rip it out. We don't
> generally keep code hanging around just on the off chance it might be
> useful some day.

I did a little more research about the origins of 03ffc4d6d.
I found this thread:

https://www.postgresql.org/message-id/flat/2988.1196271930%40sss.pgh.pa.us#a7dd1ce92f5470ba5ad2e1be03d40802

That points back to commit 0b56be834, which fixed the oversight that
OpclassOidIndexId had not been marked as a critical system index,
and claims that making LookupOpclassInfo invalidate entries would
have helped identify that.

To test that, I tried removing OpclassOidIndexId from the list of
critical system indexes. I found that (a) if I also remove the
invalidation code in LookupOpclassInfo, then things seem to proceed
normally even in CCA mode, but (b) with that code, we get into
infinite recursion, continually trying to rebuild that index's
relcache entry. Once criticalRelcachesBuilt is set, LookupOpclassInfo
assumes it can use catalog index scans for all opclasses, even those
needed for indexes it's going to rely on, which is why the recursion
happens. This might arguably be a bug in that CLOBBER_CACHE_ENABLED
code itself: maybe it needs to avoid invalidating the entries for
OID_BTREE_OPS_OID and INT2_BTREE_OPS_OID. If I make it do so, the
infinite recursion disappears even without OpclassOidIndexId as a
critical index.

So on the one hand maybe that logic is too simplistic, but on the
other hand it might help identify missed critical indexes in future
too. That puts it in the same category as the sorts of bugs that
"debug_invalidate_system_caches_always > 1" is meant to help with,
so now I'm content to change it that way.

regards, tom lane