On Sat, Feb 25, 2023 at 02:58:36PM +0900, Michael Paquier wrote: > On Fri, Feb 24, 2023 at 08:54:00PM +0000, Imseih (AWS), Sami wrote: > > I think the only thing to do here is to call this out in docs with a > > suggestion to increase pg_stat_statements.max to reduce the > > likelihood. I also attached the suggested doc enhancement as well. > > Improving the docs about that sounds like a good idea. This would be > less surprising for users, if we had some details about that. > > > Any thoughts? > > The risk of deallocation of an entry between the post-analyze hook and > the planner/utility hook represented with two calls of pgss_store() > means that this will never be able to work correctly as long as we > don't know the shape of the normalized query in the second code path > (planner, utility execution) updating the entries with the call > information, etc. And we only want to pay the cost of normalization > once, after the post-analyze where we do the query jumbling.
Note also that this is a somewhat wanted behavior (to evict queries that didn't have any planning or execution stats record), per the STICKY_DECREASE_FACTOR and related stuff. > Could things be done in a more stable way? For example, imagine that > we have an extra Query field called void *private_data that extensions > can use to store custom data associated to a query ID, then we could > do something like that: > - In the post-analyze hook, check if an entry with the query ID > calculated exists. > -- If the entry exists, grab a copy of the existing query string, > which may be normalized or not, and save it into Query->private_data. > -- If the entry does not exist, normalize the query, store it in > Query->private_data but do not yet create an entry in the hash table. > - In the planner/utility hook, fetch the normalized query from > private_data, then use it if an entry needs to be created in the hash > table. The entry may have been deallocated since the post-analyze > hook, in which case it is re-created with the normalized copy saved in > the first phase. I think the idea of a "private_data" like thing has been discussed before and rejected IIRC, as it could be quite expensive and would also need to accommodate for multiple extensions and so on. Overall, I think that if the pgss eviction rate is high enough that it's problematic for doing performance analysis, the performance overhead will be so bad that simply removing pg_stat_statements will give you a ~ x2 performance increase. I don't see much point trying to make such a performance killer scenario more usable.
