Ashutosh Bapat <ashutosh.bapat.oss@gmail.com> writes: > I read that discussion, and it may be ok for pg_upgrade/pg_dump usecase and > maybe also for IMPORT foreign schema where the SQL is generated by > PostgreSQL itself. But not for simulating statistics. In that case, if the > function happily installs statistics cooked by the user and those aren't > used anywhere, users may be misled by the plans that are generated > subsequently. Thus negating the very purpose of simulating > statistics.
I'm not sure what you think the "purpose of simulating statistics" is, but it seems like you have an extremely narrow-minded view of it. I think we should allow injecting any stats that won't actively crash the backend. Such functionality could be useful for stress-testing the planner, for example, or even just to see what it would do in a situation that is not what you have.
My reply was in the following context
For a partitioned table this value has to be true. For a normal table when setting this value to true, it should at least make sure that the table has at least one child. Otherwise it should throw an error. Blindly accepting the given value may render the statistics unusable. Prologue of the function needs to be updated accordingly.
I can see rejecting non-inherited stats for a partitioned table. The reverse, however, isn't true, because a table may end up being inherited by another, so those statistics may be legit. Having said that, a great deal of the data validation I was doing was seen as unnecessary, so I' not sure where this check would fall on that line. It's a trivial check if we do add it.
If a user installs inherited stats for a non-inherited table by accidently passing true to the corresponding argument, those stats won't be even used. The user wouldn't know that those stats are not used. Yet, they would think that any change in the plans is the result of their stats. So whatever simulation experiment they are running would lead to wrong conclusions. This could be easily avoided by raising an error. Similarly for installing non-inherited stats for a partitioned table. There might be other scenarios where the error won't be required.