Re: Support for dumping extended statistics

On 1/7/23 03:39, Bruce Momjian wrote:
> On Thu, Jan 5, 2023 at 06:29:03PM +0000, Hari krishna Maddileti wrote:
>> Hi Team,
>> In order to restore dumped extended statistics (stxdndistinct,
>> stxddependencies, stxdmcv) we need to provide input functions to parse
>> pg_distinct/pg_dependency/pg_mcv_list strings.
>>
>> Today we get the ERROR "cannot accept a value of type pg_ndistinct/
>> pg_dependencies/pg_mcv_list" when we try to do an insert of any type.
>>
>> Approch tried:
>>
>> - Using yacc grammar file (statistics_gram.y) to parse the input string to its
>> internal format for the types pg_distinct and pg_dependencies
>>
>> - We are just calling byteain() for serialized input text of type pg_mcv_list.
>>
>> Currently the changes are working locally, I would like to push the commit
>> changes to upstream if there any usecase for postgres. Would like to know if
>> there any interest from postgres side.
>
> There is certainly interest in allowing the optimizer statistics to be
> dumped and reloaded. This could be used by pg_restore and pg_upgrade.
>

Indeed, although I think it'd be better to deal with regular statistics
(which is what 99% of systems use). Furthermore, we should probably
think about differences between major versions - until now we could
change on-disk format of the statistics, because we have reset them.
It'd be silly to do dump on version X, and then fail to restore it on
(X+1) just because the statistics changed a bit.

So we need to be able to determine is the statistics has the correct
format/version, or what. And we need to do that for pg_upgrade.

At the very least we need an option to skip restoring statistics, or
something like that.

regards
--
Tomas Vondra
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