Re: Split index and table statistics into different types of stats

Hi,

On 11/16/22 9:12 PM, Andres Freund wrote:
> Hi,
> 
> On 2022-11-15 10:48:40 +0100, Drouvot, Bertrand wrote:
>> diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
>> index ae3365d917..be7f175bf1 100644
>> --- a/src/backend/utils/adt/pgstatfuncs.c
>> +++ b/src/backend/utils/adt/pgstatfuncs.c
>> @@ -36,24 +36,34 @@
>>   
>>   #define HAS_PGSTAT_PERMISSIONS(role)     (has_privs_of_role(GetUserId(), ROLE_PG_READ_ALL_STATS) ||
has_privs_of_role(GetUserId(),role))
 
>>   
>> +#define PGSTAT_FETCH_STAT_ENTRY(entry, stat_name) ((entry == NULL) ? 0 : (int64) (entry->stat_name))
>> +
>>   Datum
>> -pg_stat_get_numscans(PG_FUNCTION_ARGS)
>> +pg_stat_get_index_numscans(PG_FUNCTION_ARGS)
>>   {
>>       Oid            relid = PG_GETARG_OID(0);
>>       int64        result;
>> -    PgStat_StatTabEntry *tabentry;
>> +    PgStat_StatIndEntry *indentry = pgstat_fetch_stat_indentry(relid);
>>   
>> -    if ((tabentry = pgstat_fetch_stat_tabentry(relid)) == NULL)
>> -        result = 0;
>> -    else
>> -        result = (int64) (tabentry->numscans);
>> +    result = PGSTAT_FETCH_STAT_ENTRY(indentry, numscans);
>>   
>>       PG_RETURN_INT64(result);
>>   }
> 
> This still leaves a fair bit of boilerplate. ISTM that the function body
> really should just be a single line.
> 
> Might even be worth defining the whole function via a macro. Perhaps something like
> 
> PGSTAT_DEFINE_REL_FIELD_ACCESSOR(PGSTAT_KIND_INDEX, pg_stat_get_index, numscans);

Thanks for the feedback!

Right, what about something like the following?

"
#define PGSTAT_FETCH_STAT_ENTRY(pgstat_entry_kind, pgstat_fetch_stat_function, relid, stat_name) \
    do { \
        pgstat_entry_kind *entry = pgstat_fetch_stat_function(relid); \
        PG_RETURN_INT64(entry == NULL ? 0 : (int64) (entry->stat_name)); \
    } while (0)

Datum
pg_stat_get_index_numscans(PG_FUNCTION_ARGS)
{
    PGSTAT_FETCH_STAT_ENTRY(PgStat_StatIndEntry, pgstat_fetch_stat_indentry, PG_GETARG_OID(0), numscans);
}
"

> 
> I think the logic to infer which DB oid to use for a stats entry could be
> shared between different kinds of stats. We don't need to duplicate it.
> 

Agree, will provide a new patch version once [1] is committed.


> Is there any reason to not replace the "double lookup" in
> pgstat_fetch_stat_tabentry() with IsSharedRelation()?
> 
> 

Thanks for the suggestion!

> This should probably be done in a preparatory commit.

Proposal submitted in [1].

[1]:
https://www.postgresql.org/message-id/flat/2e4a0ae1-2696-9f0c-301c-2330e447133f%40gmail.com#e47bf5d2902121461b61ed47413628fc

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com