Sampling doesn't work on the Global activity/Parallel activity/Backend age views

[marco44]

It comes from this query:

FROM 
            (SELECT pgsa_history.srvid,
              dense_rank() OVER (ORDER BY pgsa_history.ts) AS number,
              count(*) OVER () AS total,
              ts,
              datid,
              cur_txid,
              backend_xid,
              backend_xmin,
              backend_start,
              xact_start,
              query_start,
              state,
              leader_pid
              FROM (
                SELECT *
                FROM (
                  SELECT srvid, (unnest(records)).*
                  FROM public.powa_stat_activity_history pgsah
                  WHERE coalesce_range && tstzrange('2025-01-22 08:23:04+0100', '2025-01-22 09:23:04+0100', '[]')
                  AND pgsah.srvid = '1'
                ) AS unnested
                WHERE ts <@ tstzrange('2025-01-22 08:23:04+0100', '2025-01-22 09:23:04+0100', '[]')
                UNION ALL
                SELECT srvid, (record).*
                FROM public.powa_stat_activity_history_current pgsac
                WHERE (pgsac.record).ts <@ tstzrange('2025-01-22 08:23:04+0100', '2025-01-22 09:23:04+0100', '[]')
                AND pgsac.srvid = '1'
              ) AS pgsa_history
              
            ) AS pgsa
            WHERE number % ( int8larger((total)/(100+1),1) ) = 0;

The problem is that "total" is not the amount of samples we want to return, it's the total count of records (and we have one record per active session in the snapshot. So If you have 100 sessions, total is off by a factor of 100 for the sampling, and you end up with 1 sample instead of 100

PR incoming