Message-ID: <394d309c-ece2-75b4-9959-9f4537001359@amazon.com>
Date: Thu, 18 Aug 2022 08:39:02 +0200
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Subject: Re: shared-memory based stats collector - v70
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To: Andres Freund <andres@anarazel.de>, Greg Stark <stark@mit.edu>
CC: Tom Lane <tgl@sss.pgh.pa.us>, Melanie Plageman
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	Justin Pryzby <pryzby@telsasoft.com>, Thomas Munro <thomas.munro@gmail.com>,
	"David G. Johnston" <david.g.johnston@gmail.com>, PostgreSQL Hackers
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From: "Drouvot, Bertrand" <bdrouvot@amazon.com>
In-Reply-To: <20220817233053.sppiwk4a32u3x4rd@awork3.anarazel.de>
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Hi,

On 8/18/22 1:30 AM, Andres Freund wrote:
> Hi,
>
> On 2022-08-17 15:46:42 -0400, Greg Stark wrote:
>> Isn't there also a local hash table used to find the entries to reduce
>> traffic on the shared hash table? Even if you don't take a snapshot
>> does it get entered there? There are definitely still parts of this
>> I'm working on a pretty vague understanding of :/
> Yes, there is. But it's more about code that generates stats, rather than
> reporting functions. While there's backend local pending stats we need to have
> a refcount on the shared stats item so that the stats item can't be dropped
> and then revived when those local stats are flushed.

What do you think about something along those lines for the reporting 
part only?

Datum
pgstat_fetch_all_tables_stats(PG_FUNCTION_ARGS)
{
     int         dbid = PG_ARGISNULL(0) ? -1 : (int) PG_GETARG_OID(0);
     ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
     dshash_seq_status hstat;
     PgStatShared_HashEntry *p;
     PgStatShared_Common *stats_data;

     /* Only members of pg_read_all_stats can use this function */
     if (!has_privs_of_role(GetUserId(), ROLE_PG_READ_ALL_STATS))
     {
         aclcheck_error(ACLCHECK_NO_PRIV, OBJECT_FUNCTION, 
"pgstat_fetch_all_tables_stats");
     }

     pgstat_assert_is_up();

     SetSingleFuncCall(fcinfo, 0);

     dshash_seq_init(&hstat, pgStatLocal.shared_hash, false);
     while ((p = dshash_seq_next(&hstat)) != NULL)
     {
         Datum           values[PG_STAT_GET_ALL_TABLES_STATS_COLS];
         bool            nulls[PG_STAT_GET_ALL_TABLES_STATS_COLS];
         PgStat_StatTabEntry * tabentry = NULL;

         MemSet(values, 0, sizeof(values));
         MemSet(nulls, false, sizeof(nulls));

         /* If looking for specific dbid, ignore all the others */
         if (dbid != -1 && p->key.dboid != (Oid) dbid)
             continue;

         /* If the entry is not of kind relation then ignore it */
         if (p->key.kind != PGSTAT_KIND_RELATION)
             continue;

         /* If the entry has been dropped then ignore it */
         if (p->dropped)
             continue;

         stats_data = dsa_get_address(pgStatLocal.dsa, p->body);
         LWLockAcquire(&stats_data->lock, LW_SHARED);
         tabentry = pgstat_get_entry_data(p->key.kind, stats_data);

         values[0] = ObjectIdGetDatum(p->key.dboid);
         values[1] = ObjectIdGetDatum(p->key.objoid);
         values[2]= DatumGetInt64(tabentry->tuples_inserted);
         values[3]= DatumGetInt64(tabentry->tuples_updated);
         values[4]= DatumGetInt64(tabentry->tuples_deleted);
         .
         .
         .
         tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, 
values, nulls);

         LWLockRelease(&stats_data->lock);
     }
     dshash_seq_term(&hstat);

     return (Datum) 0;
}

I also tried to make use of pgstat_get_entry_ref() but went into a 
failed assertion: pgstat_get_entry_ref -> dshash_find -> 
ASSERT_NO_PARTITION_LOCKS_HELD_BY_ME(hash_table) due to lock acquired by 
dshash_seq_next().

Regards,

-- 

Bertrand Drouvot
Amazon Web Services: https://aws.amazon.com