Re: shared-memory based stats collector - v68 - Mailing list pgsql-hackers

So, I decided to see what this would look like; the results are attached, portions of it also inlined below.

I'll admit this does introduce a terminology problem - but IMO these words are much more meaningful to the reader and code than the existing booleans.  I'm hopeful we can bikeshed something agreeable as I'm strongly in favor of making this change.

The ability to create defines for subsets nicely resolves the problem that CLUSTER and DATABASE (now OBJECT to avoid DATABASE conflict in PgStat_Kind) are generally related together - they are now grouped under the DYNAMIC label (variable, if you want) while all of the fixed entries get associated with GLOBAL.  Thus the majority of usages, since accessed_across_databases is rare, end up being either DYNAMIC or GLOBAL.  The presence of any other category should give one pause.  We could add an ALL define if we ever decide to consolidate the API - but for now it's largely used to ensure that stats of one type don't get processed by the other.  The boolean fixed does that well enough but this just seems much cleaner and more understandable to me.  Though having made up the terms and model myself, that isn't too surprising.

The only existing usage of accessed_across_databases is in the negative form, which translates to excluding objects, but only those from other actual databases.

@@ -909,7 +904,7 @@ pgstat_build_snapshot(void)
  */
  if (p->key.dboid != MyDatabaseId &&
  p->key.dboid != InvalidOid &&
- !kind_info->accessed_across_databases)
+ kind_info->kind_group == PGSTAT_OBJECT)
  continue;

The only other usage of something other than GLOBAL or DYNAMIC is the restriction on the behavior of reset_single_counter, which also has to be an object in the current database (the later condition being enforced by the presence of a valid object oid I presume).  The replacement for this below is not behavior-preserving, the proposed behavior I believe we agree is correct though.

@@ -652,7 +647,7 @@ pgstat_reset_single_counter(PgStat_Kind kind, Oid objoid)
 
- Assert(!pgstat_kind_info_for(kind)->fixed_amount);
+ Assert(pgstat_kind_info_for(kind)->kind_group == PGSTAT_OBJECT);
 

Everything else is a straight conversion of fixed_amount to CLUSTER+OBJECT

and !fixed_amount to GLOBAL

David J.