Add pgstat_drop_matching_entries() to pgstats

This allows users of the cumulative statistics to drop entries in the
shared hash stats table, deleting as well local references.  Callers of
this function can optionally define a callback able to filter which
entries to drop, similarly to pgstat_reset_matching_entries() with its
callback do_reset().

pgstat_drop_all_entries() is refactored so as it uses this new function.

Author: Lukas Fitti
Discussion: https://postgr.es/m/CAP53PkwuFbo3NkwZgxwNRMjMfqPEqidD-SggaoQ4ijotBVLJAA@mail.gmail.com

src/backend/utils/activity/pgstat_shmem.c

@@ -993,19 +993,39 @@ pgstat_drop_entry(PgStat_Kind kind, Oid dboid, uint64 objid)
 	return freed;
 }
 
+/*
+ * Scan through the shared hashtable of stats, dropping statistics if
+ * approved by the optional do_drop() function.
+ */
 void
-pgstat_drop_all_entries(void)
+pgstat_drop_matching_entries(bool (*do_drop) (PgStatShared_HashEntry *, Datum),
+							 Datum match_data)
 {
 	dshash_seq_status hstat;
 	PgStatShared_HashEntry *ps;
 	uint64		not_freed_count = 0;
 
+	/* entries are removed, take an exclusive lock */
 	dshash_seq_init(&hstat, pgStatLocal.shared_hash, true);
 	while ((ps = dshash_seq_next(&hstat)) != NULL)
 	{
 		if (ps->dropped)
 			continue;
 
+		if (do_drop != NULL && !do_drop(ps, match_data))
+			continue;
+
+		/* delete local reference */
+		if (pgStatEntryRefHash)
+		{
+			PgStat_EntryRefHashEntry *lohashent =
+				pgstat_entry_ref_hash_lookup(pgStatEntryRefHash, ps->key);
+
+			if (lohashent)
+				pgstat_release_entry_ref(lohashent->key, lohashent->entry_ref,
+										 true);
+		}
+
 		if (!pgstat_drop_entry_internal(ps, &hstat))
 			not_freed_count++;
 	}
@@ -1015,6 +1035,15 @@ pgstat_drop_all_entries(void)
 		pgstat_request_entry_refs_gc();
 }
 
+/*
+ * Scan through the shared hashtable of stats and drop all entries.
+ */
+void
+pgstat_drop_all_entries(void)
+{
+	pgstat_drop_matching_entries(NULL, 0);
+}
+
 static void
 shared_stat_reset_contents(PgStat_Kind kind, PgStatShared_Common *header,
 						   TimestampTz ts)

src/include/utils/pgstat_internal.h

@@ -718,6 +718,8 @@ extern bool pgstat_lock_entry_shared(PgStat_EntryRef *entry_ref, bool nowait);
 extern void pgstat_unlock_entry(PgStat_EntryRef *entry_ref);
 extern bool pgstat_drop_entry(PgStat_Kind kind, Oid dboid, uint64 objid);
 extern void pgstat_drop_all_entries(void);
+extern void pgstat_drop_matching_entries(bool (*do_drop) (PgStatShared_HashEntry *, Datum),
+										 Datum match_data);
 extern PgStat_EntryRef *pgstat_get_entry_ref_locked(PgStat_Kind kind, Oid dboid, uint64 objid,
 													bool nowait);
 extern void pgstat_reset_entry(PgStat_Kind kind, Oid dboid, uint64 objid, TimestampTz ts);