Set pg_class.reltuples for partitioned tables

When commit 0827e8a added auto-analyze support for partitioned
tables, it included code to obtain reltuples for the partitioned table
as a number of catalog accesses to read pg_class.reltuples for each
partition.  That's not only very inefficient, but also problematic
because autovacuum doesn't hold any locks on any of those tables -- and
doesn't want to.  Replace that code with a read of pg_class.reltuples
for the partitioned table, and make sure ANALYZE and TRUNCATE properly
maintain that value.

I found no code that would be affected by the change of relpages from
zero to non-zero for partitioned tables, and no other code that should
be maintaining it, but if there is, hopefully it'll be an easy fix.

Per buildfarm.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Zhihong Yu <zyu@yugabyte.com>
Discussion: https://postgr.es/m/1823909.1617862590@sss.pgh.pa.us

Author: alvherre

src/backend/commands/analyze.c

@@ -656,6 +656,18 @@ do_analyze_rel(Relation onerel, VacuumParams *params,
 								in_outer_xact);
 		}
 	}
+	else if (onerel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
+	{
+		/*
+		 * Partitioned tables don't have storage, so we don't set any fields in
+		 * their pg_class entries except for relpages, which is necessary for
+		 * auto-analyze to work properly.
+		 */
+		vac_update_relstats(onerel, -1, totalrows,
+							0, false, InvalidTransactionId,
+							InvalidMultiXactId,
+							in_outer_xact);
+	}
 
 	/*
 	 * Now report ANALYZE to the stats collector.  For regular tables, we do

src/backend/commands/tablecmds.c

@@ -337,6 +337,7 @@ typedef struct ForeignTruncateInfo
 static void truncate_check_rel(Oid relid, Form_pg_class reltuple);
 static void truncate_check_perms(Oid relid, Form_pg_class reltuple);
 static void truncate_check_activity(Relation rel);
+static void truncate_update_partedrel_stats(List *parted_rels);
 static void RangeVarCallbackForTruncate(const RangeVar *relation,
 										Oid relId, Oid oldRelId, void *arg);
 static List *MergeAttributes(List *schema, List *supers, char relpersistence,
@@ -1755,6 +1756,7 @@ ExecuteTruncateGuts(List *explicit_rels,
 {
 	List	   *rels;
 	List	   *seq_relids = NIL;
+	List	   *parted_rels = NIL;
 	HTAB	   *ft_htab = NULL;
 	EState	   *estate;
 	ResultRelInfo *resultRelInfos;
@@ -1908,9 +1910,15 @@ ExecuteTruncateGuts(List *explicit_rels,
 		Relation	rel = (Relation) lfirst(lc1);
 		int			extra = lfirst_int(lc2);
 
-		/* Skip partitioned tables as there is nothing to do */
+		/*
+		 * Save OID of partitioned tables for later; nothing else to do for
+		 * them here.
+		 */
 		if (rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
+		{
+			parted_rels = lappend_oid(parted_rels, RelationGetRelid(rel));
 			continue;
+		}
 
 		/*
 		 * Build the lists of foreign tables belonging to each foreign server
@@ -2061,6 +2069,9 @@ ExecuteTruncateGuts(List *explicit_rels,
 		ResetSequence(seq_relid);
 	}
 
+	/* Reset partitioned tables' pg_class.reltuples */
+	truncate_update_partedrel_stats(parted_rels);
+
 	/*
 	 * Write a WAL record to allow this set of actions to be logically
 	 * decoded.
@@ -2207,6 +2218,40 @@ truncate_check_activity(Relation rel)
 	CheckTableNotInUse(rel, "TRUNCATE");
 }
 
+/*
+ * Update pg_class.reltuples for all the given partitioned tables to 0.
+ */
+static void
+truncate_update_partedrel_stats(List *parted_rels)
+{
+	Relation	pg_class;
+	ListCell   *lc;
+
+	pg_class = table_open(RelationRelationId, RowExclusiveLock);
+
+	foreach(lc, parted_rels)
+	{
+		Oid			relid = lfirst_oid(lc);
+		HeapTuple	tuple;
+		Form_pg_class rd_rel;
+
+		tuple = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(relid));
+		if (!HeapTupleIsValid(tuple))
+			elog(ERROR, "could not find tuple for relation %u", relid);
+		rd_rel = (Form_pg_class) GETSTRUCT(tuple);
+		if (rd_rel->reltuples != (float4) 0)
+		{
+			rd_rel->reltuples = (float4) 0;
+
+			heap_inplace_update(pg_class, tuple);
+		}
+
+		heap_freetuple(tuple);
+	}
+
+	table_close(pg_class, RowExclusiveLock);
+}
+
 /*
  * storage_name
  *	  returns the name corresponding to a typstorage/attstorage enum value

src/backend/postmaster/autovacuum.c

@@ -3209,44 +3209,7 @@ relation_needs_vacanalyze(Oid relid,
 	 */
 	if (PointerIsValid(tabentry) && AutoVacuumingActive())
 	{
-		if (classForm->relkind != RELKIND_PARTITIONED_TABLE)
-		{
-			reltuples = classForm->reltuples;
-		}
-		else
-		{
-			/*
-			 * If the relation is a partitioned table, we must add up
-			 * children's reltuples.
-			 */
-			List	   *children;
-			ListCell   *lc;
-
-			reltuples = 0;
-
-			/* Find all members of inheritance set taking AccessShareLock */
-			children = find_all_inheritors(relid, AccessShareLock, NULL);
-
-			foreach(lc, children)
-			{
-				Oid			childOID = lfirst_oid(lc);
-				HeapTuple	childtuple;
-				Form_pg_class childclass;
-
-				childtuple = SearchSysCache1(RELOID, ObjectIdGetDatum(childOID));
-				childclass = (Form_pg_class) GETSTRUCT(childtuple);
-
-				/* Skip a partitioned table and foreign partitions */
-				if (RELKIND_HAS_STORAGE(childclass->relkind))
-				{
-					/* Sum up the child's reltuples for its parent table */
-					reltuples += childclass->reltuples;
-				}
-				ReleaseSysCache(childtuple);
-			}
-
-			list_free(children);
-		}
+		reltuples = classForm->reltuples;
 		vactuples = tabentry->n_dead_tuples;
 		instuples = tabentry->inserts_since_vacuum;
 		anltuples = tabentry->changes_since_analyze;