Make TRUNCATE do truncate-in-place when processing a relation that was created

or previously truncated in the current (sub)transaction.  This is safe since
if the (sub)transaction later rolls back, we'd just discard the rel's current
physical file anyway.  This avoids unreasonable growth in the number of
transient files when a relation is repeatedly truncated.  Per a performance
gripe a couple weeks ago from Todd Cook.

src/backend/catalog/heap.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/catalog/heap.c,v 1.357 2009/08/02 22:14:52 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/catalog/heap.c,v 1.358 2009/08/23 19:23:40 tgl Exp $
  *
  *
  * INTERFACE ROUTINES
@@ -2342,18 +2342,9 @@ heap_truncate(List *relids)
 	{
 		Oid			rid = lfirst_oid(cell);
 		Relation	rel;
-		Oid			toastrelid;
 
 		rel = heap_open(rid, AccessExclusiveLock);
 		relations = lappend(relations, rel);
-
-		/* If there is a toast table, add it to the list too */
-		toastrelid = rel->rd_rel->reltoastrelid;
-		if (OidIsValid(toastrelid))
-		{
-			rel = heap_open(toastrelid, AccessExclusiveLock);
-			relations = lappend(relations, rel);
-		}
 	}
 
 	/* Don't allow truncate on tables that are referenced by foreign keys */
@@ -2364,19 +2355,47 @@ heap_truncate(List *relids)
 	{
 		Relation	rel = lfirst(cell);
 
-		/* Truncate the actual file (and discard buffers) */
-		RelationTruncate(rel, 0);
+		/* Truncate the relation */
+		heap_truncate_one_rel(rel);
 
-		/* If this relation has indexes, truncate the indexes too */
-		RelationTruncateIndexes(rel);
-
-		/*
-		 * Close the relation, but keep exclusive lock on it until commit.
-		 */
+		/* Close the relation, but keep exclusive lock on it until commit */
 		heap_close(rel, NoLock);
 	}
 }
 
+/*
+ *	 heap_truncate_one_rel
+ *
+ *	 This routine deletes all data within the specified relation.
+ *
+ * This is not transaction-safe, because the truncation is done immediately
+ * and cannot be rolled back later.  Caller is responsible for having
+ * checked permissions etc, and must have obtained AccessExclusiveLock.
+ */
+void
+heap_truncate_one_rel(Relation rel)
+{
+	Oid			toastrelid;
+
+	/* Truncate the actual file (and discard buffers) */
+	RelationTruncate(rel, 0);
+
+	/* If the relation has indexes, truncate the indexes too */
+	RelationTruncateIndexes(rel);
+
+	/* If there is a toast table, truncate that too */
+	toastrelid = rel->rd_rel->reltoastrelid;
+	if (OidIsValid(toastrelid))
+	{
+		Relation	toastrel = heap_open(toastrelid, AccessExclusiveLock);
+
+		RelationTruncate(toastrel, 0);
+		RelationTruncateIndexes(toastrel);
+		/* keep the lock... */
+		heap_close(toastrel, NoLock);
+	}
+}
+
 /*
  * heap_truncate_check_FKs
  *		Check for foreign keys referencing a list of relations that

src/backend/commands/tablecmds.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.297 2009/08/12 23:00:12 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.298 2009/08/23 19:23:41 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -775,6 +775,7 @@ ExecuteTruncate(TruncateStmt *stmt)
 	EState	   *estate;
 	ResultRelInfo *resultRelInfos;
 	ResultRelInfo *resultRelInfo;
+	SubTransactionId mySubid;
 	ListCell   *cell;
 
 	/*
@@ -944,36 +945,58 @@ ExecuteTruncate(TruncateStmt *stmt)
 	/*
 	 * OK, truncate each table.
 	 */
+	mySubid = GetCurrentSubTransactionId();
+
 	foreach(cell, rels)
 	{
 		Relation	rel = (Relation) lfirst(cell);
-		Oid			heap_relid;
-		Oid			toast_relid;
 
 		/*
-		 * Create a new empty storage file for the relation, and assign it as
-		 * the relfilenode value.	The old storage file is scheduled for
-		 * deletion at commit.
+		 * Normally, we need a transaction-safe truncation here.  However,
+		 * if the table was either created in the current (sub)transaction
+		 * or has a new relfilenode in the current (sub)transaction, then
+		 * we can just truncate it in-place, because a rollback would
+		 * cause the whole table or the current physical file to be
+		 * thrown away anyway.
 		 */
-		setNewRelfilenode(rel, RecentXmin);
-
-		heap_relid = RelationGetRelid(rel);
-		toast_relid = rel->rd_rel->reltoastrelid;
-
-		/*
-		 * The same for the toast table, if any.
-		 */
-		if (OidIsValid(toast_relid))
+		if (rel->rd_createSubid == mySubid ||
+			rel->rd_newRelfilenodeSubid == mySubid)
 		{
-			rel = relation_open(toast_relid, AccessExclusiveLock);
-			setNewRelfilenode(rel, RecentXmin);
-			heap_close(rel, NoLock);
+			/* Immediate, non-rollbackable truncation is OK */
+			heap_truncate_one_rel(rel);
 		}
+		else
+		{
+			Oid			heap_relid;
+			Oid			toast_relid;
 
-		/*
-		 * Reconstruct the indexes to match, and we're done.
-		 */
-		reindex_relation(heap_relid, true);
+			/*
+			 * Need the full transaction-safe pushups.
+			 *
+			 * Create a new empty storage file for the relation, and assign it
+			 * as the relfilenode value. The old storage file is scheduled for
+			 * deletion at commit.
+			 */
+			setNewRelfilenode(rel, RecentXmin);
+
+			heap_relid = RelationGetRelid(rel);
+			toast_relid = rel->rd_rel->reltoastrelid;
+
+			/*
+			 * The same for the toast table, if any.
+			 */
+			if (OidIsValid(toast_relid))
+			{
+				rel = relation_open(toast_relid, AccessExclusiveLock);
+				setNewRelfilenode(rel, RecentXmin);
+				heap_close(rel, NoLock);
+			}
+
+			/*
+			 * Reconstruct the indexes to match, and we're done.
+			 */
+			reindex_relation(heap_relid, true);
+		}
 	}
 
 	/*

src/include/catalog/heap.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/catalog/heap.h,v 1.91 2009/06/11 14:49:09 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/catalog/heap.h,v 1.92 2009/08/23 19:23:41 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -62,6 +62,8 @@ extern void heap_drop_with_catalog(Oid relid);
 
 extern void heap_truncate(List *relids);
 
+extern void heap_truncate_one_rel(Relation rel);
+
 extern void heap_truncate_check_FKs(List *relations, bool tempTables);
 
 extern List *heap_truncate_find_FKs(List *relationIds);