Back out patch:

---------------------------------------------------------------------------

Tom Lane <tgl@sss.pgh.pa.us> writes:
> a_ogawa <a_ogawa@hi-ho.ne.jp> writes:
> > It is a reasonable idea. However, the majority part of MemSet was not
> > able to be avoided by this idea. Because the per-tuple contexts are used
> > at the early stage of executor.
>
> Drat.  Well, what about changing that?  We could introduce additional
> contexts or change the startup behavior so that the ones that are
> frequently reset don't have any data in them unless you are working
> with pass-by-ref values inside the inner loop.

That might be possible. However, I think that we should change only
aset.c about this article.
I thought further: We can check whether context was used from the last
reset even when blocks list is not empty. Please see attached patch.

src/backend/utils/mmgr/aset.c

@@ -11,7 +11,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/mmgr/aset.c,v 1.61 2005/06/04 20:14:12 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/mmgr/aset.c,v 1.62 2005/06/04 22:57:22 momjian Exp $
  *
  * NOTE:
  *	This is a new (Feb. 05, 1999) implementation of the allocation set
@@ -399,17 +399,6 @@ AllocSetReset(MemoryContext context)
 	if (block == NULL)
 		return;
 
-	/*
-	 * When blocks list has only "keeper" block and freeptr of the block
-	 * is initial value, the context is not used from last reset.
-	 */
-	if (block == set->keeper && block->next == NULL)
-	{
-		char	   *datastart = ((char *) block) + ALLOC_BLOCKHDRSZ;
-		if (block->freeptr == datastart)
-			return;
-	}
-
 	/* Clear chunk freelists */
 	MemSetAligned(set->freelist, 0, sizeof(set->freelist));