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.

The effect of the patch that I measured is as follows:

o Execution time that executed the SQL ten times.
(1)Linux(CPU: Pentium III, Compiler option: -O2)
 - original: 24.960s
 - patched : 23.114s

(2)Linux(CPU: Pentium 4, Compiler option: -O2)
 - original: 8.730s
 - patched : 7.962s

(3)Solaris(CPU: Ultra SPARC III, Compiler option: -O2)
 - original: 37.0s
 - patched : 33.7s

Atsushi Ogawa (a_ogawa)

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 1.60 2005/05/14 20:29:13 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/mmgr/aset.c,v 1.61 2005/06/04 20:14:12 momjian Exp $
  *
  * NOTE:
  *	This is a new (Feb. 05, 1999) implementation of the allocation set
@@ -399,6 +399,17 @@ 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));