Further examination of ltsReleaseBlock usage shows that it's got a

performance issue during regular merge passes not only the 'final merge'
case.  The original design contemplated that there would never be more
than about one free block per 'tape', hence no need for an efficient
method of keeping the free blocks sorted.  But given the later addition
of merge preread behavior in tuplesort.c, there is likely to be about
work_mem worth of free blocks, which is not so small ... and for that
matter the number of tapes isn't necessarily small anymore either.  So
we'd better get rid of the assumption entirely.  Instead, I'm assuming
that the usage pattern will involve alternation between merge preread
and writing of a new run.  This makes it reasonable to just add blocks
to the list without sorting during successive ltsReleaseBlock calls,
and then do a qsort() when we start getting ltsGetFreeBlock() calls.
Experimentation seems to confirm that there aren't many qsort calls
relative to the number of ltsReleaseBlock/ltsGetFreeBlock calls.

src/backend/utils/sort/logtape.c

@@ -30,8 +30,7 @@
  * For simplicity, we allocate and release space in the underlying file
  * in BLCKSZ-size blocks.  Space allocation boils down to keeping track
  * of which blocks in the underlying file belong to which logical tape,
- * plus any blocks that are free (recycled and not yet reused).  Normally
- * there are not very many free blocks, so we just keep those in a list.
+ * plus any blocks that are free (recycled and not yet reused).
  * The blocks in each logical tape are remembered using a method borrowed
  * from the Unix HFS filesystem: we store data block numbers in an
  * "indirect block".  If an indirect block fills up, we write it out to
@@ -53,6 +52,13 @@
  * not clear this helps much, but it can't hurt.  (XXX perhaps a LIFO
  * policy for free blocks would be better?)
  *
+ * To support the above policy of writing to the lowest free block,
+ * ltsGetFreeBlock sorts the list of free block numbers into decreasing
+ * order each time it is asked for a block and the list isn't currently
+ * sorted.  This is an efficient way to handle it because we expect cycles
+ * of releasing many blocks followed by re-using many blocks, due to
+ * tuplesort.c's "preread" behavior.
+ *
  * Since all the bookkeeping and buffer memory is allocated with palloc(),
  * and the underlying file(s) are made with OpenTemporaryFile, all resources
  * for a logical tape set are certain to be cleaned up even if processing
@@ -64,7 +70,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/sort/logtape.c,v 1.20 2006/03/07 19:06:49 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/sort/logtape.c,v 1.21 2006/03/07 23:46:24 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -143,15 +149,19 @@ struct LogicalTapeSet
 
 	/*
 	 * We store the numbers of recycled-and-available blocks in freeBlocks[].
-	 * When there are no such blocks, we extend the underlying file.  Note
-	 * that the block numbers in freeBlocks are always in *decreasing* order,
-	 * so that removing the last entry gives us the lowest free block.
+	 * When there are no such blocks, we extend the underlying file.
 	 *
 	 * If forgetFreeSpace is true then any freed blocks are simply forgotten
 	 * rather than being remembered in freeBlocks[].  See notes for
 	 * LogicalTapeSetForgetFreeSpace().
+	 *
+	 * If blocksSorted is true then the block numbers in freeBlocks are in
+	 * *decreasing* order, so that removing the last entry gives us the lowest
+	 * free block.  We re-sort the blocks whenever a block is demanded; this
+	 * should be reasonably efficient given the expected usage pattern.
 	 */
 	bool		forgetFreeSpace;	/* are we remembering free blocks? */
+	bool		blocksSorted;	/* is freeBlocks[] currently in order? */
 	long	   *freeBlocks;		/* resizable array */
 	int			nFreeBlocks;	/* # of currently free blocks */
 	int			freeBlocksLen;	/* current allocated length of freeBlocks[] */
@@ -223,6 +233,23 @@ ltsReadBlock(LogicalTapeSet *lts, long blocknum, void *buffer)
 						blocknum)));
 }
 
+/*
+ * qsort comparator for sorting freeBlocks[] into decreasing order.
+ */
+static int
+freeBlocks_cmp(const void *a, const void *b)
+{
+	long		ablk = *((const long *) a);
+	long		bblk = *((const long *) b);
+
+	/* can't just subtract because long might be wider than int */
+	if (ablk < bblk)
+		return 1;
+	if (ablk > bblk)
+		return -1;
+	return 0;
+}
+
 /*
  * Select a currently unused block for writing to.
  *
@@ -234,11 +261,19 @@ ltsGetFreeBlock(LogicalTapeSet *lts)
 {
 	/*
 	 * If there are multiple free blocks, we select the one appearing last in
-	 * freeBlocks[].  If there are none, assign the next block at the end of
-	 * the file.
+	 * freeBlocks[] (after sorting the array if needed).  If there are none,
+	 * assign the next block at the end of the file.
 	 */
 	if (lts->nFreeBlocks > 0)
+	{
+		if (!lts->blocksSorted)
+		{
+			qsort((void *) lts->freeBlocks, lts->nFreeBlocks,
+				  sizeof(long), freeBlocks_cmp);
+			lts->blocksSorted = true;
+		}
 		return lts->freeBlocks[--lts->nFreeBlocks];
+	}
 	else
 		return lts->nFileBlocks++;
 }
@@ -250,7 +285,6 @@ static void
 ltsReleaseBlock(LogicalTapeSet *lts, long blocknum)
 {
 	int			ndx;
-	long	   *ptr;
 
 	/*
 	 * Do nothing if we're no longer interested in remembering free space.
@@ -269,19 +303,13 @@ ltsReleaseBlock(LogicalTapeSet *lts, long blocknum)
 	}
 
 	/*
-	 * Insert blocknum into array, preserving decreasing order (so that
-	 * ltsGetFreeBlock returns the lowest available block number). This could
-	 * get fairly slow if there were many free blocks, but we don't expect
-	 * there to be very many at one time.
+	 * Add blocknum to array, and mark the array unsorted if it's no longer
+	 * in decreasing order.
 	 */
 	ndx = lts->nFreeBlocks++;
-	ptr = lts->freeBlocks + ndx;
-	while (ndx > 0 && ptr[-1] < blocknum)
-	{
-		ptr[0] = ptr[-1];
-		ndx--, ptr--;
-	}
-	ptr[0] = blocknum;
+	lts->freeBlocks[ndx] = blocknum;
+	if (ndx > 0 && lts->freeBlocks[ndx-1] < blocknum)
+		lts->blocksSorted = false;
 }
 
 /*
@@ -503,6 +531,7 @@ LogicalTapeSetCreate(int ntapes)
 	lts->pfile = BufFileCreateTemp(false);
 	lts->nFileBlocks = 0L;
 	lts->forgetFreeSpace = false;
+	lts->blocksSorted = true;	/* a zero-length array is sorted ... */
 	lts->freeBlocksLen = 32;	/* reasonable initial guess */
 	lts->freeBlocks = (long *) palloc(lts->freeBlocksLen * sizeof(long));
 	lts->nFreeBlocks = 0;