Modify logtape.c so that the initial LogicalTapeSetCreate call only

allocates the control data.  The per-tape buffers are allocated only
on first use.  This saves memory in situations where tuplesort.c
overestimates the number of tapes needed (ie, there are fewer runs
than tapes).  Also, this makes legitimate the coding in inittapes()
that includes tape buffer space in the maximum-memory calculation:
when inittapes runs, we've already expended the whole allowed memory
on tuple storage, and so we'd better not allocate all the tape buffers
until we've flushed some tuples out of memory.

src/backend/utils/sort/logtape.c

@@ -64,7 +64,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/sort/logtape.c,v 1.17 2005/10/18 22:59:37 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/sort/logtape.c,v 1.18 2006/02/19 05:58:36 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -124,10 +124,10 @@ typedef struct LogicalTape
 	 * we do need the relative block number so we can detect end-of-tape while
 	 * reading.
 	 */
+	char	   *buffer;			/* physical buffer (separately palloc'd) */
 	long		curBlockNumber; /* this block's logical blk# within tape */
 	int			pos;			/* next read/write position in buffer */
 	int			nbytes;			/* total # of valid bytes in buffer */
-	char		buffer[BLCKSZ];
 } LogicalTape;
 
 /*
@@ -156,7 +156,7 @@ struct LogicalTapeSet
 	 * is of length nTapes.
 	 */
 	int			nTapes;			/* # of logical tapes in set */
-	LogicalTape *tapes[1];		/* must be last in struct! */
+	LogicalTape	tapes[1];		/* must be last in struct! */
 };
 
 static void ltsWriteBlock(LogicalTapeSet *lts, long blocknum, void *buffer);
@@ -327,6 +327,10 @@ ltsRewindIndirectBlock(LogicalTapeSet *lts,
 					   IndirectBlock *indirect,
 					   bool freezing)
 {
+	/* Handle case of never-written-to tape */
+	if (indirect == NULL)
+		return -1L;
+
 	/* Insert sentinel if block is not full */
 	if (indirect->nextSlot < BLOCKS_PER_INDIR_BLOCK)
 		indirect->ptrs[indirect->nextSlot] = -1L;
@@ -366,6 +370,10 @@ static long
 ltsRewindFrozenIndirectBlock(LogicalTapeSet *lts,
 							 IndirectBlock *indirect)
 {
+	/* Handle case of never-written-to tape */
+	if (indirect == NULL)
+		return -1L;
+
 	/*
 	 * If block is not topmost, recurse to obtain address of first block in
 	 * this hierarchy level.  Read that one in.
@@ -399,6 +407,10 @@ ltsRecallNextBlockNum(LogicalTapeSet *lts,
 					  IndirectBlock *indirect,
 					  bool frozen)
 {
+	/* Handle case of never-written-to tape */
+	if (indirect == NULL)
+		return -1L;
+
 	if (indirect->nextSlot >= BLOCKS_PER_INDIR_BLOCK ||
 		indirect->ptrs[indirect->nextSlot] == -1L)
 	{
@@ -432,6 +444,10 @@ static long
 ltsRecallPrevBlockNum(LogicalTapeSet *lts,
 					  IndirectBlock *indirect)
 {
+	/* Handle case of never-written-to tape */
+	if (indirect == NULL)
+		return -1L;
+
 	if (indirect->nextSlot <= 1)
 	{
 		long		indirblock;
@@ -467,12 +483,12 @@ LogicalTapeSetCreate(int ntapes)
 	int			i;
 
 	/*
-	 * Create top-level struct.  First LogicalTape pointer is already counted
-	 * in sizeof(LogicalTapeSet).
+	 * Create top-level struct including per-tape LogicalTape structs.
+	 * First LogicalTape struct is already counted in sizeof(LogicalTapeSet).
 	 */
 	Assert(ntapes > 0);
 	lts = (LogicalTapeSet *) palloc(sizeof(LogicalTapeSet) +
-									(ntapes - 1) *sizeof(LogicalTape *));
+									(ntapes - 1) * sizeof(LogicalTape));
 	lts->pfile = BufFileCreateTemp(false);
 	lts->nFileBlocks = 0L;
 	lts->freeBlocksLen = 32;	/* reasonable initial guess */
@@ -481,20 +497,21 @@ LogicalTapeSetCreate(int ntapes)
 	lts->nTapes = ntapes;
 
 	/*
-	 * Create per-tape structs, including first-level indirect blocks.
+	 * Initialize per-tape structs.  Note we allocate the I/O buffer and
+	 * first-level indirect block for a tape only when it is first actually
+	 * written to.  This avoids wasting memory space when tuplesort.c
+	 * overestimates the number of tapes needed.
 	 */
 	for (i = 0; i < ntapes; i++)
 	{
-		lt = (LogicalTape *) palloc(sizeof(LogicalTape));
-		lts->tapes[i] = lt;
-		lt->indirect = (IndirectBlock *) palloc(sizeof(IndirectBlock));
-		lt->indirect->nextSlot = 0;
-		lt->indirect->nextup = NULL;
+		lt = &lts->tapes[i];
+		lt->indirect = NULL;
 		lt->writing = true;
 		lt->frozen = false;
 		lt->dirty = false;
 		lt->numFullBlocks = 0L;
 		lt->lastBlockBytes = 0;
+		lt->buffer = NULL;
 		lt->curBlockNumber = 0L;
 		lt->pos = 0;
 		lt->nbytes = 0;
@@ -516,13 +533,14 @@ LogicalTapeSetClose(LogicalTapeSet *lts)
 	BufFileClose(lts->pfile);
 	for (i = 0; i < lts->nTapes; i++)
 	{
-		lt = lts->tapes[i];
+		lt = &lts->tapes[i];
 		for (ib = lt->indirect; ib != NULL; ib = nextib)
 		{
 			nextib = ib->nextup;
 			pfree(ib);
 		}
-		pfree(lt);
+		if (lt->buffer)
+			pfree(lt->buffer);
 	}
 	pfree(lts->freeBlocks);
 	pfree(lts);
@@ -556,9 +574,19 @@ LogicalTapeWrite(LogicalTapeSet *lts, int tapenum,
 	size_t		nthistime;
 
 	Assert(tapenum >= 0 && tapenum < lts->nTapes);
-	lt = lts->tapes[tapenum];
+	lt = &lts->tapes[tapenum];
 	Assert(lt->writing);
 
+	/* Allocate data buffer and first indirect block on first write */
+	if (lt->buffer == NULL)
+		lt->buffer = (char *) palloc(BLCKSZ);
+	if (lt->indirect == NULL)
+	{
+		lt->indirect = (IndirectBlock *) palloc(sizeof(IndirectBlock));
+		lt->indirect->nextSlot = 0;
+		lt->indirect->nextup = NULL;
+	}
+
 	while (size > 0)
 	{
 		if (lt->pos >= BLCKSZ)
@@ -606,7 +634,7 @@ LogicalTapeRewind(LogicalTapeSet *lts, int tapenum, bool forWrite)
 	long		datablocknum;
 
 	Assert(tapenum >= 0 && tapenum < lts->nTapes);
-	lt = lts->tapes[tapenum];
+	lt = &lts->tapes[tapenum];
 
 	if (!forWrite)
 	{
@@ -660,13 +688,16 @@ LogicalTapeRewind(LogicalTapeSet *lts, int tapenum, bool forWrite)
 
 		Assert(!lt->writing && !lt->frozen);
 		/* Must truncate the indirect-block hierarchy down to one level. */
-		for (ib = lt->indirect->nextup; ib != NULL; ib = nextib)
+		if (lt->indirect)
 		{
-			nextib = ib->nextup;
-			pfree(ib);
+			for (ib = lt->indirect->nextup; ib != NULL; ib = nextib)
+			{
+				nextib = ib->nextup;
+				pfree(ib);
+			}
+			lt->indirect->nextSlot = 0;
+			lt->indirect->nextup = NULL;
 		}
-		lt->indirect->nextSlot = 0;
-		lt->indirect->nextup = NULL;
 		lt->writing = true;
 		lt->dirty = false;
 		lt->numFullBlocks = 0L;
@@ -691,7 +722,7 @@ LogicalTapeRead(LogicalTapeSet *lts, int tapenum,
 	size_t		nthistime;
 
 	Assert(tapenum >= 0 && tapenum < lts->nTapes);
-	lt = lts->tapes[tapenum];
+	lt = &lts->tapes[tapenum];
 	Assert(!lt->writing);
 
 	while (size > 0)
@@ -749,7 +780,7 @@ LogicalTapeFreeze(LogicalTapeSet *lts, int tapenum)
 	long		datablocknum;
 
 	Assert(tapenum >= 0 && tapenum < lts->nTapes);
-	lt = lts->tapes[tapenum];
+	lt = &lts->tapes[tapenum];
 	Assert(lt->writing);
 
 	/*
@@ -793,7 +824,7 @@ LogicalTapeBackspace(LogicalTapeSet *lts, int tapenum, size_t size)
 	int			newpos;
 
 	Assert(tapenum >= 0 && tapenum < lts->nTapes);
-	lt = lts->tapes[tapenum];
+	lt = &lts->tapes[tapenum];
 	Assert(lt->frozen);
 
 	/*
@@ -858,7 +889,7 @@ LogicalTapeSeek(LogicalTapeSet *lts, int tapenum,
 	LogicalTape *lt;
 
 	Assert(tapenum >= 0 && tapenum < lts->nTapes);
-	lt = lts->tapes[tapenum];
+	lt = &lts->tapes[tapenum];
 	Assert(lt->frozen);
 	Assert(offset >= 0 && offset <= BLCKSZ);
 
@@ -921,7 +952,7 @@ LogicalTapeTell(LogicalTapeSet *lts, int tapenum,
 	LogicalTape *lt;
 
 	Assert(tapenum >= 0 && tapenum < lts->nTapes);
-	lt = lts->tapes[tapenum];
+	lt = &lts->tapes[tapenum];
 	*blocknum = lt->curBlockNumber;
 	*offset = lt->pos;
 }