/*-------------------------------------------------------------------------
 * nbtsort.c
 *		Build a btree from sorted input by loading leaf pages sequentially.
 *
 * NOTES
 *
 * We use tuplesort.c to sort the given index tuples into order.
 * Then we scan the index tuples in order and build the btree pages
 * for each level.  We load source tuples into leaf-level pages.
 * Whenever we fill a page at one level, we add a link to it to its
 * parent level (starting a new parent level if necessary).  When
 * done, we write out each final page on each level, adding it to
 * its parent level.  When we have only one page on a level, it must be
 * the root -- it can be attached to the btree metapage and we are done.
 *
 * This code is moderately slow (~10% slower) compared to the regular
 * btree (insertion) build code on sorted or well-clustered data.  On
 * random data, however, the insertion build code is unusable -- the
 * difference on a 60MB heap is a factor of 15 because the random
 * probes into the btree thrash the buffer pool.  (NOTE: the above
 * "10%" estimate is probably obsolete, since it refers to an old and
 * not very good external sort implementation that used to exist in
 * this module.  tuplesort.c is almost certainly faster.)
 *
 * It is not wise to pack the pages entirely full, since then *any*
 * insertion would cause a split (and not only of the leaf page; the need
 * for a split would cascade right up the tree).  The steady-state load
 * factor for btrees is usually estimated at 70%.  We choose to pack leaf
 * pages to 90% and upper pages to 70%.  This gives us reasonable density
 * (there aren't many upper pages if the keys are reasonable-size) without
 * incurring a lot of cascading splits during early insertions.
 *
 *
 * Portions Copyright (c) 1996-2000, PostgreSQL, Inc
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *	  $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtsort.c,v 1.56 2000/07/21 22:14:09 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/nbtree.h"
#include "utils/tuplesort.h"


/*
 * Status record for spooling.
 */
struct BTSpool
{
	Tuplesortstate *sortstate;	/* state data for tuplesort.c */
	Relation	index;
	bool		isunique;
};

/*
 * Status record for a btree page being built.  We have one of these
 * for each active tree level.
 *
 * The reason we need to store a copy of the minimum key is that we'll
 * need to propagate it to the parent node when this page is linked
 * into its parent.  However, if the page is not a leaf page, the first
 * entry on the page doesn't need to contain a key, so we will not have
 * stored the key itself on the page.  (You might think we could skip
 * copying the minimum key on leaf pages, but actually we must have a
 * writable copy anyway because we'll poke the page's address into it
 * before passing it up to the parent...)
 */
typedef struct BTPageState
{
	Buffer		btps_buf;		/* current buffer & page */
	Page		btps_page;
	BTItem		btps_minkey;	/* copy of minimum key (first item) on page */
	OffsetNumber btps_lastoff;	/* last item offset loaded */
	int			btps_level;		/* tree level (0 = leaf) */
	Size		btps_full;		/* "full" if less than this much free space */
	struct BTPageState *btps_next; /* link to parent level, if any */
} BTPageState;


#define BTITEMSZ(btitem) \
	((btitem) ? \
	 (IndexTupleDSize((btitem)->bti_itup) + \
	  (sizeof(BTItemData) - sizeof(IndexTupleData))) : \
	 0)


static void _bt_blnewpage(Relation index, Buffer *buf, Page *page, int flags);
static BTPageState *_bt_pagestate(Relation index, int flags, int level);
static void _bt_slideleft(Relation index, Buffer buf, Page page);
static void _bt_sortaddtup(Page page, Size itemsize,
						   BTItem btitem, OffsetNumber itup_off);
static void _bt_buildadd(Relation index, BTPageState *state, BTItem bti);
static void _bt_uppershutdown(Relation index, BTPageState *state);
static void _bt_load(Relation index, BTSpool *btspool);


/*
 * Interface routines
 */


/*
 * create and initialize a spool structure
 */
BTSpool    *
_bt_spoolinit(Relation index, bool isunique)
{
	BTSpool    *btspool = (BTSpool *) palloc(sizeof(BTSpool));

	MemSet((char *) btspool, 0, sizeof(BTSpool));

	btspool->index = index;
	btspool->isunique = isunique;

	btspool->sortstate = tuplesort_begin_index(index, isunique, false);

	/*
	 * Currently, tuplesort provides sort functions on IndexTuples. If we
	 * kept anything in a BTItem other than a regular IndexTuple, we'd
	 * need to modify tuplesort to understand BTItems as such.
	 */
	Assert(sizeof(BTItemData) == sizeof(IndexTupleData));

	return btspool;
}

/*
 * clean up a spool structure and its substructures.
 */
void
_bt_spooldestroy(BTSpool *btspool)
{
	tuplesort_end(btspool->sortstate);
	pfree((void *) btspool);
}

/*
 * spool a btitem into the sort file.
 */
void
_bt_spool(BTItem btitem, BTSpool *btspool)
{
	/* A BTItem is really just an IndexTuple */
	tuplesort_puttuple(btspool->sortstate, (void *) btitem);
}

/*
 * given a spool loaded by successive calls to _bt_spool,
 * create an entire btree.
 */
void
_bt_leafbuild(BTSpool *btspool)
{
#ifdef BTREE_BUILD_STATS
	if (Show_btree_build_stats)
	{
		fprintf(StatFp, "BTREE BUILD (Spool) STATISTICS\n");
		ShowUsage();
		ResetUsage();
	}
#endif /* BTREE_BUILD_STATS */
	tuplesort_performsort(btspool->sortstate);

	_bt_load(btspool->index, btspool);
}


/*
 * Internal routines.
 */


/*
 * allocate a new, clean btree page, not linked to any siblings.
 */
static void
_bt_blnewpage(Relation index, Buffer *buf, Page *page, int flags)
{
	BTPageOpaque opaque;

	*buf = _bt_getbuf(index, P_NEW, BT_WRITE);
	*page = BufferGetPage(*buf);
	_bt_pageinit(*page, BufferGetPageSize(*buf));
	opaque = (BTPageOpaque) PageGetSpecialPointer(*page);
	opaque->btpo_prev = opaque->btpo_next = P_NONE;
	opaque->btpo_flags = flags;
}

/*
 * allocate and initialize a new BTPageState.  the returned structure
 * is suitable for immediate use by _bt_buildadd.
 */
static BTPageState *
_bt_pagestate(Relation index, int flags, int level)
{
	BTPageState *state = (BTPageState *) palloc(sizeof(BTPageState));

	MemSet((char *) state, 0, sizeof(BTPageState));

	/* create initial page */
	_bt_blnewpage(index, &(state->btps_buf), &(state->btps_page), flags);

	state->btps_minkey = (BTItem) NULL;
	/* initialize lastoff so first item goes into P_FIRSTKEY */
	state->btps_lastoff = P_HIKEY;
	state->btps_level = level;
	/* set "full" threshold based on level.  See notes at head of file. */
	if (level > 0)
		state->btps_full = (PageGetPageSize(state->btps_page) * 3) / 10;
	else
		state->btps_full = PageGetPageSize(state->btps_page) / 10;
	/* no parent level, yet */
	state->btps_next = (BTPageState *) NULL;

	return state;
}

/*
 * slide an array of ItemIds back one slot (from P_FIRSTKEY to
 * P_HIKEY, overwriting P_HIKEY).  we need to do this when we discover
 * that we have built an ItemId array in what has turned out to be a
 * P_RIGHTMOST page.
 */
static void
_bt_slideleft(Relation index, Buffer buf, Page page)
{
	OffsetNumber off;
	OffsetNumber maxoff;
	ItemId		previi;
	ItemId		thisii;

	if (!PageIsEmpty(page))
	{
		maxoff = PageGetMaxOffsetNumber(page);
		previi = PageGetItemId(page, P_HIKEY);
		for (off = P_FIRSTKEY; off <= maxoff; off = OffsetNumberNext(off))
		{
			thisii = PageGetItemId(page, off);
			*previi = *thisii;
			previi = thisii;
		}
		((PageHeader) page)->pd_lower -= sizeof(ItemIdData);
	}
}

/*
 * Add an item to a page being built.
 *
 * The main difference between this routine and a bare PageAddItem call
 * is that this code knows that the leftmost data item on a non-leaf
 * btree page doesn't need to have a key.  Therefore, it strips such
 * items down to just the item header.
 *
 * This is almost like nbtinsert.c's _bt_pgaddtup(), but we can't use
 * that because it assumes that P_RIGHTMOST() will return the correct
 * answer for the page.  Here, we don't know yet if the page will be
 * rightmost.  Offset P_FIRSTKEY is always the first data key.
 */
static void
_bt_sortaddtup(Page page,
			   Size itemsize,
			   BTItem btitem,
			   OffsetNumber itup_off)
{
	BTPageOpaque opaque = (BTPageOpaque) PageGetSpecialPointer(page);
	BTItemData truncitem;

	if (! P_ISLEAF(opaque) && itup_off == P_FIRSTKEY)
	{
		memcpy(&truncitem, btitem, sizeof(BTItemData));
		truncitem.bti_itup.t_info = sizeof(BTItemData);
		btitem = &truncitem;
		itemsize = sizeof(BTItemData);
	}

	if (PageAddItem(page, (Item) btitem, itemsize, itup_off,
					LP_USED) == InvalidOffsetNumber)
		elog(FATAL, "btree: failed to add item to the page in _bt_sort");
}

/*----------
 * Add an item to a disk page from the sort output.
 *
 * We must be careful to observe the page layout conventions of nbtsearch.c:
 * - rightmost pages start data items at P_HIKEY instead of at P_FIRSTKEY.
 * - on non-leaf pages, the key portion of the first item need not be
 *   stored, we should store only the link.
 *
 * A leaf page being built looks like:
 *
 * +----------------+---------------------------------+
 * | PageHeaderData | linp0 linp1 linp2 ...			  |
 * +-----------+----+---------------------------------+
 * | ... linpN |									  |
 * +-----------+--------------------------------------+
 * |	 ^ last										  |
 * |												  |
 * +-------------+------------------------------------+
 * |			 | itemN ...						  |
 * +-------------+------------------+-----------------+
 * |		  ... item3 item2 item1 | "special space" |
 * +--------------------------------+-----------------+
 *
 * Contrast this with the diagram in bufpage.h; note the mismatch
 * between linps and items.  This is because we reserve linp0 as a
 * placeholder for the pointer to the "high key" item; when we have
 * filled up the page, we will set linp0 to point to itemN and clear
 * linpN.  On the other hand, if we find this is the last (rightmost)
 * page, we leave the items alone and slide the linp array over.
 *
 * 'last' pointer indicates the last offset added to the page.
 *----------
 */
static void
_bt_buildadd(Relation index, BTPageState *state, BTItem bti)
{
	Buffer		nbuf;
	Page		npage;
	OffsetNumber last_off;
	Size		pgspc;
	Size		btisz;

	nbuf = state->btps_buf;
	npage = state->btps_page;
	last_off = state->btps_lastoff;

	pgspc = PageGetFreeSpace(npage);
	btisz = BTITEMSZ(bti);
	btisz = MAXALIGN(btisz);

	/*
	 * Check whether the item can fit on a btree page at all. (Eventually,
	 * we ought to try to apply TOAST methods if not.) We actually need to
	 * be able to fit three items on every page, so restrict any one item
	 * to 1/3 the per-page available space. Note that at this point, btisz
	 * doesn't include the ItemId.
	 *
	 * NOTE: similar code appears in _bt_insertonpg() to defend against
	 * oversize items being inserted into an already-existing index. But
	 * during creation of an index, we don't go through there.
	 */
	if (btisz > (PageGetPageSize(npage) - sizeof(PageHeaderData) - MAXALIGN(sizeof(BTPageOpaqueData))) / 3 - sizeof(ItemIdData))
		elog(ERROR, "btree: index item size %d exceeds maximum %ld",
			 btisz,
			 (PageGetPageSize(npage) - sizeof(PageHeaderData) - MAXALIGN(sizeof(BTPageOpaqueData))) /3 - sizeof(ItemIdData));

	if (pgspc < btisz || pgspc < state->btps_full)
	{
		/*
		 * Item won't fit on this page, or we feel the page is full enough
		 * already.  Finish off the page and write it out.
		 */
		Buffer		obuf = nbuf;
		Page		opage = npage;
		ItemId		ii;
		ItemId		hii;
		BTItem		obti;

		/* Create new page */
		_bt_blnewpage(index, &nbuf, &npage,
					  (state->btps_level > 0) ? 0 : BTP_LEAF);

		/*
		 * We copy the last item on the page into the new page, and then
		 * rearrange the old page so that the 'last item' becomes its high
		 * key rather than a true data item.  There had better be at least
		 * two items on the page already, else the page would be empty of
		 * useful data.  (Hence, we must allow pages to be packed at least
		 * 2/3rds full; the 70% figure used above is close to minimum.)
		 */
		Assert(last_off > P_FIRSTKEY);
		ii = PageGetItemId(opage, last_off);
		obti = (BTItem) PageGetItem(opage, ii);
		_bt_sortaddtup(npage, ItemIdGetLength(ii), obti, P_FIRSTKEY);

		/*
		 * Move 'last' into the high key position on opage
		 */
		hii = PageGetItemId(opage, P_HIKEY);
		*hii = *ii;
		ii->lp_flags &= ~LP_USED;
		((PageHeader) opage)->pd_lower -= sizeof(ItemIdData);

		/*
		 * Link the old buffer into its parent, using its minimum key.
		 * If we don't have a parent, we have to create one;
		 * this adds a new btree level.
		 */
		if (state->btps_next == (BTPageState *) NULL)
		{
			state->btps_next =
				_bt_pagestate(index, 0, state->btps_level + 1);
		}
		Assert(state->btps_minkey != NULL);
		ItemPointerSet(&(state->btps_minkey->bti_itup.t_tid),
					   BufferGetBlockNumber(obuf), P_HIKEY);
		_bt_buildadd(index, state->btps_next, state->btps_minkey);
		pfree((void *) state->btps_minkey);

		/*
		 * Save a copy of the minimum key for the new page.  We have to
		 * copy it off the old page, not the new one, in case we are
		 * not at leaf level.
		 */
		state->btps_minkey = _bt_formitem(&(obti->bti_itup));

		/*
		 * Set the sibling links for both pages, and parent links too.
		 *
		 * It's not necessary to set the parent link at all, because it's
		 * only used for handling concurrent root splits, but we may as well
		 * do it as a debugging aid.  Note we set new page's link as well
		 * as old's, because if the new page turns out to be the last of
		 * the level, _bt_uppershutdown won't change it.  The links may be
		 * out of date by the time the build finishes, but that's OK; they
		 * need only point to a left-sibling of the true parent.  See the
		 * README file for more info.
		 */
		{
			BTPageOpaque oopaque = (BTPageOpaque) PageGetSpecialPointer(opage);
			BTPageOpaque nopaque = (BTPageOpaque) PageGetSpecialPointer(npage);

			oopaque->btpo_next = BufferGetBlockNumber(nbuf);
			nopaque->btpo_prev = BufferGetBlockNumber(obuf);
			nopaque->btpo_next = P_NONE;
			oopaque->btpo_parent = nopaque->btpo_parent =
				BufferGetBlockNumber(state->btps_next->btps_buf);
		}

		/*
		 * Write out the old page.  We never want to see it again, so we
		 * can give up our lock (if we had one; most likely BuildingBtree
		 * is set, so we aren't locking).
		 */
		_bt_wrtbuf(index, obuf);

		/*
		 * Reset last_off to point to new page
		 */
		last_off = P_FIRSTKEY;
	}

	/*
	 * If the new item is the first for its page, stash a copy for later.
	 * Note this will only happen for the first item on a level; on later
	 * pages, the first item for a page is copied from the prior page
	 * in the code above.
	 */
	if (last_off == P_HIKEY)
	{
		Assert(state->btps_minkey == NULL);
		state->btps_minkey = _bt_formitem(&(bti->bti_itup));
	}

	/*
	 * Add the new item into the current page.
	 */
	last_off = OffsetNumberNext(last_off);
	_bt_sortaddtup(npage, btisz, bti, last_off);

	state->btps_buf = nbuf;
	state->btps_page = npage;
	state->btps_lastoff = last_off;
}

/*
 * Finish writing out the completed btree.
 */
static void
_bt_uppershutdown(Relation index, BTPageState *state)
{
	BTPageState *s;

	/*
	 * Each iteration of this loop completes one more level of the tree.
	 */
	for (s = state; s != (BTPageState *) NULL; s = s->btps_next)
	{
		BlockNumber blkno;
		BTPageOpaque opaque;

		blkno = BufferGetBlockNumber(s->btps_buf);
		opaque = (BTPageOpaque) PageGetSpecialPointer(s->btps_page);

		/*
		 * We have to link the last page on this level to somewhere.
		 *
		 * If we're at the top, it's the root, so attach it to the metapage.
		 * Otherwise, add an entry for it to its parent using its minimum
		 * key.  This may cause the last page of the parent level to split,
		 * but that's not a problem -- we haven't gotten to it yet.
		 */
		if (s->btps_next == (BTPageState *) NULL)
		{
			opaque->btpo_flags |= BTP_ROOT;
			_bt_metaproot(index, blkno, s->btps_level + 1);
		}
		else
		{
			Assert(s->btps_minkey != NULL);
			ItemPointerSet(&(s->btps_minkey->bti_itup.t_tid),
						   blkno, P_HIKEY);
			_bt_buildadd(index, s->btps_next, s->btps_minkey);
			pfree((void *) s->btps_minkey);
			s->btps_minkey = NULL;
		}

		/*
		 * This is the rightmost page, so the ItemId array needs to be
		 * slid back one slot.  Then we can dump out the page.
		 */
		_bt_slideleft(index, s->btps_buf, s->btps_page);
		_bt_wrtbuf(index, s->btps_buf);
	}
}

/*
 * Read tuples in correct sort order from tuplesort, and load them into
 * btree leaves.
 */
static void
_bt_load(Relation index, BTSpool *btspool)
{
	BTPageState *state = NULL;

	for (;;)
	{
		BTItem		bti;
		bool		should_free;

		bti = (BTItem) tuplesort_getindextuple(btspool->sortstate, true,
											   &should_free);
		if (bti == (BTItem) NULL)
			break;

		/* When we see first tuple, create first index page */
		if (state == NULL)
			state = _bt_pagestate(index, BTP_LEAF, 0);

		_bt_buildadd(index, state, bti);

		if (should_free)
			pfree((void *) bti);
	}

	/* Close down final pages, if we had any data at all */
	if (state != NULL)
		_bt_uppershutdown(index, state);
}
