nbtsearch.c

PostgreSQL7.4.6 for Linux

/*-------------------------------------------------------------------------
 *
 * nbtsearch.c
 *	  Search code for postgres btrees.
 *
 *
 * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *	  $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.80 2003/08/08 21:41:27 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/genam.h"
#include "access/nbtree.h"


static Buffer _bt_walk_left(Relation rel, Buffer buf);
static bool _bt_endpoint(IndexScanDesc scan, ScanDirection dir);


/*
 *	_bt_search() -- Search the tree for a particular scankey,
 *		or more precisely for the first leaf page it could be on.
 *
 * Return value is a stack of parent-page pointers.  *bufP is set to the
 * address of the leaf-page buffer, which is read-locked and pinned.
 * No locks are held on the parent pages, however!
 *
 * NOTE that the returned buffer is read-locked regardless of the access
 * parameter.  However, access = BT_WRITE will allow an empty root page
 * to be created and returned.	When access = BT_READ, an empty index
 * will result in *bufP being set to InvalidBuffer.
 */
BTStack
_bt_search(Relation rel, int keysz, ScanKey scankey,
		   Buffer *bufP, int access)
{
	BTStack		stack_in = NULL;

	/* Get the root page to start with */
	*bufP = _bt_getroot(rel, access);

	/* If index is empty and access = BT_READ, no root page is created. */
	if (!BufferIsValid(*bufP))
		return (BTStack) NULL;

	/* Loop iterates once per level descended in the tree */
	for (;;)
	{
		Page		page;
		BTPageOpaque opaque;
		OffsetNumber offnum;
		ItemId		itemid;
		BTItem		btitem;
		IndexTuple	itup;
		BlockNumber blkno;
		BlockNumber par_blkno;
		BTStack		new_stack;

		/*
		 * Race -- the page we just grabbed may have split since we read
		 * its pointer in the parent (or metapage).  If it has, we may
		 * need to move right to its new sibling.  Do that.
		 */
		*bufP = _bt_moveright(rel, *bufP, keysz, scankey, BT_READ);

		/* if this is a leaf page, we're done */
		page = BufferGetPage(*bufP);
		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
		if (P_ISLEAF(opaque))
			break;

		/*
		 * Find the appropriate item on the internal page, and get the
		 * child page that it points to.
		 */
		offnum = _bt_binsrch(rel, *bufP, keysz, scankey);
		itemid = PageGetItemId(page, offnum);
		btitem = (BTItem) PageGetItem(page, itemid);
		itup = &(btitem->bti_itup);
		blkno = ItemPointerGetBlockNumber(&(itup->t_tid));
		par_blkno = BufferGetBlockNumber(*bufP);

		/*
		 * We need to save the location of the index entry we chose in the
		 * parent page on a stack. In case we split the tree, we'll use
		 * the stack to work back up to the parent page.  We also save the
		 * actual downlink (TID) to uniquely identify the index entry, in
		 * case it moves right while we're working lower in the tree.  See
		 * the paper by Lehman and Yao for how this is detected and
		 * handled. (We use the child link to disambiguate duplicate keys
		 * in the index -- Lehman and Yao disallow duplicate keys.)
		 */
		new_stack = (BTStack) palloc(sizeof(BTStackData));
		new_stack->bts_blkno = par_blkno;
		new_stack->bts_offset = offnum;
		memcpy(&new_stack->bts_btitem, btitem, sizeof(BTItemData));
		new_stack->bts_parent = stack_in;

		/* drop the read lock on the parent page, acquire one on the child */
		_bt_relbuf(rel, *bufP);
		*bufP = _bt_getbuf(rel, blkno, BT_READ);

		/* okay, all set to move down a level */
		stack_in = new_stack;
	}

	return stack_in;
}

/*
 *	_bt_moveright() -- move right in the btree if necessary.
 *
 *		When we follow a pointer to reach a page, it is possible that
 *		the page has changed in the meanwhile.	If this happens, we're
 *		guaranteed that the page has "split right" -- that is, that any
 *		data that appeared on the page originally is either on the page
 *		or strictly to the right of it.
 *
 *		This routine decides whether or not we need to move right in the
 *		tree by examining the high key entry on the page.  If that entry
 *		is strictly less than one we expect to be on the page, then our
 *		picture of the page is incorrect and we need to move right.
 *
 *		On entry, we have the buffer pinned and a lock of the proper type.
 *		If we move right, we release the buffer and lock and acquire the
 *		same on the right sibling.	Return value is the buffer we stop at.
 */
Buffer
_bt_moveright(Relation rel,
			  Buffer buf,
			  int keysz,
			  ScanKey scankey,
			  int access)
{
	Page		page;
	BTPageOpaque opaque;

	page = BufferGetPage(buf);
	opaque = (BTPageOpaque) PageGetSpecialPointer(page);

	/*
	 * If the scan key that brought us to this page is > the high key
	 * stored on the page, then the page has split and we need to move
	 * right.  (If the scan key is equal to the high key, we might or
	 * might not need to move right; have to scan the page first anyway.)
	 * It could even have split more than once, so scan as far as needed.
	 *
	 * We also have to move right if we followed a link that brought us to a
	 * dead page.
	 */
	while (!P_RIGHTMOST(opaque) &&
		   (P_IGNORE(opaque) ||
			_bt_compare(rel, keysz, scankey, page, P_HIKEY) > 0))
	{
		/* step right one page */
		BlockNumber rblkno = opaque->btpo_next;

		_bt_relbuf(rel, buf);
		buf = _bt_getbuf(rel, rblkno, access);
		page = BufferGetPage(buf);
		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
	}

	if (P_IGNORE(opaque))
		elog(ERROR, "fell off the end of \"%s\"",
			 RelationGetRelationName(rel));

	return buf;
}

/*
 *	_bt_binsrch() -- Do a binary search for a key on a particular page.
 *
 * The scankey we get has the compare function stored in the procedure
 * entry of each data struct.  We invoke this regproc to do the
 * comparison for every key in the scankey.
 *
 * On a leaf page, _bt_binsrch() returns the OffsetNumber of the first
 * key >= given scankey.  (NOTE: in particular, this means it is possible
 * to return a value 1 greater than the number of keys on the page,
 * if the scankey is > all keys on the page.)
 *
 * On an internal (non-leaf) page, _bt_binsrch() returns the OffsetNumber
 * of the last key < given scankey.  (Since _bt_compare treats the first
 * data key of such a page as minus infinity, there will be at least one
 * key < scankey, so the result always points at one of the keys on the
 * page.)  This key indicates the right place to descend to be sure we
 * find all leaf keys >= given scankey.
 *
 * This procedure is not responsible for walking right, it just examines
 * the given page.	_bt_binsrch() has no lock or refcount side effects
 * on the buffer.
 */
OffsetNumber
_bt_binsrch(Relation rel,
			Buffer buf,
			int keysz,
			ScanKey scankey)
{
	TupleDesc	itupdesc;
	Page		page;
	BTPageOpaque opaque;
	OffsetNumber low,
				high;
	int32		result;

	itupdesc = RelationGetDescr(rel);
	page = BufferGetPage(buf);
	opaque = (BTPageOpaque) PageGetSpecialPointer(page);

	low = P_FIRSTDATAKEY(opaque);
	high = PageGetMaxOffsetNumber(page);

	/*
	 * If there are no keys on the page, return the first available slot.
	 * Note this covers two cases: the page is really empty (no keys), or
	 * it contains only a high key.  The latter case is possible after
	 * vacuuming.  This can never happen on an internal page, however,
	 * since they are never empty (an internal page must have children).
	 */
	if (high < low)
		return low;

	/*
	 * Binary search to find the first key on the page >= scan key. Loop
	 * invariant: all slots before 'low' are < scan key, all slots at or
	 * after 'high' are >= scan key.  We can fall out when high == low.
	 */
	high++;						/* establish the loop invariant for high */

	while (high > low)
	{
		OffsetNumber mid = low + ((high - low) / 2);

		/* We have low <= mid < high, so mid points at a real slot */

		result = _bt_compare(rel, keysz, scankey, page, mid);

		if (result > 0)
			low = mid + 1;
		else
			high = mid;
	}

	/*
	 * At this point we have high == low, but be careful: they could point
	 * past the last slot on the page.
	 *
	 * On a leaf page, we always return the first key >= scan key (which
	 * could be the last slot + 1).
	 */
	if (P_ISLEAF(opaque))
		return low;

	/*
	 * On a non-leaf page, return the last key < scan key. There must be
	 * one if _bt_compare() is playing by the rules.
	 */
	Assert(low > P_FIRSTDATAKEY(opaque));

	return OffsetNumberPrev(low);
}

/*----------
 *	_bt_compare() -- Compare scankey to a particular tuple on the page.
 *
 *	keysz: number of key conditions to be checked (might be less than the
 *	total length of the scan key!)
 *	page/offnum: location of btree item to be compared to.
 *
 *		This routine returns:
 *			<0 if scankey < tuple at offnum;
 *			 0 if scankey == tuple at offnum;
 *			>0 if scankey > tuple at offnum.
 *		NULLs in the keys are treated as sortable values.  Therefore
 *		"equality" does not necessarily mean that the item should be
 *		returned to the caller as a matching key!
 *
 * CRUCIAL NOTE: on a non-leaf page, the first data key is assumed to be
 * "minus infinity": this routine will always claim it is less than the
 * scankey.  The actual key value stored (if any, which there probably isn't)
 * does not matter.  This convention allows us to implement the Lehman and
 * Yao convention that the first down-link pointer is before the first key.
 * See backend/access/nbtree/README for details.
 *----------
 */
int32
_bt_compare(Relation rel,
			int keysz,
			ScanKey scankey,
			Page page,
			OffsetNumber offnum)
{
	TupleDesc	itupdesc = RelationGetDescr(rel);
	BTPageOpaque opaque = (BTPageOpaque) PageGetSpecialPointer(page);
	BTItem		btitem;
	IndexTuple	itup;
	int			i;

	/*
	 * Force result ">" if target item is first data item on an internal
	 * page --- see NOTE above.
	 */
	if (!P_ISLEAF(opaque) && offnum == P_FIRSTDATAKEY(opaque))
		return 1;

	btitem = (BTItem) PageGetItem(page, PageGetItemId(page, offnum));
	itup = &(btitem->bti_itup);

	/*
	 * The scan key is set up with the attribute number associated with
	 * each term in the key.  It is important that, if the index is
	 * multi-key, the scan contain the first k key attributes, and that
	 * they be in order.  If you think about how multi-key ordering works,
	 * you'll understand why this is.
	 *
	 * We don't test for violation of this condition here, however.  The
	 * initial setup for the index scan had better have gotten it right
	 * (see _bt_first).
	 */

	for (i = 0; i < keysz; i++)
	{
		ScanKey		entry = &scankey[i];
		Datum		datum;
		bool		isNull;
		int32		result;

		datum = index_getattr(itup, entry->sk_attno, itupdesc, &isNull);

		/* see comments about NULLs handling in btbuild */
		if (entry->sk_flags & SK_ISNULL)		/* key is NULL */
		{
			if (isNull)
				result = 0;		/* NULL "=" NULL */
			else
				result = 1;		/* NULL ">" NOT_NULL */
		}
		else if (isNull)		/* key is NOT_NULL and item is NULL */
		{
			result = -1;		/* NOT_NULL "<" NULL */
		}
		else
		{
			result = DatumGetInt32(FunctionCall2(&entry->sk_func,
												 entry->sk_argument,
												 datum));
		}

		/* if the keys are unequal, return the difference */
		if (result != 0)
			return result;
	}

	/* if we get here, the keys are equal */
	return 0;
}

/*
 *	_bt_next() -- Get the next item in a scan.
 *
 *		On entry, we have a valid currentItemData in the scan, and a
 *		read lock and pin count on the page that contains that item.
 *		We return the next item in the scan, or false if no more.
 *		On successful exit, the page containing the new item is locked
 *		and pinned; on failure exit, no lock or pin is held.
 */
bool
_bt_next(IndexScanDesc scan, ScanDirection dir)
{
	Relation	rel;
	Buffer		buf;
	Page		page;
	OffsetNumber offnum;
	ItemPointer current;
	BTItem		btitem;
	IndexTuple	itup;
	BTScanOpaque so;
	bool		continuescan;

	rel = scan->indexRelation;
	so = (BTScanOpaque) scan->opaque;
	current = &(scan->currentItemData);

	/* we still have the buffer pinned and locked */
	buf = so->btso_curbuf;
	Assert(BufferIsValid(buf));

	do
	{
		/* step one tuple in the appropriate direction */
		if (!_bt_step(scan, &buf, dir))
			return false;

		/* current is the next candidate tuple to return */
		offnum = ItemPointerGetOffsetNumber(current);
		page = BufferGetPage(buf);
		btitem = (BTItem) PageGetItem(page, PageGetItemId(page, offnum));
		itup = &btitem->bti_itup;

		if (_bt_checkkeys(scan, itup, dir, &continuescan))
		{
			/* tuple passes all scan key conditions, so return it */
			scan->xs_ctup.t_self = itup->t_tid;