BTStack
_bt_search(Relation rel, Relation heaprel, BTScanInsert key, Buffer *bufP,
		   int access, bool returnstack)
{
…
	/* Get the root page to start with */
	*bufP = _bt_getroot(rel, heaprel, 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;
		IndexTuple	itup;
		BlockNumber child;
		BTStack		new_stack;

…
		*bufP = _bt_moveright(rel, heaprel, key, *bufP, (access == BT_WRITE),
							  stack_in, page_access);

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

		/*
		 * Find the appropriate pivot tuple on this page.  Its downlink points
		 * to the child page that we're about to descend to.
		 */
		offnum = _bt_binsrch(rel, key, *bufP);
		itemid = PageGetItemId(page, offnum);
		itup = (IndexTuple) PageGetItem(page, itemid);
		Assert(BTreeTupleIsPivot(itup) || !key->heapkeyspace);
		child = BTreeTupleGetDownLink(itup);
…
		/* drop the read lock on the page, then acquire one on its child */
		*bufP = _bt_relandgetbuf(rel, *bufP, child, page_access);

		/* okay, all set to move down a level */
	}

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