Use PageIndexTupleOverwrite() within nbtree.

Use the PageIndexTupleOverwrite() bufpage.c routine within nbtree
instead of deleting a tuple and re-inserting its replacement.  This
makes the intent of affected code slightly clearer.  It also makes
CREATE INDEX slightly faster, since there is no longer a need to shift
every leaf page's line pointer array back and forth during index builds.

Author: Peter Geoghegan, Anastasia Lubennikova
Reviewed-By: Anastasia Lubennikova
Discussion: https://postgr.es/m/CAH2-Wz=Zk=B9+Vwm376WuO7YTjFc2SSskifQm4Nme3RRRPtOSQ@mail.gmail.com

src/backend/access/nbtree/nbtpage.c

@@ -1653,8 +1653,7 @@ _bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack)
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 	opaque->btpo_flags |= BTP_HALF_DEAD;
 
-	PageIndexTupleDelete(page, P_HIKEY);
-	Assert(PageGetMaxOffsetNumber(page) == 0);
+	Assert(PageGetMaxOffsetNumber(page) == P_HIKEY);
 	MemSet(&trunctuple, 0, sizeof(IndexTupleData));
 	trunctuple.t_info = sizeof(IndexTupleData);
 	if (target != leafblkno)
@@ -1662,9 +1661,9 @@ _bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack)
 	else
 		BTreeTupleSetTopParent(&trunctuple, InvalidBlockNumber);
 
-	if (PageAddItem(page, (Item) &trunctuple, sizeof(IndexTupleData), P_HIKEY,
-					false, false) == InvalidOffsetNumber)
-		elog(ERROR, "could not add dummy high key to half-dead page");
+	if (!PageIndexTupleOverwrite(page, P_HIKEY, (Item) &trunctuple,
+								 IndexTupleSize(&trunctuple)))
+		elog(ERROR, "could not overwrite high key in half-dead page");
 
 	/* Must mark buffers dirty before XLogInsert */
 	MarkBufferDirty(topparent);

src/backend/access/nbtree/nbtsort.c

@@ -943,7 +943,6 @@ _bt_buildadd(BTWriteState *wstate, BTPageState *state, IndexTuple itup)
 		{
 			IndexTuple	lastleft;
 			IndexTuple	truncated;
-			Size		truncsz;
 
 			/*
 			 * Truncate away any unneeded attributes from high key on leaf
@@ -961,26 +960,18 @@ _bt_buildadd(BTWriteState *wstate, BTPageState *state, IndexTuple itup)
 			 * choosing a split point here for a benefit that is bound to be
 			 * much smaller.
 			 *
-			 * Since the truncated tuple is often smaller than the original
-			 * tuple, it cannot just be copied in place (besides, we want to
-			 * actually save space on the leaf page).  We delete the original
-			 * high key, and add our own truncated high key at the same
-			 * offset.
-			 *
-			 * Note that the page layout won't be changed very much.  oitup is
-			 * already located at the physical beginning of tuple space, so we
-			 * only shift the line pointer array back and forth, and overwrite
-			 * the tuple space previously occupied by oitup.  This is fairly
-			 * cheap.
+			 * Overwrite the old item with new truncated high key directly.
+			 * oitup is already located at the physical beginning of tuple
+			 * space, so this should directly reuse the existing tuple space.
 			 */
 			ii = PageGetItemId(opage, OffsetNumberPrev(last_off));
 			lastleft = (IndexTuple) PageGetItem(opage, ii);
 
 			truncated = _bt_truncate(wstate->index, lastleft, oitup,
 									 wstate->inskey);
-			truncsz = IndexTupleSize(truncated);
-			PageIndexTupleDelete(opage, P_HIKEY);
-			_bt_sortaddtup(opage, truncsz, truncated, P_HIKEY);
+			if (!PageIndexTupleOverwrite(opage, P_HIKEY, (Item) truncated,
+										 IndexTupleSize(truncated)))
+				elog(ERROR, "failed to add high key to the index page");
 			pfree(truncated);
 
 			/* oitup should continue to point to the page's high key */