Improve comment about pins & locks on step to the left.

src/backend/access/nbtree/nbtsearch.c

@@ -1383,19 +1383,26 @@ _bt_steppage(IndexScanDesc scan, ScanDirection dir)
 		/* Remember we left a page with data */
 		so->currPos.moreRight = true;
 
-		/* XXX: Can walking left be lighter on the locking and pins? */
-		if (BTScanPosIsPinned(so->currPos))
-			LockBuffer(so->currPos.buf, BT_READ);
-		else
-			so->currPos.buf = _bt_getbuf(rel, so->currPos.currPage, BT_READ);
-
 		/*
 		 * Walk left to the next page with data.  This is much more complex
 		 * than the walk-right case because of the possibility that the page
 		 * to our left splits while we are in flight to it, plus the
 		 * possibility that the page we were on gets deleted after we leave
 		 * it.  See nbtree/README for details.
+		 *
+		 * It might be possible to rearrange this code to have less overhead
+		 * in pinning and locking, but that would require capturing the left
+		 * pointer when the page is initially read, and using it here, along
+		 * with big changes to _bt_walk_left() and the code below.  It is not
+		 * clear whether this would be a win, since if the page immediately to
+		 * the left splits after we read this page and before we step left, we
+		 * would need to visit more pages than with the current code.
 		 */
+		if (BTScanPosIsPinned(so->currPos))
+			LockBuffer(so->currPos.buf, BT_READ);
+		else
+			so->currPos.buf = _bt_getbuf(rel, so->currPos.currPage, BT_READ);
+
 		for (;;)
 		{
 			/* Done if we know there are no matching keys to the left */