Avoid looping through line pointers twice in PageRepairFragmentation().

There doesn't seem to be any good reason to do the filling of the
itemidbase[] array separately from the first traversal of the pointers.
It's certainly not a win if there are any line pointers with storage,
and even if there aren't, this change doesn't insert code into the part
of the first loop that will be traversed in that case.  So let's just
merge the two loops.

Yura Sokolov, reviewed by Claudio Freire

Discussion: https://postgr.es/m/e49befcc6f1d7099834c6fdf5c675a60@postgrespro.ru

src/backend/storage/page/bufpage.c

@@ -481,6 +481,8 @@ PageRepairFragmentation(Page page)
 	Offset		pd_lower = ((PageHeader) page)->pd_lower;
 	Offset		pd_upper = ((PageHeader) page)->pd_upper;
 	Offset		pd_special = ((PageHeader) page)->pd_special;
+	itemIdSortData itemidbase[MaxHeapTuplesPerPage];
+	itemIdSort	itemidptr;
 	ItemId		lp;
 	int			nline,
 				nstorage,
@@ -505,15 +507,31 @@ PageRepairFragmentation(Page page)
 				 errmsg("corrupted page pointers: lower = %u, upper = %u, special = %u",
 						pd_lower, pd_upper, pd_special)));
 
+	/*
+	 * Run through the line pointer array and collect data about live items.
+	 */
 	nline = PageGetMaxOffsetNumber(page);
-	nunused = nstorage = 0;
+	itemidptr = itemidbase;
+	nunused = totallen = 0;
 	for (i = FirstOffsetNumber; i <= nline; i++)
 	{
 		lp = PageGetItemId(page, i);
 		if (ItemIdIsUsed(lp))
 		{
 			if (ItemIdHasStorage(lp))
-				nstorage++;
+			{
+				itemidptr->offsetindex = i - 1;
+				itemidptr->itemoff = ItemIdGetOffset(lp);
+				if (unlikely(itemidptr->itemoff < (int) pd_upper ||
+							 itemidptr->itemoff >= (int) pd_special))
+					ereport(ERROR,
+							(errcode(ERRCODE_DATA_CORRUPTED),
+							 errmsg("corrupted item pointer: %u",
+									itemidptr->itemoff)));
+				itemidptr->alignedlen = MAXALIGN(ItemIdGetLength(lp));
+				totallen += itemidptr->alignedlen;
+				itemidptr++;
+			}
 		}
 		else
 		{
@@ -523,6 +541,7 @@ PageRepairFragmentation(Page page)
 		}
 	}
 
+	nstorage = itemidptr - itemidbase;
 	if (nstorage == 0)
 	{
 		/* Page is completely empty, so just reset it quickly */
@@ -531,29 +550,6 @@ PageRepairFragmentation(Page page)
 	else
 	{
 		/* Need to compact the page the hard way */
-		itemIdSortData itemidbase[MaxHeapTuplesPerPage];
-		itemIdSort	itemidptr = itemidbase;
-
-		totallen = 0;
-		for (i = 0; i < nline; i++)
-		{
-			lp = PageGetItemId(page, i + 1);
-			if (ItemIdHasStorage(lp))
-			{
-				itemidptr->offsetindex = i;
-				itemidptr->itemoff = ItemIdGetOffset(lp);
-				if (itemidptr->itemoff < (int) pd_upper ||
-					itemidptr->itemoff >= (int) pd_special)
-					ereport(ERROR,
-							(errcode(ERRCODE_DATA_CORRUPTED),
-							 errmsg("corrupted item pointer: %u",
-									itemidptr->itemoff)));
-				itemidptr->alignedlen = MAXALIGN(ItemIdGetLength(lp));
-				totallen += itemidptr->alignedlen;
-				itemidptr++;
-			}
-		}
-
 		if (totallen > (Size) (pd_special - pd_lower))
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),