Fix typos in comments

The changes done in this commit impact comments with no direct
user-visible changes, with fixes for incorrect function, variable or
structure names.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/e8c38840-596a-83d6-bd8d-cebc51111572@gmail.com

src/backend/access/heap/heapam.c

@@ -1223,7 +1223,7 @@ heap_set_tidrange(TableScanDesc sscan, ItemPointer mintid,
 	 * Calculate the first block and the number of blocks we must scan. We
 	 * could be more aggressive here and perform some more validation to try
 	 * and further narrow the scope of blocks to scan by checking if the
-	 * lowerItem has an offset above MaxOffsetNumber.  In this case, we could
+	 * lowestItem has an offset above MaxOffsetNumber.  In this case, we could
 	 * advance startBlk by one.  Likewise, if highestItem has an offset of 0
 	 * we could scan one fewer blocks.  However, such an optimization does not
 	 * seem worth troubling over, currently.

src/backend/access/heap/heapam_handler.c

@@ -816,7 +816,7 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
 				 * If the last pages of the scan were empty, we would go to
 				 * the next phase while heap_blks_scanned != heap_blks_total.
 				 * Instead, to ensure that heap_blks_scanned is equivalent to
-				 * total_heap_blks after the table scan phase, this parameter
+				 * heap_blks_total after the table scan phase, this parameter
 				 * is manually updated to the correct value when the table
 				 * scan finishes.
 				 */

src/backend/access/heap/pruneheap.c

@@ -198,8 +198,7 @@ heap_page_prune_opt(Relation relation, Buffer buffer)
 		/*
 		 * Now that we have buffer lock, get accurate information about the
 		 * page's free space, and recheck the heuristic about whether to
-		 * prune. (We needn't recheck PageIsPrunable, since no one else could
-		 * have pruned while we hold pin.)
+		 * prune.
 		 */
 		if (PageIsFull(page) || PageGetHeapFreeSpace(page) < minfree)
 		{
@@ -490,7 +489,7 @@ heap_page_prune(Relation relation, Buffer buffer,
  *
  * Due to its cost we also only want to call
  * TransactionIdLimitedForOldSnapshots() if necessary, i.e. we might not have
- * done so in heap_hot_prune_opt() if pd_prune_xid was old enough. But we
+ * done so in heap_page_prune_opt() if pd_prune_xid was old enough. But we
  * still want to be able to remove rows that are too new to be removed
  * according to prstate->vistest, but that can be removed based on
  * old_snapshot_threshold. So we call TransactionIdLimitedForOldSnapshots() on

src/backend/access/heap/vacuumlazy.c

@@ -2575,7 +2575,7 @@ lazy_vacuum_heap_page(LVRelState *vacrel, BlockNumber blkno, Buffer buffer,
 	END_CRIT_SECTION();
 
 	/*
-	 * Now that we have removed the LD_DEAD items from the page, once again
+	 * Now that we have removed the LP_DEAD items from the page, once again
 	 * check if the page has become all-visible.  The page is already marked
 	 * dirty, exclusively locked, and, if needed, a full page image has been
 	 * emitted.