Add additional information in the vacuum error context.

The additional information added will be an offset number for heap
operations. This information will help us in finding the exact tuple due
to which the error has occurred.

Author: Mahendra Singh Thalor and Amit Kapila
Reviewed-by: Sawada Masahiko, Justin Pryzby and Amit Kapila
Discussion: https://postgr.es/m/CAKYtNApK488TDF4bMbw+1QH8HJf9cxdNDXquhU50TK5iv_FtCQ@mail.gmail.com

src/backend/access/heap/pruneheap.c

@@ -188,7 +188,7 @@ heap_page_prune_opt(Relation relation, Buffer buffer)
 			/* OK to prune */
 			(void) heap_page_prune(relation, buffer, vistest,
 								   limited_xmin, limited_ts,
-								   true, &ignore);
+								   true, &ignore, NULL);
 		}
 
 		/* And release buffer lock */
@@ -213,6 +213,9 @@ heap_page_prune_opt(Relation relation, Buffer buffer)
  * send its own new total to pgstats, and we don't want this delta applied
  * on top of that.)
  *
+ * off_loc is the offset location required by the caller to use in error
+ * callback.
+ *
  * Returns the number of tuples deleted from the page and sets
  * latestRemovedXid.
  */
@@ -221,7 +224,8 @@ heap_page_prune(Relation relation, Buffer buffer,
 				GlobalVisState *vistest,
 				TransactionId old_snap_xmin,
 				TimestampTz old_snap_ts,
-				bool report_stats, TransactionId *latestRemovedXid)
+				bool report_stats, TransactionId *latestRemovedXid,
+				OffsetNumber *off_loc)
 {
 	int			ndeleted = 0;
 	Page		page = BufferGetPage(buffer);
@@ -262,6 +266,13 @@ heap_page_prune(Relation relation, Buffer buffer,
 		if (prstate.marked[offnum])
 			continue;
 
+		/*
+		 * Set the offset number so that we can display it along with any
+		 * error that occurred while processing this tuple.
+		 */
+		if (off_loc)
+			*off_loc = offnum;
+
 		/* Nothing to do if slot is empty or already dead */
 		itemid = PageGetItemId(page, offnum);
 		if (!ItemIdIsUsed(itemid) || ItemIdIsDead(itemid))
@@ -271,6 +282,10 @@ heap_page_prune(Relation relation, Buffer buffer,
 		ndeleted += heap_prune_chain(buffer, offnum, &prstate);
 	}
 
+	/* Clear the offset information once we have processed the given page. */
+	if (off_loc)
+		*off_loc = InvalidOffsetNumber;
+
 	/* Any error while applying the changes is critical */
 	START_CRIT_SECTION();
 

src/include/access/heapam.h

@@ -178,7 +178,8 @@ extern int	heap_page_prune(Relation relation, Buffer buffer,
 							struct GlobalVisState *vistest,
 							TransactionId limited_oldest_xmin,
 							TimestampTz limited_oldest_ts,
-							bool report_stats, TransactionId *latestRemovedXid);
+							bool report_stats, TransactionId *latestRemovedXid,
+							OffsetNumber *off_loc);
 extern void heap_page_prune_execute(Buffer buffer,
 									OffsetNumber *redirected, int nredirected,
 									OffsetNumber *nowdead, int ndead,