amcheck: Reduce debug message verbosity.

Empty sibling pages can occasionally be much more common than any other
event that we report on at elevel DEBUG1.  Increase the elevel for
relevant cases to DEBUG2 to avoid overwhelming the user with relatively
insignificant details.

Author: petergeoghegan

contrib/amcheck/verify_nbtree.c

@@ -721,7 +721,7 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
 			else
 				ereport(DEBUG1,
 						(errcode(ERRCODE_NO_DATA),
-						 errmsg_internal("block %u of index \"%s\" ignored",
+						 errmsg_internal("block %u of index \"%s\" concurrently deleted",
 								current, RelationGetRelationName(state->rel))));
 			goto nextpage;
 		}
@@ -1592,14 +1592,18 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		if (!P_IGNORE(opaque) || P_RIGHTMOST(opaque))
 			break;
 
-		/* We landed on a deleted page, so step right to find a live page */
-		targetnext = opaque->btpo_next;
-		ereport(DEBUG1,
+		/*
+		 * We landed on a deleted or half-dead sibling page.  Step right until
+		 * we locate a live sibling page.
+		 */
+		ereport(DEBUG2,
 				(errcode(ERRCODE_NO_DATA),
-				 errmsg_internal("level %u leftmost page of index \"%s\" was found deleted or half dead",
-						opaque->btpo_level, RelationGetRelationName(state->rel)),
+				 errmsg_internal("level %u sibling page in block %u of index \"%s\" was found deleted or half dead",
+						opaque->btpo_level, targetnext, RelationGetRelationName(state->rel)),
 				 errdetail_internal("Deleted page found when building scankey from right sibling.")));
 
+		targetnext = opaque->btpo_next;
+
 		/* Be slightly more pro-active in freeing this memory, just in case */
 		pfree(rightpage);
 	}
@@ -1722,7 +1726,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		 * possible that it's an internal page with only a negative infinity
 		 * item.
 		 */
-		ereport(DEBUG1,
+		ereport(DEBUG2,
 				(errcode(ERRCODE_NO_DATA),
 				 errmsg_internal("%s block %u of index \"%s\" has no first data item",
 						P_ISLEAF(opaque) ? "leaf" : "internal", targetnext,