[PGPRO-4074] Skip equality check of dropped column during apply process.

Replication worker don't send dropped attribute (attisdropped == true) to a
replica. But in the heap it may be not null. If we apply UPDATE operation in the
REPLICA IDENTITY FULL mode, we will do SeqScan on the table and check equality
of each attribute of each tuple. Tuple in the heap will have value of
attr->isnull == false, tuple that passed from another node - true.

(cherry picked from commit 8d4c732f0e653ae6c65b4764f7de4f31456284ff)

tags: multimaster
(cherry picked from commit 2825fa678e82ba0fd6888bacc95f2b112401a46c)

Author: danolivo

src/backend/executor/execReplication.c

@@ -244,6 +244,15 @@ tuples_equal(TupleTableSlot *slot1, TupleTableSlot *slot2,
 		Form_pg_attribute att;
 		TypeCacheEntry *typentry;
 
+		/* The case of ALTER TABLE ... DROP COLUMN */
+		if (TupleDescAttr(slot1->tts_tupleDescriptor, attrnum)->attisdropped ||
+			TupleDescAttr(slot1->tts_tupleDescriptor, attrnum)->attgenerated)
+		{
+			Assert(slot1->tts_tupleDescriptor->attrs[attrnum].attisdropped ==
+					slot2->tts_tupleDescriptor->attrs[attrnum].attisdropped);
+			continue;
+		}
+
 		/*
 		 * If one value is NULL and other is not, then they are certainly not
 		 * equal