Support PG/EPAS 12.

Earlier commit missed to change the Makefile.meta and .legacy
for the v12 support.  So this commit fix the same as well as
fix few other warning due to v12 API changes.

Makefile.legacy

@@ -47,6 +47,6 @@ ifndef MAJORVERSION
     MAJORVERSION := $(basename $(VERSION))
 endif
 
-ifeq (,$(findstring $(MAJORVERSION), 9.3 9.4 9.5 9.6 10.0 11.0))
-    $(error PostgreSQL 9.3, 9.4, 9.5, 9.6 10.0 or 11.0 is required to compile this extension)
+ifeq (,$(findstring $(MAJORVERSION), 9.3 9.4 9.5 9.6 10.0 11.0 12.0))
+    $(error PostgreSQL 9.3, 9.4, 9.5, 9.6 10.0 11.0 or 12.0 is required to compile this extension)
 endif

Makefile.meta

@@ -43,6 +43,6 @@ ifndef MAJORVERSION
     MAJORVERSION := $(basename $(VERSION))
 endif
 
-ifeq (,$(findstring $(MAJORVERSION), 9.3 9.4 9.5 9.6 10.0 11.0))
-    $(error PostgreSQL 9.3, 9.4, 9.5, 9.6 10.0 or 11.0 is required to compile this extension)
+ifeq (,$(findstring $(MAJORVERSION), 9.3 9.4 9.5 9.6 10.0 11.0 12.0))
+    $(error PostgreSQL 9.3, 9.4, 9.5, 9.6 10.0 11.0 or 12.0 is required to compile this extension)
 endif

mongo_fdw.c

@@ -2016,7 +2016,6 @@ MongoAcquireSampleRows(Relation relation, int errorLevel,
 	bool                     *columnNulls = NULL;
 	Oid                      foreignTableId = InvalidOid;
 	TupleDesc                tupleDescriptor = NULL;
-	Form_pg_attribute        *attributesPtr = NULL;
 	AttrNumber               columnCount = 0;
 	AttrNumber               columnId = 0;
 	HTAB                     *columnMappingHash = NULL;
@@ -2035,16 +2034,22 @@ MongoAcquireSampleRows(Relation relation, int errorLevel,
 	/* create list of columns in the relation */
 	tupleDescriptor = RelationGetDescr(relation);
 	columnCount = tupleDescriptor->natts;
-	attributesPtr = tupleDescriptor->attrs;
 
 	for (columnId = 1; columnId <= columnCount; columnId++)
 	{
 		Var *column = (Var *) palloc0(sizeof(Var));
+#if PG_VERSION_NUM >= 120000
+		Form_pg_attribute attr = TupleDescAttr(tupleDescriptor, columnId-1);
 
+		column->varattno = columnId;
+		column->vartype = attr->atttypid;
+		column->vartypmod = attr->atttypmod;
+#else
 		/* only assign required fields for column mapping hash */
 		column->varattno = columnId;
-		column->vartype = attributesPtr[columnId-1]->atttypid;
-		column->vartypmod = attributesPtr[columnId-1]->atttypmod;
+		column->vartype = tupleDescriptor->attrs[columnId-1]->atttypid;
+		column->vartypmod = tupleDescriptor->attrs[columnId-1]->atttypmod;
+#endif
 
 		columnList = lappend(columnList, column);
 	}