Fix: Core happend when calling pg_relation_size on root partitioned table with PAX AM (#1128)

In the `calculate_relation_size` method, whether to directly callback the
table AM(access method) is determined by `RelationIsNonblockRelation`.
However, for root partitioned tables, the table AM is always be NULL.

When the AM of a root partitioned table is PAX, `RelationIsNonblockRelation`
always returns TRUE. But for AO tables, `RelationIsNonblockRelation` will
returns FALSE. It because AO table are determined directly through `rel->rd_amhandler`
(PAX as a plugin, cannot register its `amhandler` in the CBDB kernel).

Author: jiaqizho

contrib/pax_storage/expected/ddl.out

@@ -71,3 +71,21 @@ drop table pax_test.t3;
 create table pax_test.t4 (v1 text) with(compresstype=zstd, compresslevel=1);
 alter table pax_test.t4 add column v2 text;
 drop table pax_test.t4;
+-- test pg_relation_size
+CREATE TABLE pt_pax (id int, date date) using pax DISTRIBUTED BY (id)
+PARTITION BY RANGE (date)
+( PARTITION subpt1 START (date '2016-01-01') INCLUSIVE ,
+  PARTITION subpt2 START (date '2016-02-01') INCLUSIVE);
+SELECT pg_catalog.pg_relation_size('pt_pax'::regclass);
+ pg_relation_size 
+------------------
+                0
+(1 row)
+
+select 1 from (select count(*) from gp_toolkit.gp_size_of_schema_disk) empty_out;
+ ?column? 
+----------
+        1
+(1 row)
+
+drop table pt_pax;

contrib/pax_storage/sql/ddl.sql

@@ -48,3 +48,13 @@ drop table pax_test.t3;
 create table pax_test.t4 (v1 text) with(compresstype=zstd, compresslevel=1);
 alter table pax_test.t4 add column v2 text;
 drop table pax_test.t4;
+
+-- test pg_relation_size
+CREATE TABLE pt_pax (id int, date date) using pax DISTRIBUTED BY (id)
+PARTITION BY RANGE (date)
+( PARTITION subpt1 START (date '2016-01-01') INCLUSIVE ,
+  PARTITION subpt2 START (date '2016-02-01') INCLUSIVE);
+
+SELECT pg_catalog.pg_relation_size('pt_pax'::regclass);
+select 1 from (select count(*) from gp_toolkit.gp_size_of_schema_disk) empty_out;
+drop table pt_pax;

src/include/utils/rel.h

@@ -563,8 +563,9 @@ typedef struct ViewOptions
  *      True iff relation(table) should run the code path as AO/CO
  */
 #define RelationIsNonblockRelation(relation) \
+	((relation)->rd_tableam && \
 	(RelationIsAppendOptimized(relation) || \
-	 RelationIsPax(relation))
+	 RelationIsPax(relation)))
 
 /*
  * RelationIsBitmapIndex