Enable insert multiple file on AO tables.

Changes:

When enable_parallel is on, the insertMultiFiles flag is set to true when:
gp_appendonly_insert_files GUC value is greater than 1 (indicating multiple files used)
and the insert is not reserving a specific segment.
When insertMultiFiles is true, data will be appended to multiple file segments in parallel.
The default value of gp_appendonly_insert_files GUC is increased from 0 to 4, to enable up to 4 parallel insert files by default.
This allows append-only tables to scale insert performance by writing to multiple file segments in parallel. The number of parallel segments can be configured using the gp_appendonly_insert_files GUC.

src/backend/access/aocs/aocsam.c

@@ -1068,7 +1068,9 @@ aocs_insert_init(Relation rel, int segno)
 											rel, segno, tupleDesc->natts, true);
 
 	/* should not enable insertMultiFiles if the table is created by own transaction */
-	desc->insertMultiFiles = ((gp_appendonly_insert_files > 1) && !ShouldUseReservedSegno(rel, CHOOSE_MODE_WRITE));
+	desc->insertMultiFiles = enable_parallel &&
+							gp_appendonly_insert_files > 1 &&
+							!ShouldUseReservedSegno(rel, CHOOSE_MODE_WRITE);
 	return desc;
 }
 

src/backend/access/appendonly/appendonlyam.c

@@ -2852,7 +2852,9 @@ aoInsertDesc->appendOnlyMetaDataSnapshot, //CONCERN:Safe to assume all block dir
 											rel, segno, 1, false);
 
 	/* should not enable insertMultiFiles if the table is created by own transaction */
-	aoInsertDesc->insertMultiFiles = ((gp_appendonly_insert_files > 1) && !ShouldUseReservedSegno(rel, CHOOSE_MODE_WRITE));
+	aoInsertDesc->insertMultiFiles = enable_parallel &&
+									gp_appendonly_insert_files > 1 &&
+									!ShouldUseReservedSegno(rel, CHOOSE_MODE_WRITE);
 	return aoInsertDesc;
 }
 

src/backend/utils/misc/guc_gp.c

@@ -3165,7 +3165,7 @@ struct config_int ConfigureNamesInt_gp[] =
 			NULL
 		},
 		&gp_appendonly_insert_files,
-		0, 0, 127,
+		4, 0, 127,
 		NULL, NULL, NULL
 	},