Stabilize the test added by commit 022564f60c.

The test was unstable in branches 14 and 15 as we were relying on the
number of changes in the table having a toast column to start streaming.
On branches >= 16, we have a GUC debug_logical_replication_streaming which
can stream each change, so the test was stable in those branches.

Change the test to use PREPARE TRANSACTION as that should make the result
consistent and test the code changed in 022564f60c.

Reported-by: Daniel Gustafsson as per buildfarm
Author: Hou Zhijie, Amit Kapila
Backpatch-through: 14
Discussion: https://postgr.es/m/8C2F86AA-981E-4803-B14D-E264C0255330@yesql.se

contrib/test_decoding/expected/stream.out

@@ -128,28 +128,6 @@ SELECT count(*) FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL,
      5
 (1 row)
 
--- Test that accessing a TOAST table in streaming mode is allowed.
--- Create a table with a column that uses a TOASTed default value.
--- (temporarily hide query, to avoid the long CREATE TABLE stmt)
-\set ECHO none
-SET debug_logical_replication_streaming = immediate;
-BEGIN;
-INSERT INTO test_tab VALUES(1);
--- Force WAL flush, so that the above changes will be streamed.
-SELECT 'force flush' FROM pg_switch_wal();
-  ?column?   
--------------
- force flush
-(1 row)
-
-SELECT count(*) FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1', 'stream-changes', '1');
- count 
--------
-     3
-(1 row)
-
-COMMIT;
-RESET debug_logical_replication_streaming;
 DROP TABLE stream_test;
 SELECT pg_drop_replication_slot('regression_slot');
  pg_drop_replication_slot 

contrib/test_decoding/expected/twophase.out

@@ -205,11 +205,34 @@ SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'inc
  COMMIT
 (3 rows)
 
+-- Test that accessing a TOAST table is permitted during the decoding of a
+-- prepared transaction.
+-- Create a table with a column that uses a TOASTed default value.
+-- (temporarily hide query, to avoid the long CREATE TABLE stmt)
+\set ECHO none
+BEGIN;
+INSERT INTO test_tab VALUES('test');
+PREPARE TRANSACTION 'test_toast_table_access';
+SELECT count(*) FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1', 'stream-changes', '1');
+ count 
+-------
+     3
+(1 row)
+
+COMMIT PREPARED 'test_toast_table_access';
+-- consume commit prepared
+SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1', 'stream-changes', '1');
+                   data                    
+-------------------------------------------
+ COMMIT PREPARED 'test_toast_table_access'
+(1 row)
+
 -- Test 8:
 -- cleanup and make sure results are also empty
 DROP TABLE test_prepared1;
 DROP TABLE test_prepared2;
 DROP TABLE test_prepared_savepoint;
+DROP TABLE test_tab;
 -- show results. There should be nothing to show
 SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
  data 

contrib/test_decoding/sql/stream.sql

@@ -59,27 +59,5 @@ ROLLBACK TO s1;
 COMMIT;
 SELECT count(*) FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1', 'stream-changes', '1');
 
--- Test that accessing a TOAST table in streaming mode is allowed.
-
--- Create a table with a column that uses a TOASTed default value.
--- (temporarily hide query, to avoid the long CREATE TABLE stmt)
-\set ECHO none
-SELECT 'CREATE TABLE test_tab (a text DEFAULT ''' || string_agg('toast value', '') || ''');' FROM generate_series(1, 4000)
-\gexec
-\set ECHO all
-
-SET debug_logical_replication_streaming = immediate;
-
-BEGIN;
-INSERT INTO test_tab VALUES(1);
-
--- Force WAL flush, so that the above changes will be streamed.
-SELECT 'force flush' FROM pg_switch_wal();
-
-SELECT count(*) FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1', 'stream-changes', '1');
-COMMIT;
-
-RESET debug_logical_replication_streaming;
-
 DROP TABLE stream_test;
 SELECT pg_drop_replication_slot('regression_slot');

contrib/test_decoding/sql/twophase.sql

@@ -104,11 +104,33 @@ COMMIT PREPARED 'test_prepared_nodecode';
 -- should be decoded now
 SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
 
+-- Test that accessing a TOAST table is permitted during the decoding of a
+-- prepared transaction.
+
+-- Create a table with a column that uses a TOASTed default value.
+-- (temporarily hide query, to avoid the long CREATE TABLE stmt)
+\set ECHO none
+SELECT 'CREATE TABLE test_tab (a text DEFAULT ''' || string_agg('toast value', '') || ''');' FROM generate_series(1, 4000)
+\gexec
+\set ECHO all
+
+BEGIN;
+INSERT INTO test_tab VALUES('test');
+PREPARE TRANSACTION 'test_toast_table_access';
+
+SELECT count(*) FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1', 'stream-changes', '1');
+
+COMMIT PREPARED 'test_toast_table_access';
+
+-- consume commit prepared
+SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1', 'stream-changes', '1');
+
 -- Test 8:
 -- cleanup and make sure results are also empty
 DROP TABLE test_prepared1;
 DROP TABLE test_prepared2;
 DROP TABLE test_prepared_savepoint;
+DROP TABLE test_tab;
 -- show results. There should be nothing to show
 SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');