Improve readability of some tests in strings.sql

c251336 has added some tests to check if a toast relation should be
empty or not, hardcoding the toast relation name when calling
pg_relation_size().  pg_class.reltoastrelid offers the same information,
so simplify the tests to use that.

Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/20190403065949.GH3298@paquier.xyz

src/test/regress/expected/strings.out

@@ -1190,9 +1190,10 @@ INSERT INTO toasttest values (repeat('1234567890',300));
 INSERT INTO toasttest values (repeat('1234567890',300));
 INSERT INTO toasttest values (repeat('1234567890',300));
 -- expect >0 blocks
-select 0 = pg_relation_size('pg_toast.pg_toast_'||(select oid from pg_class where relname = 'toasttest'))/current_setting('block_size')::integer as blocks;
- blocks 
---------
+SELECT pg_relation_size(reltoastrelid) = 0 AS is_empty
+  FROM pg_class where relname = 'toasttest';
+ is_empty 
+----------
  f
 (1 row)
 
@@ -1203,9 +1204,10 @@ INSERT INTO toasttest values (repeat('1234567890',300));
 INSERT INTO toasttest values (repeat('1234567890',300));
 INSERT INTO toasttest values (repeat('1234567890',300));
 -- expect 0 blocks
-select 0 = pg_relation_size('pg_toast.pg_toast_'||(select oid from pg_class where relname = 'toasttest'))/current_setting('block_size')::integer as blocks;
- blocks 
---------
+SELECT pg_relation_size(reltoastrelid) = 0 AS is_empty
+  FROM pg_class where relname = 'toasttest';
+ is_empty 
+----------
  t
 (1 row)
 

src/test/regress/sql/strings.sql

@@ -375,7 +375,8 @@ INSERT INTO toasttest values (repeat('1234567890',300));
 INSERT INTO toasttest values (repeat('1234567890',300));
 INSERT INTO toasttest values (repeat('1234567890',300));
 -- expect >0 blocks
-select 0 = pg_relation_size('pg_toast.pg_toast_'||(select oid from pg_class where relname = 'toasttest'))/current_setting('block_size')::integer as blocks;
+SELECT pg_relation_size(reltoastrelid) = 0 AS is_empty
+  FROM pg_class where relname = 'toasttest';
 
 TRUNCATE TABLE toasttest;
 ALTER TABLE toasttest set (toast_tuple_target = 4080);
@@ -384,7 +385,8 @@ INSERT INTO toasttest values (repeat('1234567890',300));
 INSERT INTO toasttest values (repeat('1234567890',300));
 INSERT INTO toasttest values (repeat('1234567890',300));
 -- expect 0 blocks
-select 0 = pg_relation_size('pg_toast.pg_toast_'||(select oid from pg_class where relname = 'toasttest'))/current_setting('block_size')::integer as blocks;
+SELECT pg_relation_size(reltoastrelid) = 0 AS is_empty
+  FROM pg_class where relname = 'toasttest';
 
 DROP TABLE toasttest;