Fix PostgreSQL 17/18 compatibility using WHERE clauses

- Use WHERE clauses to completely exclude checkpoint queries for PG17+
- Avoids query parsing errors with non-existent columns
- For PG17+: Shows informational note about pg_stat_checkpointer
- For PG<17: Shows normal checkpoint statistics from pg_stat_bgwriter
- No DDL required, purely conditional query execution

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: NikolayS

sql/0_node.sql

@@ -49,40 +49,30 @@ select 'Started At', pg_postmaster_start_time()::timestamptz(0)::text
 union all
 select 'Uptime', (now() - pg_postmaster_start_time())::interval(0)::text
 union all
-select
-  'Checkpoints',
-  case 
-    when current_setting('server_version_num')::int >= 170000
-    then 'See pg_stat_checkpointer (PG17+)'
-    else (select (checkpoints_timed + checkpoints_req)::text from pg_stat_bgwriter)
-  end
-union all
-select
-  'Forced Checkpoints',
+select 'Checkpoints Note', 
   case 
     when current_setting('server_version_num')::int >= 170000
-    then 'See pg_stat_checkpointer (PG17+)'
-    else (
-      select round(100.0 * checkpoints_req::numeric /
-        (nullif(checkpoints_timed + checkpoints_req, 0)), 1)::text || '%'
-      from pg_stat_bgwriter
-    )
+    then 'Use pg_stat_checkpointer view for checkpoint statistics (PostgreSQL 17+)'
+    else 'Checkpoint statistics from pg_stat_bgwriter'
   end
 union all
-select
-  'Checkpoint MB/sec',
-  case 
-    when current_setting('server_version_num')::int >= 170000
-    then 'See pg_stat_checkpointer (PG17+)'
-    else (
-      select round((nullif(buffers_checkpoint::numeric, 0) /
-        ((1024.0 * 1024 /
-          (current_setting('block_size')::numeric))
-            * extract('epoch' from now() - stats_reset)
-        ))::numeric, 6)::text
-      from pg_stat_bgwriter
-    )
-  end
+select 'Checkpoints', (select (checkpoints_timed + checkpoints_req)::text from pg_stat_bgwriter)
+  where current_setting('server_version_num')::int < 170000
+union all
+select 'Forced Checkpoints', (
+  select round(100.0 * checkpoints_req::numeric /
+    (nullif(checkpoints_timed + checkpoints_req, 0)), 1)::text || '%'
+  from pg_stat_bgwriter
+) where current_setting('server_version_num')::int < 170000
+union all
+select 'Checkpoint MB/sec', (
+  select round((nullif(buffers_checkpoint::numeric, 0) /
+    ((1024.0 * 1024 /
+      (current_setting('block_size')::numeric))
+        * extract('epoch' from now() - stats_reset)
+    ))::numeric, 6)::text
+  from pg_stat_bgwriter
+) where current_setting('server_version_num')::int < 170000
 union all
 select repeat('-', 33), repeat('-', 88)
 union all