Add a GUC check hook to ensure summarize_wal cannot be enabled when wal_level is minimal

Hi,

The documentation states that "WAL summarization cannot be enabled when wal_level is set to minimal." Therefore, at startup, the postmaster checks these settings and exits with an error if they are not configured properly.

However, I found that summarize_wal can still be enabled while the server is running with wal_level=minimal. Please see the following example to cause this situation. I think this is a bug.

=# SHOW wal_level;
wal_level
-----------
minimal
(1 row)

=# SELECT * FROM pg_get_wal_summarizer_state();
summarized_tli | summarized_lsn | pending_lsn | summarizer_pid
----------------+----------------+-------------+----------------
0 | 0/0 | 0/0 | (null)
(1 row)

=# ALTER SYSTEM SET summarize_wal TO on;
ALTER SYSTEM

=# SELECT pg_reload_conf();
pg_reload_conf
----------------
t
(1 row)

=# SELECT * FROM pg_get_wal_summarizer_state();
summarized_tli | summarized_lsn | pending_lsn | summarizer_pid
----------------+----------------+-------------+----------------
1 | 0/1492D80 | 0/1492DF8 | 12228
(1 row)

The attached patch adds a GUC check hook to ensure summarize_wal cannot be enabled when wal_level is minimal, fixing the issue.

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION