Re: Minimal logical decoding on standbys

According to the feature LGTM and all tests passed. Documentation is also OK. Some minor comments:

+    <para>
+     A logical replication slot can also be created on a hot standby. To prevent
+     <command>VACUUM</command> from removing required rows from the system
+     catalogs, <varname>hot_standby_feedback</varname> should be set on the
+     standby. In spite of that, if any required rows get removed, the slot gets
+     dropped.  Existing logical slots on standby also get dropped if wal_level
+     on primary is reduced to less than 'logical'.
+    </para>

Remove extra space before "Existing logical slots..."

+            pg_stat_get_db_conflict_logicalslot(D.oid) AS confl_logicalslot,

Move it to the end of pg_stat_database_conflicts columns

+            * is being reduced.  Hence this extra check.

Remove extra space before "Hence this..."

+       /* Send the other backend, a conflict recovery signal */
+
+       SetInvalidVirtualTransactionId(vxid);

Remove extra empty line

+               if (restart_lsn % XLOG_BLCKSZ != 0)
+                   elog(ERROR, "invalid replay pointer");

Add an empty line after this "IF" for code readability 

+void
+ResolveRecoveryConflictWithLogicalSlots(Oid dboid, TransactionId xid,
+                                       char *conflict_reason)
+{
+   int         i;
+   bool        found_conflict = false;
+
+   if (max_replication_slots <= 0)
+       return;

What about adding an "Assert(max_replication_slots >= 0);" before the replication slots check?

One last thing is about the name of TAP tests, we should rename them because there are other TAP tests starting with 022_ and 023_. It should be renamed to:

src/test/recovery/t/022_standby_logical_decoding_xmins.pl -> src/test/recovery/t/024_standby_logical_decoding_xmins.pl

src/test/recovery/t/023_standby_logical_decoding_conflicts.pl -> src/test/recovery/t/025_standby_logical_decoding_conflicts.pl

Regards,