Prevent log_replication_commands from causing SQL commands to be logged.

Commit 7c4f5240 allowed walsender to execute normal SQL commands
to support table sync feature in logical replication. Previously
while log_statement caused such SQL commands to be logged,
log_replication_commands caused them to be logged, too.
That is, such SQL commands were logged twice unexpectedly
when those settings were both enabled.

This commit forces log_replication_commands to log only replication
commands, to prevent normal SQL commands from being logged twice.

Author: Masahiko Sawada
Reviewed-by: Kyotaro Horiguchi
Reported-by: Fujii Masao
Discussion: http://postgr.es/m/CAHGQGwFDWh_Qr-q_GEMpD+qH=vYPMdVqw=ZOSY3kX_Pna9R9SA@mail.gmail.com

src/backend/replication/walsender.c

@@ -1368,14 +1368,6 @@ exec_replication_command(const char *cmd_string)
 	MemoryContext cmd_context;
 	MemoryContext old_context;
 
-	/*
-	 * Log replication command if log_replication_commands is enabled. Even
-	 * when it's disabled, log the command with DEBUG1 level for backward
-	 * compatibility.
-	 */
-	ereport(log_replication_commands ? LOG : DEBUG1,
-			(errmsg("received replication command: %s", cmd_string)));
-
 	/*
 	 * CREATE_REPLICATION_SLOT ... LOGICAL exports a snapshot until the next
 	 * command arrives. Clean up the old stuff if there's anything.
@@ -1399,6 +1391,16 @@ exec_replication_command(const char *cmd_string)
 
 	cmd_node = replication_parse_result;
 
+	/*
+	 * Log replication command if log_replication_commands is enabled. Even
+	 * when it's disabled, log the command with DEBUG1 level for backward
+	 * compatibility. Note that SQL commands are not logged here, and will be
+	 * logged later if log_statement is enabled.
+	 */
+	if (cmd_node->type != T_SQLCmd)
+		ereport(log_replication_commands ? LOG : DEBUG1,
+				(errmsg("received replication command: %s", cmd_string)));
+
 	/*
 	 * CREATE_REPLICATION_SLOT ... LOGICAL exports a snapshot. If it was
 	 * called outside of transaction the snapshot should be cleared here.