PostgreSQL日志中的SQL记录时机 —— log_statement 和 log_min_duration_statement

/*
 * exec_simple_query
 *
 * Execute a "simple Query" protocol message.
 */
static void
exec_simple_query(const char *query_string)
{
	CommandDest dest = whereToSendOutput;
	MemoryContext oldcontext;
	List	   *parsetree_list;
	ListCell   *parsetree_item;
	bool		save_log_statement_stats = log_statement_stats;
	bool		was_logged = false;
	bool		use_implicit_block;
	char		msec_str[32];
...

	/*
	 * Do basic parsing of the query or queries (this should be safe even if
	 * we are in aborted transaction state!)
	 */
	parsetree_list = pg_parse_query(query_string);

	/* Log immediately if dictated by log_statement */
	if (check_log_statement(parsetree_list))
	{
		ereport(LOG,
				(errmsg("statement: %s", query_string),
				 errhidestmt(true),
				 errdetail_execute(parsetree_list)));
		was_logged = true;
	}

pg_parse_query函数用于SQL解析，符合log_statement记录的语句发生在解析完成后
check_log_statement函数就用于检查log_statement的设置，标记是否需要记录
was_logged=true表示已记录SQL语句，主要是给后面的慢SQL记录函数，提示其不需重复记录SQL文本

/*
 * check_log_statement
 *		Determine whether command should be logged because of log_statement
 *
 * stmt_list can be either raw grammar output or a list of planned
 * statements
 */
static bool
check_log_statement(List *stmt_list)
{
	ListCell   *stmt_item;

	if (log_statement == LOGSTMT_NONE)
		return false;
	if (log_statement == LOGSTMT_ALL)
		return true;

	/* Else we have to inspect the statement(s) to see whether to log */
	foreach(stmt_item, stmt_list)
	{
		Node	   *stmt = (Node *) lfirst(stmt_item);

		if (GetCommandLogLevel(stmt) <= log_statement)
			return true;
	}

	return false;
}

后面一大堆是SQL执行相关的代码，非本次重点，直接拉到函数末尾，可以看到check_log_duration函数，看名字都能猜到是它了，也符合在SQL执行完才记录。

...

	/*
	 * Emit duration logging if appropriate.
	 */
	switch (check_log_duration(msec_str, was_logged))
	{
		case 1:
			ereport(LOG,
					(errmsg("duration: %s ms", msec_str),
					 errhidestmt(true)));
			break;
		case 2:
			ereport(LOG,
					(errmsg("duration: %s ms  statement: %s",
							msec_str, query_string),
					 errhidestmt(true),
					 errdetail_execute(parsetree_list)));
			break;
	}

	if (save_log_statement_stats)
		ShowUsage("QUERY STATISTICS");

	TRACE_POSTGRESQL_QUERY_DONE(query_string);

	debug_query_string = NULL;
}

check_log_duration函数

可以看到注释中给出了上面case 1,2的含义：

1：仅记录duration，即前面提到的两个参数均生效的场景
2：记录duration和语句：即前面提到的仅log_min_duration_statement生效的场景

另外是否记录慢sql与 log_duration、log_min_duration_sample 这些参数也有关。

/*
 * check_log_duration
 *		Determine whether current command's duration should be logged
 *		We also check if this statement in this transaction must be logged
 *		(regardless of its duration).
 *
 * Returns:
 *		0 if no logging is needed
 *		1 if just the duration should be logged
 *		2 if duration and query details should be logged
 *
 * If logging is needed, the duration in msec is formatted into msec_str[],
 * which must be a 32-byte buffer.
 *
 * was_logged should be true if caller already logged query details (this
 * essentially prevents 2 from being returned).
 */
int
check_log_duration(char *msec_str, bool was_logged)
{
	if (log_duration || log_min_duration_sample >= 0 ||
		log_min_duration_statement >= 0 || xact_is_sampled)
	{
		long		secs;
		int			usecs;
		int			msecs;
		bool		exceeded_duration;
		bool		exceeded_sample_duration;
		bool		in_sample = false;

		TimestampDifference(GetCurrentStatementStartTimestamp(),
							GetCurrentTimestamp(),
							&secs, &usecs);
		msecs = usecs / 1000;

		/*
		 * This odd-looking test for log_min_duration_* being exceeded is
		 * designed to avoid integer overflow with very long durations: don't
		 * compute secs * 1000 until we've verified it will fit in int.
		 */
		exceeded_duration = (log_min_duration_statement == 0 ||
							 (log_min_duration_statement > 0 &&
							  (secs > log_min_duration_statement / 1000 ||
							   secs * 1000 + msecs >= log_min_duration_statement)));

		exceeded_sample_duration = (log_min_duration_sample == 0 ||
									(log_min_duration_sample > 0 &&
									 (secs > log_min_duration_sample / 1000 ||
									  secs * 1000 + msecs >= log_min_duration_sample)));

		/*
		 * Do not log if log_statement_sample_rate = 0. Log a sample if
		 * log_statement_sample_rate <= 1 and avoid unnecessary random() call
		 * if log_statement_sample_rate = 1.
		 */
		if (exceeded_sample_duration)
			in_sample = log_statement_sample_rate != 0 &&
				(log_statement_sample_rate == 1 ||
				 random() <= log_statement_sample_rate * MAX_RANDOM_VALUE);

		if (exceeded_duration || in_sample || log_duration || xact_is_sampled)
		{
			snprintf(msec_str, 32, "%ld.%03d",
					 secs * 1000 + msecs, usecs % 1000);
			if ((exceeded_duration || in_sample || xact_is_sampled) && !was_logged)
				return 2;
			else
				return 1;
		}
	}

	return 0;
}

参考：

https://www.postgresql.org/docs/current/run time–config-logging.html#GUC-LOG-STATEMENT
https://www.postgresql.org/docs/current/protocol-flow.html#PROTOCOL-FLOW-EXT-QUERY
https://blog.csdn.net/qq_35423190/article/details/129138740
https://iwmj.wordpress.com/2018/04/03/postgresql-%E5%89%8D%E5%90%8E%E7%AB%AF%E5%8D%8F%E8%AE%AE%E4%B8%AD%E7%9A%84%E6%9F%A5%E8%AF%A2%E6%96%B9%E5%BC%8F%EF%BC%9A%E7%AE%80%E5%8D%95%E6%9F%A5%E8%AF%A2%E3%80%81%E6%89%A9%E5%B1%95%E6%9F%A5%E8%AF%A2/