Flink 为日期和时间提供了丰富的数据类型， 包括 DATE， TIME， TIMESTAMP， TIMESTAMP_LTZ， INTERVAL YEAR TO MONTH， INTERVAL DAY TO SECOND 。 Flink 支持在 session （会话）级别设置时区。 Flink 对多种时间类型和时区的支持使得跨时区的数据处理变得非常容易。
1. TIMESTAMP vs TIMESTAMP_LTZ
1.1 TIMESTAMP 类型
TIMESTAMP(p)是TIMESTAMP(p) WITHOUT TIME ZONE的简写， 精度p支持的范围是0-9， 默认是6。TIMESTAMP用于描述年， 月， 日， 小时， 分钟， 秒 和 小数秒对应的时间戳。TIMESTAMP可以通过一个字符串来指定，例如：
Flink SQL> SELECT TIMESTAMP '1970-01-01 00:00:04.001';
+-------------------------+
| 1970-01-01 00:00:04.001 |
+-------------------------+1.2 TIMESTAMP_LTZ 类型
TIMESTAMP_LTZ(p)是TIMESTAMP(p) WITH LOCAL TIME ZONE的简写， 精度p支持的范围是0-9， 默认是6。TIMESTAMP_LTZ用于描述时间线上的绝对时间点， 使用 long 保存从 epoch 至今的毫秒数， 使用int保存毫秒中的纳秒数。 epoch 时间是从 java 的标准 epoch 时间1970-01-01T00:00:00Z开始计算。 在计算和可视化时， 每个TIMESTAMP_LTZ类型的数据都是使用的 session （会话）中配置的时区。TIMESTAMP_LTZ没有字符串表达形式因此无法通过字符串来指定， 可以通过一个 long 类型的 epoch 时间来转化(例如: 通过 Java 来产生一个 long 类型的 epoch 时间System.currentTimeMillis())
Flink SQL> CREATE VIEW T1 AS SELECT TO_TIMESTAMP_LTZ(4001, 3);
Flink SQL> SET 'table.local-time-zone' = 'UTC';
Flink SQL> SELECT * FROM T1;
+---------------------------+
| TO_TIMESTAMP_LTZ(4001, 3) |
+---------------------------+
| 1970-01-01 00:00:04.001 |
+---------------------------+Flink SQL> SET 'table.local-time-zone' = 'Asia/Shanghai';
Flink SQL> SELECT * FROM T1;
+---------------------------+
| TO_TIMESTAMP_LTZ(4001, 3) |
+---------------------------+
| 1970-01-01 08:00:04.001 |
+---------------------------+TIMESTAMP_LTZ可以用于跨时区的计算，因为它是一个基于 epoch 的绝对时间点（比如上例中的4001毫秒）代表的就是不同时区的同一个绝对时间点。 补充一个背景知识：在同一个时间点， 全世界所有的机器上执行System.currentTimeMillis()都会返回同样的值。 (比如上例中的4001milliseconds), 这就是绝对时间的定义。
2. 时区的作用
本地时区定义了当前 session（会话）所在的时区， 你可以在 Sql client 或者应用程序中配置。
-- 设置为 UTC 时区
Flink SQL> SET 'table.local-time-zone' = 'UTC';-- 设置为上海时区
Flink SQL> SET 'table.local-time-zone' = 'Asia/Shanghai';-- 设置为Los_Angeles时区
Flink SQL> SET 'table.local-time-zone' = 'America/Los_Angeles';
session（会话）的时区设置在 Flink SQL 中非常有用， 它的主要用法如下:
2.1 确定时间函数的返回值
session （会话）中配置的时区会对以下函数生效。
- LOCALTIME
- LOCALTIMESTAMP
- CURRENT_DATE
- CURRENT_TIME
- CURRENT_TIMESTAMP
- CURRENT_ROW_TIMESTAMP()
- NOW()
- PROCTIME()
Flink SQL> SET 'sql-client.execution.result-mode' = 'tableau';
Flink SQL> CREATE VIEW MyView1 AS SELECT LOCALTIME, LOCALTIMESTAMP, CURRENT_DATE, CURRENT_TIME, CURRENT_TIMESTAMP, CURRENT_ROW_TIMESTAMP(), NOW(), PROCTIME();
Flink SQL> DESC MyView1;
+------------------------+-----------------------------+-------+-----+--------+-----------+
| name | type | null | key | extras | watermark |
+------------------------+-----------------------------+-------+-----+--------+-----------+
| LOCALTIME | TIME(0) | false | | | |
| LOCALTIMESTAMP | TIMESTAMP(3) | false | | | |
| CURRENT_DATE | DATE | false | | | |
| CURRENT_TIME | TIME(0) | false | | | |
| CURRENT_TIMESTAMP | TIMESTAMP_LTZ(3) | false | | | |
|CURRENT_ROW_TIMESTAMP() | TIMESTAMP_LTZ(3) | false | | | |
| NOW() | TIMESTAMP_LTZ(3) | false | | | |
| PROCTIME() | TIMESTAMP_LTZ(3) *PROCTIME* | false | | | |
+------------------------+-----------------------------+-------+-----+--------+-----------+Flink SQL> SET 'table.local-time-zone' = 'UTC';
Flink SQL> SELECT * FROM MyView1;
+-----------+-------------------------+--------------+--------------+-------------------------+-------------------------+-------------------------+-------------------------+
| LOCALTIME | LOCALTIMESTAMP | CURRENT_DATE | CURRENT_TIME | CURRENT_TIMESTAMP | CURRENT_ROW_TIMESTAMP() | NOW() | PROCTIME() |
+-----------+-------------------------+--------------+--------------+-------------------------+-------------------------+-------------------------+-------------------------+
| 15:18:36 | 2021-04-15 15:18:36.384 | 2021-04-15 | 15:18:36 | 2021-04-15 15:18:36.384 | 2021-04-15 15:18:36.384 | 2021-04-15 15:18:36.384 | 2021-04-15 15:18:36.384 |
+-----------+-------------------------+--------------+--------------+-------------------------+-------------------------+-------------------------+-------------------------+
Flink SQL> SET 'table.local-time-zone' = 'Asia/Shanghai';
Flink SQL> SELECT * FROM MyView1;
+-----------+-------------------------+--------------+--------------+-------------------------+-------------------------+-------------------------+-------------------------+
| LOCALTIME | LOCALTIMESTAMP | CURRENT_DATE | CURRENT_TIME | CURRENT_TIMESTAMP | CURRENT_ROW_TIMESTAMP() | NOW() | PROCTIME() |
+-----------+-------------------------+--------------+--------------+-------------------------+-------------------------+-------------------------+-------------------------+
| 23:18:36 | 2021-04-15 23:18:36.384 | 2021-04-15 | 23:18:36 | 2021-04-15 23:18:36.384 | 2021-04-15 23:18:36.384 | 2021-04-15 23:18:36.384 | 2021-04-15 23:18:36.384 |
+-----------+-------------------------+--------------+--------------+-------------------------+-------------------------+-------------------------+-------------------------+2.2 TIMESTAMP_LTZ 字符串表示
当一个 TIMESTAMP_LTZ 值转为 string 格式时， session 中配置的时区会生效。 例如打印这个值，将类型强制转化为 STRING 类型， 将类型强制转换为 TIMESTAMP ，将 TIMESTAMP 的值转化为 TIMESTAMP_LTZ 类型：
Flink SQL> CREATE VIEW MyView2 AS SELECT TO_TIMESTAMP_LTZ(4001, 3) AS ltz, TIMESTAMP '1970-01-01 00:00:01.001' AS ntz;
Flink SQL> DESC MyView2;
+------+------------------+-------+-----+--------+-----------+
| name | type | null | key | extras | watermark |
+------+------------------+-------+-----+--------+-----------+
| ltz | TIMESTAMP_LTZ(3) | true | | | |
| ntz | TIMESTAMP(3) | false | | | |
+------+------------------+-------+-----+--------+-----------+
Flink SQL> SET 'table.local-time-zone' = 'UTC';
Flink SQL> SELECT * FROM MyView2;
+-------------------------+-------------------------+
| ltz | ntz |
+-------------------------+-------------------------+
| 1970-01-01 00:00:04.001 | 1970-01-01 00:00:01.001 |
+-------------------------+-------------------------+
Flink SQL> SET 'table.local-time-zone' = 'Asia/Shanghai';
Flink SQL> SELECT * FROM MyView2;
+-------------------------+-------------------------+
| ltz | ntz |
+-------------------------+-------------------------+
| 1970-01-01 08:00:04.001 | 1970-01-01 00:00:01.001 |
+-------------------------+-------------------------+
Flink SQL> CREATE VIEW MyView3 AS SELECT ltz, CAST(ltz AS TIMESTAMP(3)), CAST(ltz AS STRING), ntz, CAST(ntz AS TIMESTAMP_LTZ(3)) FROM MyView2;
Flink SQL> DESC MyView3;
+-------------------------------+------------------+-------+-----+--------+-----------+
| name | type | null | key | extras | watermark |
+-------------------------------+------------------+-------+-----+--------+-----------+
| ltz | TIMESTAMP_LTZ(3) | true | | | |
| CAST(ltz AS TIMESTAMP(3)) | TIMESTAMP(3) | true | | | |
| CAST(ltz AS STRING) | STRING | true | | | |
| ntz | TIMESTAMP(3) | false | | | |
| CAST(ntz AS TIMESTAMP_LTZ(3)) | TIMESTAMP_LTZ(3) | false | | | |
+-------------------------------+------------------+-------+-----+--------+-----------+
Flink SQL> SELECT * FROM MyView3;
+-------------------------+---------------------------+-------------------------+-------------------------+-------------------------------+
| ltz | CAST(ltz AS TIMESTAMP(3)) | CAST(ltz AS STRING) | ntz | CAST(ntz AS TIMESTAMP_LTZ(3)) |
+-------------------------+---------------------------+-------------------------+-------------------------+-------------------------------+
| 1970-01-01 08:00:04.001 | 1970-01-01 08:00:04.001 | 1970-01-01 08:00:04.001 | 1970-01-01 00:00:01.001 | 1970-01-01 00:00:01.001 |
+-------------------------+---------------------------+-------------------------+-------------------------+-------------------------------+
