Flink (十七) ：Table API & SQL (五) 时区

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：

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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())

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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 4001 milliseconds), 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。

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-- 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()

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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;

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+------------------------+-----------------------------+-------+-----+--------+-----------+
|                   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 |     |        |           |
+------------------------+-----------------------------+-------+-----+--------+-----------+

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Flink SQL> SET 'table.local-time-zone' = 'UTC';
Flink SQL> SELECT * FROM MyView1;

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+-----------+-------------------------+--------------+--------------+-------------------------+-------------------------+-------------------------+-------------------------+
| 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 |
+-----------+-------------------------+--------------+--------------+-------------------------+-------------------------+-------------------------+-------------------------+

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Flink SQL> SET 'table.local-time-zone' = 'Asia/Shanghai';
Flink SQL> SELECT * FROM MyView1;

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+-----------+-------------------------+--------------+--------------+-------------------------+-------------------------+-------------------------+-------------------------+
| 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：

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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;

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+------+------------------+-------+-----+--------+-----------+
| name |             type |  null | key | extras | watermark |
+------+------------------+-------+-----+--------+-----------+
|  ltz | TIMESTAMP_LTZ(3) |  true |     |        |           |
|  ntz |     TIMESTAMP(3) | false |     |        |           |
+------+------------------+-------+-----+--------+-----------+

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Flink SQL> SET 'table.local-time-zone' = 'UTC';
Flink SQL> SELECT * FROM MyView2;

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+-------------------------+-------------------------+
|                     ltz |                     ntz |
+-------------------------+-------------------------+
| 1970-01-01 00:00:04.001 | 1970-01-01 00:00:01.001 |
+-------------------------+-------------------------+

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Flink SQL> SET 'table.local-time-zone' = 'Asia/Shanghai';
Flink SQL> SELECT * FROM MyView2;

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+-------------------------+-------------------------+
|                     ltz |                     ntz |
+-------------------------+-------------------------+
| 1970-01-01 08:00:04.001 | 1970-01-01 00:00:01.001 |
+-------------------------+-------------------------+

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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;

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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;

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+-------------------------+---------------------------+-------------------------+-------------------------+-------------------------------+
|                     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 |
+-------------------------+---------------------------+-------------------------+-------------------------+-------------------------------+

Flink (十七) ：Table API & SQL (五) 时区

1. TIMESTAMP vs TIMESTAMP_LTZ

1.1 TIMESTAMP 类型

1.2 TIMESTAMP_LTZ 类型

2. 时区的作用

2.1 确定时间函数的返回值

2.2 TIMESTAMP_LTZ 字符串表示

`2.2 TIMESTAMP_LTZ` 字符串表示