reference/sql: add some details to the document of auto_random #2478
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TomShawn
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Mar 17, 2020
| + 如果该 `INSERT` 语句没有指定整型主键列（`a` 列）的值，TiDB 会为该列自动分配值。该值不保证自增，不保证连续，只保证唯一，避免了连续的行 ID 带来的热点问题。 | ||
| + 如果该 `INSERT` 语句显式指定了整型主键列的值，和 `AUTO_INCREMENT` 属性类似，TiDB 会保存该值。 | ||
| + 如果该 `INSERT` 语句没有指定整型主键列（`a` 列）的值，或者指定为 `NULL`，TiDB 会为该列自动分配值。该值不保证自增，不保证连续，只保证唯一，避免了连续的行 ID 带来的热点问题。 | ||
| + 如果该 `INSERT` 语句显式指定了整型主键列的值，和 `AUTO_INCREMENT` 属性类似，TiDB 会保存该值。注意，如果系统变量 `@@sql_mode` 中没有设置 `NO_AUTO_VALUE_ON_ZERO`， 即使显式指定为 0，TiDB 也会为该列自动分配值。 |
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| + 如果该 `INSERT` 语句显式指定了整型主键列的值，和 `AUTO_INCREMENT` 属性类似，TiDB 会保存该值。注意，如果系统变量 `@@sql_mode` 中没有设置 `NO_AUTO_VALUE_ON_ZERO`， 即使显式指定为 0，TiDB 也会为该列自动分配值。 | |
| + 如果该 `INSERT` 语句显式指定了整型主键列的值，和 `AUTO_INCREMENT` 属性类似，TiDB 会保存该值。注意，如果未在系统变量 `@@sql_mode` 中设置 `NO_AUTO_VALUE_ON_ZERO`， 即使显式指定整型主键列的值为 `0`，TiDB 也会为该列自动分配值。 |
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| 自动分配值的计算方式如下： | ||
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| 该行值在二进制形式下的最高五位（称为 shard bits）由当前事务的开始时间决定，剩下的位数按照自增顺序分配数值。 | ||
| 该行值在二进制形式下，除去最高位（无论是 unsigned 还是 signed）的次高 5 位（称为 shard bits）由当前事务的开始时间决定，剩下的位数按照自增的顺序分配。 |
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| 该行值在二进制形式下，除去最高位（无论是 unsigned 还是 signed）的次高 5 位（称为 shard bits）由当前事务的开始时间决定，剩下的位数按照自增的顺序分配。 | |
| 该行值在二进制形式下，除去最高位（无论是 unsigned 还是 signed）的次高五位（称为 shard bits）由当前事务的开始时间决定，剩下的位数按照自增的顺序分配。 |
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| 另外，含有 `AUTO_RANDOM` 属性的表在系统表 `information_schema.tables` 中 `TIDB_ROW_ID_SHARDING_INFO` 一列的值为 `PK_AUTO_RANDOM_BITS=x`，其中 `x` 为 shard bits 的数量。 | ||
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| 要获取上一次 TiDB 隐式分配的 id，可以使用 `select last_insert_id()` 查看，例如： |
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| 要获取上一次 TiDB 隐式分配的 id，可以使用 `select last_insert_id()` 查看，例如： | |
| 要获取上一次 TiDB 隐式分配的 ID，可以使用 `select last_insert_id()` 查看，例如： |
TomShawn
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Your auto merge job has been accepted, waiting for 2512, 2513, 2514 |
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/run-all-tests |
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cherry pick to release-3.1 in PR #2516 |
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What is changed, added or deleted? (Required)
Add some details to the document of auto_random.
Which TiDB version(s) do your changes apply to? (Required)
If you select two or more versions from above, to trigger the bot to cherry-pick this PR to your desired release version branch(es), you must add corresponding labels such as needs-cherry-pick-3.1, needs-cherry-pick-3.0, and needs-cherry-pick-2.1.
What is the related PR or file link(s)?
pingcap/tidb#15145: fix last_insert_id in auto_random mode
pingcap/tidb#15262: show max allocate times when an auto_random table is created