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distributed-database-middle.../umn/source/sql_syntax/global_sequence/overview.rst
proposalbot 78ac771d84 Changes to ddm_umn from docs/doc-exports#613 (DDM UMN 1st version 
DDM UMN 1st v

Reviewed-by: Wagner, Fabian <fabian.wagner@t-systems.com>
Co-authored-by: proposalbot <proposalbot@otc-service.com>
Co-committed-by: proposalbot <proposalbot@otc-service.com>
2024-01-08 13:37:20 +00:00

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Overview

Global sequences are mainly database-based global sequences.

Note

  • The start auto-increment SN can be modified.
  • Global sequence provides sequence numbers that are globally unique but may not increase continuously.
Table 1 Table types supported by global sequence
Table Type Sharded Broadcast Unsharded
DB-based Supported Supported Not supported

Creating an Auto-Increment Sequence

  1. Log in to the required DDM instance using a client.

  2. Open the required schema.

  3. Run the following command to create an auto-increment sequence:

    create sequence <seq_name>;

    Note

    • <seq_name> indicates the sequence name.
    • The auto-increment key should be a BIGINT value. To avoid duplicate values, do not use TINYINT, SMALLINT, MEDIUMINT, INTEGER, or INT as the auto-increment key.
    • Run show sequences to view the usage of the auto-increment sequence. If the usage reaches 100%, do not insert data anymore.

Dropping an Auto-Increment Sequence

  1. Log in to the required DDM instance using a client.

  2. Open the required schema.

  3. Run show sequences to view all global sequences.

  4. Run the following command to drop an auto-increment sequence:

    drop sequence <seq_name> ;

    drop sequence DB.<seq_name>;

    Note

    • The sequence name is case-insensitive.
    • If an auto-increment sequence is inherent to a table, the sequence cannot be deleted.

Modifying the Start Value of an Auto-Increment Sequence

  1. Log in to the required DDM instance using a client.

  2. Open the required schema.

  3. Run show sequences to view all global sequences.

  4. Run the command to change the start value:

    alter sequence <seq_name> START WITH <start_value>;

    Note

    • <seq_name> indicates the sequence name.
    • <start_value> indicates the start value of the target sequence.

Querying an Auto-Increment Sequence

  1. Log in to the required DDM instance using a client.

  2. Open the required schema.

  3. Run show sequences to view all global sequences.

    show sequences;

    image1

Modifying the Auto-Increment Cache Value

Important

This feature is only available in kernel 3.0.3 and later versions.

  1. Log in to the required DDM instance using a client.
  2. Open the required schema.
  3. Run command alter sequence test cache 5000 to modify the global sequence cache value of table test.
  4. Run command show sequences to view the cache value (INCREMENT value) of table test.

Updating Auto-Increment Sequences of All Tables

Important

This feature is available only in kernel 3.0.4.1 or later.

  1. Log in to the required DDM instance using a client.
  2. Run command fresh all sequence start value to change sequences of all schemas.