Transactions in databases refer to logical units of work that consist of one or more database operations. A transaction is a sequence of database operations, such as reads (queries) and writes (updates or inserts), that are treated as a single indivisible and consistent unit. The main purpose of transactions is to ensure data integrity and maintain the ACID (Atomicity, Consistency, Isolation, Durability) properties of the database.

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Read-only transactions

RisingWave supports read-only transactions, where all reads within a transaction are executed against the consistent Hummock snapshot. Hummock is the LSM-Tree-based storage engine in RisingWave that is specifically optimized for streaming workloads.

To initiate a transaction, use either the START TRANSACTION READ ONLY or BEGIN READ ONLY command. Subsequently, you can execute queries to read data from the consistent snapshot. To finalize the transaction and submit the queries as a single unit, use the COMMIT command.

Please note that data modifications are not allowed while a transaction is initiated but not yet committed. The statements listed below are not allowed within a transaction:

  • All DDL statements (CREATE, ALTER, and DROP)
  • Most of DML statements (INSERT, UPDATE, and DELETE)
  • Statements related to USER. This category may overlap with DDL statements.
  • All privilege-related statements, including GRANT and REVOKE.

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Transactions within a CDC table

When you create a table to ingest CDC streams, you can enable this feature by setting transactional to true in the WITH clause of the CREATE TABLE statement. Note that this feature is only available if you are using the native MySQL CDC or PostgreSQL CDC connectors.

For performance considerations, transactions involving changes to more than 4096 rows cannot be guaranteed.