![]() The identifier must be written as a string literal in this command, for example '000003A1-1'. That function returns a snapshot identifier, which must be given to SET TRANSACTION SNAPSHOT to specify which snapshot is to be imported. The pre-existing transaction must have exported its snapshot with the pg_export_snapshot() function. The SET TRANSACTION SNAPSHOT command allows a new transaction to run with the same snapshot as an existing transaction. Because Greenplum Database does not support serializable transactions, the DEFERRABLE transaction property has no effect in Greenplum Database. This mode is well suited for long-running reports or backups. When all of these properties are set on a transaction, the transaction may block when first acquiring its snapshot, after which it is able to run without the normal overhead of a SERIALIZABLE transaction and without any risk of contributing to or being cancelled by a serialization failure. The DEFERRABLE transaction property has no effect unless the transaction is also SERIALIZABLE and READ ONLY. This is a high-level notion of read-only that does not prevent all writes to disk. When a transaction is read-only, the following SQL commands are disallowed: INSERT, UPDATE, DELETE, and COPY FROM if the table they would write to is not a temporary table all CREATE, ALTER, and DROP commands COMMENT, GRANT, REVOKE, TRUNCATE and EXPLAIN ANALYZE and EXECUTE if the command they would run is among those listed. The transaction access mode determines whether the transaction is read/write or read-only. The transaction isolation level cannot be changed after the first query or data-modification statement ( SELECT, INSERT, DELETE, UPDATE, FETCH, or COPY) of a transaction has been run. If you specify SERIALIZABLE, Greenplum Database falls back to REPEATABLE READ. In Greenplum Database, READ UNCOMMITTED is treated as READ COMMITTED. The SQL standard defines two additional levels, READ UNCOMMITTED and SERIALIZABLE. ![]() REPEATABLE READ - All statements in the current transaction can only see rows committed before the first query or data-modification statement run in the transaction.READ COMMITTED - A statement can only see rows committed before it began.The isolation level of a transaction determines what data the transaction can see when other transactions are running concurrently. Greenplum Database does not support serializable transactions, so including the DEFERRABLE clause has no effect. Noteĭeferrable transactions require the transaction to be serializable. In addition, a snapshot can be selected, though only for the current transaction, not as a session default. The available transaction characteristics are the transaction isolation level, the transaction access mode (read/write or read-only), and the deferrable mode. These defaults can be overridden by SET TRANSACTION for an individual transaction. SET SESSION CHARACTERISTICS sets the default transaction characteristics for subsequent transactions of a session. It has no effect on any subsequent transactions. The SET TRANSACTION command sets the characteristics of the current transaction. ISOLATION LEVEL Īnd is the id of the existing transaction whose snapshot you want this transaction to run with. SET SESSION CHARACTERISTICS AS TRANSACTION Sets the characteristics of the current transaction.
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