Synopsis
Use the BEGIN statement to start a transaction with the default (or specified) isolation level.
Syntax
Grammar
Diagrambegin ::= BEGIN [ TRANSACTION | WORK ] [ transaction_mode [ ... ] ]
transaction_mode ::= isolation_level
| read_write_mode
| deferrable_mode
isolation_level ::= ISOLATION LEVEL { READ UNCOMMITTED
| READ COMMITTED
| REPEATABLE READ
| SERIALIZABLE }
read_write_mode ::= READ ONLY | READ WRITE
deferrable_mode ::= [ NOT ] DEFERRABLE
Semantics
begin
BEGIN [ TRANSACTION | WORK ] [ transaction_mode [ ... ] ]
WORK
Add optional keyword — has no effect.
TRANSACTION
Add optional keyword — has no effect.
transaction_mode
Supports isolation level, read write, and deferrable modes using the transaction mode syntax of ISOLATION LEVEL, READ ONLY, READ WRITE, and [NOT] DEFERRABLE.
For isolation level, you can specify Serializable, Snapshot, and Read Committed EA
Isolation using the PostgreSQL isolation level syntax of SERIALIZABLE, REPEATABLE READ, and READ COMMITTED respectively. PostgreSQL's READ UNCOMMITTED also maps to Read Committed Isolation.
Read Committed Isolation is supported only if the YB-TServer flag yb_enable_read_committed_isolation is set to true. By default this flag is false and in this case the Read Committed isolation level of YugabyteDB's transactional layer falls back to the stricter Snapshot Isolation (in which case READ COMMITTED and READ UNCOMMITTED of YSQL also in turn use Snapshot Isolation).
Examples
Create a sample table.
CREATE TABLE sample(k1 int, k2 int, v1 int, v2 text, PRIMARY KEY (k1, k2));
Begin a transaction and insert some rows.
BEGIN TRANSACTION; SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
INSERT INTO sample(k1, k2, v1, v2) VALUES (1, 2.0, 3, 'a'), (1, 3.0, 4, 'b');
Start a new shell with ysqlsh and begin another transaction to insert some more rows.
BEGIN TRANSACTION; SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
INSERT INTO sample(k1, k2, v1, v2) VALUES (2, 2.0, 3, 'a'), (2, 3.0, 4, 'b');
In each shell, check the only the rows from the current transaction are visible.
1st shell.
yugabyte=# SELECT * FROM sample; -- run in first shell
k1 | k2 | v1 | v2
----+----+----+----
1 | 2 | 3 | a
1 | 3 | 4 | b
(2 rows)
2nd shell
yugabyte=# SELECT * FROM sample; -- run in second shell
k1 | k2 | v1 | v2
----+----+----+----
2 | 2 | 3 | a
2 | 3 | 4 | b
(2 rows)
Commit the first transaction and abort the second one.
COMMIT TRANSACTION; -- run in first shell.
Abort the current transaction (from the first shell).
ABORT TRANSACTION; -- run second shell.
In each shell check that only the rows from the committed transaction are visible.
yugabyte=# SELECT * FROM sample; -- run in first shell.
k1 | k2 | v1 | v2
----+----+----+----
1 | 2 | 3 | a
1 | 3 | 4 | b
(2 rows)
yugabyte=# SELECT * FROM sample; -- run in second shell.
k1 | k2 | v1 | v2
----+----+----+----
1 | 2 | 3 | a
1 | 3 | 4 | b
(2 rows)
Start a READ WRITE transaction using SERIALIZABLE isolation level and making foreign keys DEFERRABLE:
yugabyte=# BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE READ WRITE DEFERRABLE;
yugabyte=# -- run queries
yugabyte=# COMMIT;