BEGIN

Synopsis

Use the BEGIN statement to start a transaction with the default (or given) isolation level.

Syntax

begin ::= BEGIN [ TRANSACTION | WORK ] [ transaction_mode [ ... ] ]

begin

BEGINTRANSACTIONWORKtransaction_mode

Semantics

begin

BEGIN [ TRANSACTION | WORK ] [ transaction_mode [ ... ] ]

WORK

Add optional keyword — has no effect.

TRANSACTION

Add optional keyword — has no effect.

transaction_mode

Supports Serializable, Snapshot, and Read Committed 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 support is currently in Beta. Read Committed Isolation is supported only if the gflag yb_enable_read_committed_isolation is set to true. By default this gflag is false and in this case the Read Committed isolation level of Yugabyte'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)

See also