SET TRANSACTION
Synopsis
Use the SET TRANSACTION statement to set the current transaction isolation level.
Syntax
set_transaction ::= SET TRANSACTION 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
set_transaction
transaction_mode
isolation_level
read_write_mode
deferrable_mode
Semantics
Supports both Serializable and Snapshot Isolation using the PostgreSQL isolation level syntax of SERIALIZABLE and REPEATABLE READ respectively. Even READ COMMITTED and READ UNCOMMITTED isolation levels are mapped to Snapshot Isolation.
transaction_mode
Set the transaction mode to one of the following.
ISOLATION LEVELclause- Access mode
DEFERRABLEmode
ISOLATION LEVEL clause
SERIALIZABLE
Default in ANSI SQL standard.
REPEATABLE READ
Also referred to as "snapshot isolation" in YugabyteDB. Default.
READ COMMITTED
A statement can only see rows committed before it begins.
READ COMMITTED is mapped to REPEATABLE READ.
Default in PostgreSQL.
READ UNCOMMITTED
READ UNCOMMITTED is mapped to REPEATABLE READ.
In PostgreSQL, READ UNCOMMITTED is mapped to READ COMMITTED.
READ WRITE mode
Default.
READ ONLY mode
The READ ONLY mode does not prevent all writes to disk.
When a transaction is READ ONLY, the following SQL statements are:
-
Disallowed if the table they would write to is not a temporary table.
- INSERT
- UPDATE
- DELETE
- COPY FROM
-
Always disallowed
- COMMENT
- GRANT
- REVOKE
- TRUNCATE
-
Disallowed when the statement that would be executed is one of the above
- EXECUTE
- EXPLAIN ANALYZE
DEFERRABLE mode
Use to defer a transaction only when both SERIALIZABLE and READ ONLY modes are also selected. If used, then 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 canceled by a serialization failure.
The DEFERRABLE mode may be useful for long-running reports or back-ups.
Examples
Create a sample table.
yugabyte=# CREATE TABLE sample(k1 int, k2 int, v1 int, v2 text, PRIMARY KEY (k1, k2));
Begin a transaction and insert some rows.
yugabyte=# BEGIN TRANSACTION; SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
yugabyte=# 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.
yugabyte=# BEGIN TRANSACTION; SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
yugabyte=# 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.
yugabyte=# COMMIT TRANSACTION; -- run in first shell.
Abort the current transaction (from the first shell).
yugabyte=# 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)