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Transactions

TiDB supports complete distributed transactions. Both optimistic transaction model and pessimistic transaction model(introduced in TiDB 3.0) are available. This document introduces transaction-related statements, explicit and implicit transactions, isolation levels, lazy check for constraints, and transaction sizes.

The common variables include autocommit, tidb_disable_txn_auto_retry, and tidb_retry_limit.

Common syntax

BEGIN, START TRANSACTION and START TRANSACTION WITH CONSISTENT SNAPSHOT

Syntax:

BEGIN;

START TRANSACTION;

START TRANSACTION WITH CONSISTENT SNAPSHOT;

All of the above three statements are used to start a transaction with the same effect. You can explicitly start a new transaction by executing one of these statements. If the current session is in the process of a transaction when one of these statements is executed, TiDB automatically commits the current transaction before starting a new transaction.

COMMIT

Syntax:

COMMIT;

You can use this statement to commit the current transaction, including all updates between [BEGIN|START TRANSACTION] and COMMIT.

ROLLBACK

Syntax:

ROLLBACK;

You can use this statement to roll back the current transaction and cancels all updates between [BEGIN | START TRANSACTION] and ROLLBACK.

Autocommit

Syntax:

SET autocommit = {0 | 1}

When autocommit = 1 (default), the status of the current session is autocommit. That is, statements are automatically committed immediately following their execution.

When autocommit = 0, the status of the current session is non-autocommit. That is, statements are only committed when you manually execute the COMMIT statement.

autocommit is also a system variable. You can update the current session or the Global value using the following variable assignment statement:

SET @@SESSION.autocommit = {0 | 1};

SET @@GLOBAL.autocommit = {0 | 1};

Explicit and implicit transaction

TiDB supports explicit transactions (use [BEGIN|START TRANSACTION] and COMMIT to define the start and end of the transaction) and implicit transactions (SET autocommit = 1).

If you set the value of autocommit to 1 and start a new transaction through the [BEGIN|START TRANSACTION] statement, the autocommit is disabled before COMMIT or ROLLBACK which makes the transaction becomes explicit.

For DDL statements, the transaction is committed automatically and does not support rollback. If you run the DDL statement while the current session is in the process of a transaction, the DDL statement is executed after the current transaction is committed.

Transaction isolation level

TiDB only supports SNAPSHOT ISOLATION. You can set the isolation level of the current session to READ COMMITTED using the following statement. However, TiDB is only compatible with the READ COMMITTED isolation level in syntax and transactions are still executed at the SNAPSHOT ISOLATION level.

SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;

Lazy check of constraints

Lazy check means that by default TiDB will not check primary key or unique constraints when an INSERT statement is executed, but instead checks when the transaction is committed. In TiDB, the lazy check is performed for values written by ordinary INSERT statements.

For example:

CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY);
INSERT INTO t1 VALUES (1);
START TRANSACTION;
INSERT INTO t1 VALUES (1); -- MySQL returns an error; TiDB returns success.
INSERT INTO t1 VALUES (2);
COMMIT; -- It is successfully committed in MySQL; TiDB returns an error and the transaction rolls back.
SELECT * FROM t1; -- MySQL returns 1 2; TiDB returns 1.

The lazy check is important because if you perform a unique constraint check on every INSERT statement in a transaction, it can cause high network overhead. A batch check when the transaction is committed can greatly improve performance.

Statement rollback

If you execute a statement within a transaction, the statement does not take effect when an error occurs.

begin;
insert into test values (1);
insert into tset values (2);  // This statement does not take effect because "test" is misspelled as "tset".
insert into test values (3);
commit;

In the above example, the second insert statement fails, while the other two insert statements (1 & 3) can be successfully committed.

begin;
insert into test values (1);
insert into tset values (2);  // This statement does not take effect because "test" is misspelled as "tset".
insert into test values (3);
rollback;

In the above example, the second insert statement fails, and this transaction does not insert any data into the database because rollback is called.

Transaction sizes

In TiDB, a transaction either too small or too large can impair the overall performance.

Small transactions

TiDB uses the default autocommit setting (that is, autocommit = 1), which automatically issues a commit when executing each SQL statement. Therefore, each of the following three statements is treated as a transaction:

UPDATE my_table SET a = 'new_value' WHERE id = 1;
UPDATE my_table SET a = 'newer_value' WHERE id = 2;
UPDATE my_table SET a = 'newest_value' WHERE id = 3;

In this case, the latency is increased because each statement, as a transaction, uses the two-phase commit which consumes more execution time.

To improve the execution efficiency, you can use an explicit transaction instead, that is, to execute the above three statements within a transaction:

START TRANSACTION;
UPDATE my_table SET a = 'new_value' WHERE id = 1;
UPDATE my_table SET a = 'newer_value' WHERE id = 2;
UPDATE my_table SET a = 'newest_value' WHERE id = 3;
COMMIT;

Similarly, it is recommended to execute INSERT statements within an explicit transaction.

Large transaction

Due to the requirement of the two-phase commit, a large transaction can lead to the following issues:

  • OOM (Out of Memory) when excessive data is written in the memory
  • More conflicts in the prewrite phase
  • Long duration before transactions are actually committed

Therefore, TiDB intentionally imposes some limits on transaction sizes:

  • The total number of SQL statements in a transaction is no more than 5,000 (default)
  • Each key-value pair is no more than 6 MB
  • The total number of key-value entries is no more than 300,000
  • The total size of key-value entries is no more than 100 MB

For each transaction, it is recommended to keep the number of SQL statements between 100 to 500 to achieve an optimal performance.

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