Squeal.PostgreSQL.Session.Transaction.Unsafe

transaction control language permitting arbitrary IO

Transaction

transactionally

Arguments

:: (MonadMask tx, MonadPQ db tx)

=> TransactionMode

-> tx x

run inside a transaction

-> tx x

Run a computation transactionally; first begin, then run the computation, onException rollback and rethrow the exception, otherwise commit and return the result.

transactionally_

Arguments

:: (MonadMask tx, MonadPQ db tx)

=> tx x

run inside a transaction

-> tx x

Run a computation transactionally_, in defaultMode.

transactionallyRetry

Arguments

:: (MonadMask tx, MonadPQ db tx)

=> TransactionMode

-> tx x

run inside a transaction

-> tx x

transactionallyRetry a computation;

• first begin,
• then try the computation,
• if it raises a serialization failure or deadlock detection, then rollback and restart the transaction,
• if it raises any other exception then rollback and rethrow the exception,
• otherwise commit and return the result.

transactionallyRetry_

Arguments

:: (MonadMask tx, MonadPQ db tx)

=> tx x

run inside a transaction

-> tx x

transactionallyRetry in retryMode.

ephemerally

Arguments

:: (MonadMask tx, MonadPQ db tx)

=> TransactionMode

-> tx x

run inside an ephemeral transaction

-> tx x

Run a computation ephemerally; Like transactionally but always rollback, useful in testing.

ephemerally_

Arguments

:: (MonadMask tx, MonadPQ db tx)

=> tx x

run inside an ephemeral transaction

-> tx x

Run a computation ephemerally in defaultMode.

begin :: TransactionMode -> Manipulation_ db () ()

BEGIN a transaction.

commit :: Manipulation_ db () ()

COMMIT a transaction.

rollback :: Manipulation_ db () ()

ROLLBACK a transaction.

withSavepoint

Arguments

:: MonadPQ db tx

=> ByteString

savepoint name

-> tx (Either e x)

-> tx (Either e x)

withSavepoint, used in a transaction block, allows a form of nested transactions, creating a savepoint, then running a transaction, rolling back to the savepoint if it returned Left, then releasing the savepoint and returning transaction's result.

Make sure to run withSavepoint in a transaction block, not directly or you will provoke a SQL exception.

Transaction Mode

data TransactionMode

The available transaction characteristics are the transaction IsolationLevel, the transaction AccessMode (ReadWrite or ReadOnly), and the DeferrableMode.

Constructors

TransactionMode

Fields

• isolationLevel :: IsolationLevel
• accessMode :: AccessMode
• deferrableMode :: DeferrableMode

Instances

Show TransactionMode

Defined in Squeal.PostgreSQL.Session.Transaction.Unsafe

Methods

showsPrec :: Int -> TransactionMode -> ShowS

show :: TransactionMode -> String

showList :: [TransactionMode] -> ShowS

Eq TransactionMode

Defined in Squeal.PostgreSQL.Session.Transaction.Unsafe

Methods

(==) :: TransactionMode -> TransactionMode -> Bool

(/=) :: TransactionMode -> TransactionMode -> Bool

RenderSQL TransactionMode

Render a TransactionMode.

Defined in Squeal.PostgreSQL.Session.Transaction.Unsafe

Methods

renderSQL :: TransactionMode -> ByteString

defaultMode :: TransactionMode

TransactionMode with a ReadCommitted IsolationLevel, ReadWrite AccessMode and NotDeferrable DeferrableMode.

retryMode :: TransactionMode

TransactionMode with a Serializable IsolationLevel, ReadWrite AccessMode and NotDeferrable DeferrableMode, appropriate for short-lived queries or manipulations.

longRunningMode :: TransactionMode

TransactionMode with a Serializable IsolationLevel, ReadOnly AccessMode and Deferrable DeferrableMode. This mode is well suited for long-running reports or backups.

data IsolationLevel

The SQL standard defines four levels of transaction isolation. The most strict is Serializable, which is defined by the standard in a paragraph which says that any concurrent execution of a set of Serializable transactions is guaranteed to produce the same effect as running them one at a time in some order. The other three levels are defined in terms of phenomena, resulting from interaction between concurrent transactions, which must not occur at each level. The phenomena which are prohibited at various levels are:

Dirty read: A transaction reads data written by a concurrent uncommitted transaction.

Nonrepeatable read: A transaction re-reads data it has previously read and finds that data has been modified by another transaction (that committed since the initial read).

Phantom read: A transaction re-executes a query returning a set of rows that satisfy a search condition and finds that the set of rows satisfying the condition has changed due to another recently-committed transaction.

Serialization anomaly: The result of successfully committing a group of transactions is inconsistent with all possible orderings of running those transactions one at a time.

In PostgreSQL, you can request any of the four standard transaction isolation levels, but internally only three distinct isolation levels are implemented, i.e. PostgreSQL's ReadUncommitted mode behaves like ReadCommitted. This is because it is the only sensible way to map the standard isolation levels to PostgreSQL's multiversion concurrency control architecture.

Constructors

Serializable

Dirty read is not possible. Nonrepeatable read is not possible. Phantom read is not possible. Serialization anomaly is not possible.

RepeatableRead

Dirty read is not possible. Nonrepeatable read is not possible. Phantom read is not possible. Serialization anomaly is possible.

ReadCommitted

Dirty read is not possible. Nonrepeatable read is possible. Phantom read is possible. Serialization anomaly is possible.

ReadUncommitted

Dirty read is not possible. Nonrepeatable read is possible. Phantom read is possible. Serialization anomaly is possible.

Instances

Show IsolationLevel

Defined in Squeal.PostgreSQL.Session.Transaction.Unsafe

Methods

showsPrec :: Int -> IsolationLevel -> ShowS

show :: IsolationLevel -> String

showList :: [IsolationLevel] -> ShowS

Eq IsolationLevel

Defined in Squeal.PostgreSQL.Session.Transaction.Unsafe

Methods

(==) :: IsolationLevel -> IsolationLevel -> Bool

(/=) :: IsolationLevel -> IsolationLevel -> Bool

RenderSQL IsolationLevel

Render an IsolationLevel.

Defined in Squeal.PostgreSQL.Session.Transaction.Unsafe

Methods

renderSQL :: IsolationLevel -> ByteString

data AccessMode

The transaction access mode determines whether the transaction is ReadWrite or ReadOnly. ReadWrite is the default. When a transaction is ReadOnly, 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 execute is among those listed. This is a high-level notion of ReadOnly that does not prevent all writes to disk.

Constructors

ReadWrite

ReadOnly

Instances

Show AccessMode

Defined in Squeal.PostgreSQL.Session.Transaction.Unsafe

Methods

showsPrec :: Int -> AccessMode -> ShowS

show :: AccessMode -> String

showList :: [AccessMode] -> ShowS

Eq AccessMode

Defined in Squeal.PostgreSQL.Session.Transaction.Unsafe

Methods

(==) :: AccessMode -> AccessMode -> Bool

(/=) :: AccessMode -> AccessMode -> Bool

RenderSQL AccessMode

Render an AccessMode.

Defined in Squeal.PostgreSQL.Session.Transaction.Unsafe

Methods

renderSQL :: AccessMode -> ByteString

data DeferrableMode

The Deferrable transaction property has no effect unless the transaction is also Serializable and ReadOnly. When all three of these properties are selected for 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 canceled by a serialization failure. This longRunningMode is well suited for long-running reports or backups.

Constructors

Deferrable

NotDeferrable

Instances

Show DeferrableMode

Defined in Squeal.PostgreSQL.Session.Transaction.Unsafe

Methods

showsPrec :: Int -> DeferrableMode -> ShowS

show :: DeferrableMode -> String

showList :: [DeferrableMode] -> ShowS

Eq DeferrableMode

Defined in Squeal.PostgreSQL.Session.Transaction.Unsafe

Methods

(==) :: DeferrableMode -> DeferrableMode -> Bool

(/=) :: DeferrableMode -> DeferrableMode -> Bool

RenderSQL DeferrableMode

Render a DeferrableMode.

Defined in Squeal.PostgreSQL.Session.Transaction.Unsafe

Methods

renderSQL :: DeferrableMode -> ByteString