Squeal.PostgreSQL.Transaction

Squeal transaction control language.

Transaction

transactionally

Arguments

:: (MonadBaseControl IO tx, MonadPQ schema tx)

=> TransactionMode

-> tx x

run inside a transaction

-> tx x

Run a schema invariant computation transactionally.

transactionally_

Arguments

:: (MonadBaseControl IO tx, MonadPQ schema tx)

=> tx x

run inside a transaction

-> tx x

Run a schema invariant computation transactionally_ in defaultMode.

begin :: MonadPQ schema tx => TransactionMode -> tx (K Result NilRelation)

BEGIN a transaction.

commit :: MonadPQ schema tx => tx (K Result NilRelation)

COMMIT a schema invariant transaction.

rollback :: MonadPQ schema tx => tx (K Result NilRelation)

ROLLBACK a schema invariant transaction.

transactionallySchema :: MonadBaseControl IO io => TransactionMode -> PQ schema0 schema1 io x -> PQ schema0 schema1 io x

Run a schema changing computation transactionallySchema.

transactionallySchema_ :: MonadBaseControl IO io => PQ schema0 schema1 io x -> PQ schema0 schema1 io x

Run a schema changing computation transactionallySchema_ in DefaultMode.

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

Eq TransactionMode

Methods

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

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

Show TransactionMode

Methods

showsPrec :: Int -> TransactionMode -> ShowS

show :: TransactionMode -> String

showList :: [TransactionMode] -> ShowS

defaultMode :: TransactionMode

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

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

Eq IsolationLevel

Methods

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

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

Show IsolationLevel

Methods

showsPrec :: Int -> IsolationLevel -> ShowS

show :: IsolationLevel -> String

showList :: [IsolationLevel] -> ShowS

renderIsolationLevel :: IsolationLevel -> ByteString

Render an IsolationLevel.

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

Eq AccessMode

Methods

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

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

Show AccessMode

Methods

showsPrec :: Int -> AccessMode -> ShowS

show :: AccessMode -> String

showList :: [AccessMode] -> ShowS

renderAccessMode :: AccessMode -> ByteString

Render an AccessMode.

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

Eq DeferrableMode

Methods

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

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

Show DeferrableMode

Methods

showsPrec :: Int -> DeferrableMode -> ShowS

show :: DeferrableMode -> String

showList :: [DeferrableMode] -> ShowS

renderDeferrableMode :: DeferrableMode -> ByteString

Render a DeferrableMode.