Database.PostgreSQL.LibPQ

postgresql-libpq-0.9.0.1: low-level binding to libpq

Copyright | (c) 2010 Grant Monroe, (c) 2011 Leon P Smith
License | BSD3
Maintainer | leon@melding-monads.com
Stability | experimental
Safe Haskell | None
Language | Haskell98

Database.PostgreSQL.LibPQ

Contents

- Database Connection Control Functions
- Connection Status Functions
- Command Execution Functions
- Retrieving Query Result Information
- Escaping Strings for Inclusion in SQL Commands
- Escaping Binary Strings for Inclusion in SQL Commands
- Escaping Identifiers for Inclusion in SQL Commands
- Using COPY
- Asynchronous Command Processing
- Cancelling Queries in Progress
- Asynchronous Notification
- Control Functions
- Large Objects

Description

This is a binding to libpq: the C application programmer's interface to PostgreSQL. libpq is a set of library functions that allow client programs to pass queries to the PostgreSQL backend server and to receive the results of these queries.

This is intended to be a very low-level interface to libpq. It provides memory management and a somewhat more consistent interface to error conditions. Application code should typically use a higher-level PostgreSQL binding.

This interface is not safe, because libpq unfortunately conflates explicit disconnects with memory management. A use-after-free memory fault will result if a connection is used in any way after `finish` is called. This will likely cause a segfault, or return an error if memory has not yet been reused. Other more bizarre behaviors are possible, though unlikely by chance. Higher-level bindings need to be aware of this issue and need to ensure that application code cannot cause the functions in this module to be called on an `finish`ed connection.

One possibility is to represent a connection in a higher-level interface as `MVar (Maybe Connection)`, using `Nothing` to represent an explicitly disconnected state. This was done in an earlier incarnation of this library, however this was removed because a higher level binding is likely to use a similar construct to deal with other issues. Thus incorporating that in this module results in extra layers of indirection for relatively little functionality.

Synopsis

- data Connection
- connectdb :: ByteString -> IO Connection
- connectStart :: ByteString -> IO Connection
- connectPoll :: Connection -> IO PollingStatus
- newNullConnection :: IO Connection
- isNullConnection :: Connection -> Bool
- reset :: Connection -> IO ()
- resetStart :: Connection -> IO Bool
- resetPoll :: Connection -> IO PollingStatus
- data PollingStatus
  - = PollingFailed
  - | PollingReading
  - | PollingWriting
  - | PollingOk
- finish :: Connection -> IO ()
- db :: Connection -> IO (Maybe ByteString)
- user :: Connection -> IO (Maybe ByteString)
- pass :: Connection -> IO (Maybe ByteString)
- host :: Connection -> IO (Maybe ByteString)
- port :: Connection -> IO (Maybe ByteString)
- options :: Connection -> IO (Maybe ByteString)
- data ConnStatus
  - = ConnectionOk
  - | ConnectionBad
  - | ConnectionStarted
  - | ConnectionMade
  - | ConnectionAwaitingResponse
  - | ConnectionAuthOk
  - | ConnectionSetEnv
  - | ConnectionSSLStartup
- status :: Connection -> IO ConnStatus
- data TransactionStatus
  - = TransIdle
  - | TransActive
  - | TransInTrans
  - | TransInError
  - | TransUnknown
- transactionStatus :: Connection -> IO TransactionStatus
- parameterStatus :: Connection -> ByteString -> IO (Maybe ByteString)
- protocolVersion :: Connection -> IO Int
- serverVersion :: Connection -> IO Int
- errorMessage :: Connection -> IO (Maybe ByteString)
- socket :: Connection -> IO (Maybe Fd)
- backendPID :: Connection -> IO CPid
- connectionNeedsPassword :: Connection -> IO Bool
- connectionUsedPassword :: Connection -> IO Bool
- data Result
- exec :: Connection -> ByteString -> IO (Maybe Result)
- data Format
  - = Text
  - | Binary
- newtype Oid = Oid CUInt
- invalidOid :: Oid
- execParams :: Connection -> ByteString -> [Maybe (Oid, ByteString, Format)] -> Format -> IO (Maybe Result)
- prepare :: Connection -> ByteString -> ByteString -> Maybe [Oid] -> IO (Maybe Result)
- execPrepared :: Connection -> ByteString -> [Maybe (ByteString, Format)] -> Format -> IO (Maybe Result)
- describePrepared :: Connection -> ByteString -> IO (Maybe Result)
- describePortal :: Connection -> ByteString -> IO (Maybe Result)
- data ExecStatus
  - = EmptyQuery
  - | CommandOk
  - | TuplesOk
  - | CopyOut
  - | CopyIn
  - | BadResponse
  - | NonfatalError
  - | FatalError
- resultStatus :: Result -> IO ExecStatus
- resStatus :: ExecStatus -> IO ByteString
- resultErrorMessage :: Result -> IO (Maybe ByteString)
- data FieldCode
  - = DiagSeverity
  - | DiagSqlstate
  - | DiagMessagePrimary
  - | DiagMessageDetail
  - | DiagMessageHint
  - | DiagStatementPosition
  - | DiagInternalPosition
  - | DiagInternalQuery
  - | DiagContext
  - | DiagSourceFile
  - | DiagSourceLine
  - | DiagSourceFunction
- resultErrorField :: Result -> FieldCode -> IO (Maybe ByteString)
- unsafeFreeResult :: Result -> IO ()
- ntuples :: Result -> IO Row
- nfields :: Result -> IO Column
- newtype Row = Row CInt
- newtype Column = Col CInt
- toRow :: Integral a => a -> Row
- toColumn :: Integral a => a -> Column
- fname :: Result -> Column -> IO (Maybe ByteString)
- fnumber :: Result -> ByteString -> IO (Maybe Column)
- ftable :: Result -> Column -> IO Oid
- ftablecol :: Result -> Column -> IO Column
- fformat :: Result -> Column -> IO Format
- ftype :: Result -> Column -> IO Oid
- fmod :: Result -> Column -> IO Int
- fsize :: Result -> Column -> IO Int
- getvalue :: Result -> Row -> Column -> IO (Maybe ByteString)
- getvalue' :: Result -> Row -> Column -> IO (Maybe ByteString)
- getisnull :: Result -> Row -> Column -> IO Bool
- getlength :: Result -> Row -> Column -> IO Int
- nparams :: Result -> IO Int
- paramtype :: Result -> Int -> IO Oid
- cmdStatus :: Result -> IO (Maybe ByteString)
- cmdTuples :: Result -> IO (Maybe ByteString)
- escapeStringConn :: Connection -> ByteString -> IO (Maybe ByteString)
- escapeByteaConn :: Connection -> ByteString -> IO (Maybe ByteString)
- unescapeBytea :: ByteString -> IO (Maybe ByteString)
- escapeIdentifier :: Connection -> ByteString -> IO (Maybe ByteString)
- data CopyInResult
  - = CopyInOk
  - | CopyInError
  - | CopyInWouldBlock
- putCopyData :: Connection -> ByteString -> IO CopyInResult
- putCopyEnd :: Connection -> Maybe ByteString -> IO CopyInResult
- data CopyOutResult
  - = CopyOutRow !ByteString
  - | CopyOutWouldBlock
  - | CopyOutDone
  - | CopyOutError
- getCopyData :: Connection -> Bool -> IO CopyOutResult
- sendQuery :: Connection -> ByteString -> IO Bool
- sendQueryParams :: Connection -> ByteString -> [Maybe (Oid, ByteString, Format)] -> Format -> IO Bool
- sendPrepare :: Connection -> ByteString -> ByteString -> Maybe [Oid] -> IO Bool
- sendQueryPrepared :: Connection -> ByteString -> [Maybe (ByteString, Format)] -> Format -> IO Bool
- sendDescribePrepared :: Connection -> ByteString -> IO Bool
- sendDescribePortal :: Connection -> ByteString -> IO Bool
- getResult :: Connection -> IO (Maybe Result)
- consumeInput :: Connection -> IO Bool
- isBusy :: Connection -> IO Bool
- setnonblocking :: Connection -> Bool -> IO Bool
- isnonblocking :: Connection -> IO Bool
- data FlushStatus
  - = FlushOk
  - | FlushFailed
  - | FlushWriting
- flush :: Connection -> IO FlushStatus
- data Cancel
- getCancel :: Connection -> IO (Maybe Cancel)
- cancel :: Cancel -> IO (Either ByteString ())
- data Notify = Notify {
  - notifyRelname :: !ByteString
  - notifyBePid :: !CPid
  - notifyExtra :: !ByteString
  }
- notifies :: Connection -> IO (Maybe Notify)
- clientEncoding :: Connection -> IO ByteString
- setClientEncoding :: Connection -> ByteString -> IO Bool
- data Verbosity
  - = ErrorsTerse
  - | ErrorsDefault
  - | ErrorsVerbose
- setErrorVerbosity :: Connection -> Verbosity -> IO Verbosity
- newtype LoFd = LoFd CInt
- loCreat :: Connection -> IO (Maybe Oid)
- loCreate :: Connection -> Oid -> IO (Maybe Oid)
- loImport :: Connection -> FilePath -> IO (Maybe Oid)
- loImportWithOid :: Connection -> FilePath -> Oid -> IO (Maybe Oid)
- loExport :: Connection -> Oid -> FilePath -> IO (Maybe ())
- loOpen :: Connection -> Oid -> IOMode -> IO (Maybe LoFd)
- loWrite :: Connection -> LoFd -> ByteString -> IO (Maybe Int)
- loRead :: Connection -> LoFd -> Int -> IO (Maybe ByteString)
- loSeek :: Connection -> LoFd -> SeekMode -> Int -> IO (Maybe Int)
- loTell :: Connection -> LoFd -> IO (Maybe Int)
- loTruncate :: Connection -> LoFd -> Int -> IO (Maybe ())
- loClose :: Connection -> LoFd -> IO (Maybe ())
- loUnlink :: Connection -> Oid -> IO (Maybe ())

# Database Connection Control Functions

The following functions deal with making a connection to a PostgreSQL backend server. An application program can have several backend connections open at one time. (One reason to do that is to access more than one database.) Each connection is represented by a `Connection`, which is obtained from the function `connectdb`, or `connectStart`. The `status` function should be called to check whether a connection was successfully made before queries are sent via the connection object.

data Connection

`Connection` encapsulates a connection to the backend.

Instances

Eq Connection |

connectdb

Arguments

:: ByteString | Connection Info
-> IO Connection |

Makes a new connection to the database server.

This function opens a new database connection using the parameters taken from the string conninfo. Its nonblocking analogues are `connectStart` and `connectPoll`.

The passed string can be empty to use all default parameters, or it can contain one or more parameter settings separated by whitespace. Each parameter setting is in the form keyword = value. Spaces around the equal sign are optional. To write an empty value or a value containing spaces, surround it with single quotes, e.g., keyword = 'a value'. Single quotes and backslashes within the value must be escaped with a backslash, i.e., ' and \.

connectStart

Arguments

:: ByteString | Connection Info
-> IO Connection |

Make a connection to the database server in a nonblocking manner.

connectPoll :: Connection -> IO PollingStatus

If `connectStart` succeeds, the next stage is to poll libpq so that it can proceed with the connection sequence. Use `socket` to obtain the `Fd` of the socket underlying the database connection. Loop thus: If `connectPoll` last returned `PollingReading`, wait until the socket is ready to read (as indicated by select(), poll(), or similar system function). Then call `connectPoll` again. Conversely, if `connectPoll` last returned `PollingWriting`, wait until the socket is ready to write, then call `connectPoll` again. If you have yet to call `connectPoll`, i.e., just after the call to `connectStart`, behave as if it last returned `PollingWriting`. Continue this loop until `connectPoll` returns `PollingFailed`, indicating the connection procedure has failed, or `PollingOk`, indicating the connection has been successfully made.

newNullConnection :: IO Connection

Allocate a Null Connection, which all libpq functions should safely fail on.

isNullConnection :: Connection -> Bool

Test if a connection is the Null Connection.

reset :: Connection -> IO ()

Resets the communication channel to the server.

This function will close the connection to the server and attempt to reestablish a new connection to the same server, using all the same parameters previously used. This might be useful for error recovery if a working connection is lost.

resetStart :: Connection -> IO Bool

Reset the communication channel to the server, in a nonblocking manner.

resetPoll :: Connection -> IO PollingStatus

To initiate a connection reset, call `resetStart`. If it returns `False`, the reset has failed. If it returns `True`, poll the reset using `resetPoll` in exactly the same way as you would create the connection using `connectPoll`.

data PollingStatus

Constructors

PollingFailed |
PollingReading |
PollingWriting |
PollingOk |

Instances

Eq PollingStatus |
Show PollingStatus |

finish :: Connection -> IO ()

Closes the connection to the server.

Note that the `Connection` must not be used again after `finish` has been called.

# Connection Status Functions

These functions can be used to interrogate the status of an existing database connection object.

db :: Connection -> IO (Maybe ByteString)

Returns the database name of the connection.

user :: Connection -> IO (Maybe ByteString)

Returns the user name of the connection.

pass :: Connection -> IO (Maybe ByteString)

Returns the password of the connection.

host :: Connection -> IO (Maybe ByteString)

Returns the server host name of the connection.

port :: Connection -> IO (Maybe ByteString)

Returns the port of the connection.

options :: Connection -> IO (Maybe ByteString)

Returns the command-line options passed in the connection request.

data ConnStatus

Constructors

ConnectionOk | The `Connection` is ready.
ConnectionBad | The connection procedure has failed.
ConnectionStarted | Waiting for connection to be made.
ConnectionMade | Connection OK; waiting to send.
ConnectionAwaitingResponse | Waiting for a response from the server.
ConnectionAuthOk | Received authentication; waiting for backend start-up to finish.
ConnectionSetEnv | Negotiating environment-driven parameter settings.
ConnectionSSLStartup | Negotiating SSL encryption.

Instances

Eq ConnStatus |
Show ConnStatus |

status :: Connection -> IO ConnStatus

Returns the status of the connection.

The status can be one of a number of values. However, only two of these are seen outside of an asynchronous connection procedure: `ConnectionOk` and `ConnectionBad`. A good connection to the database has the status `ConnectionOk`. A failed connection attempt is signaled by status `ConnectionBad`. Ordinarily, an OK status will remain so until `finish`, but a communications failure might result in the status changing to `ConnectionBad` prematurely. In that case the application could try to recover by calling `reset`.

See the entry for `connectStart` and `connectPoll` with regards to other status codes that might be seen.

data TransactionStatus

Constructors

TransIdle | currently idle
TransActive | a command is in progress
TransInTrans | idle, in a valid transaction block
TransInError | idle, in a failed transaction block
TransUnknown | the connection is bad

Instances

Eq TransactionStatus |
Show TransactionStatus |

transactionStatus :: Connection -> IO TransactionStatus

Returns the current in-transaction status of the server.

`TransActive` is reported only when a query has been sent to the server and not yet completed.

parameterStatus

Arguments

:: Connection |
-> ByteString | paramName
-> IO (Maybe ByteString) |

Looks up a current parameter setting of the server.

Certain parameter values are reported by the server automatically at connection startup or whenever their values change. `parameterStatus` can be used to interrogate these settings. It returns the current value of a parameter if known, or `Nothing` if the parameter is not known.

protocolVersion :: Connection -> IO Int

Interrogates the frontend/backend protocol being used.

Applications might wish to use this to determine whether certain features are supported. Currently, the possible values are 2 (2.0 protocol), 3 (3.0 protocol), or zero (connection bad). This will not change after connection startup is complete, but it could theoretically change during a connection reset. The 3.0 protocol will normally be used when communicating with PostgreSQL 7.4 or later servers; pre-7.4 servers support only protocol 2.0. (Protocol 1.0 is obsolete and not supported by libpq.)

serverVersion :: Connection -> IO Int

Returns an integer representing the backend version.

Applications might use this to determine the version of the database server they are connected to. The number is formed by converting the major, minor, and revision numbers into two-decimal-digit numbers and appending them together. For example, version 8.1.5 will be returned as 80105, and version 8.2 will be returned as 80200 (leading zeroes are not shown). Zero is returned if the connection is bad.

errorMessage :: Connection -> IO (Maybe ByteString)

Returns the error message most recently generated by an operation on the connection.

Nearly all libpq functions will set a message for `errorMessage` if they fail. Note that by libpq convention, a nonempty `errorMessage` result can be multiple lines, and will include a trailing newline. The result string should not be expected to remain the same across operations on the `Connection`.

socket :: Connection -> IO (Maybe Fd)

Obtains the file descriptor number of the connection socket to the server. (This will not change during normal operation, but could change during connection setup or reset.)

backendPID :: Connection -> IO CPid

Returns the process `CPid` of the backend server process handling this connection.

The backend PID is useful for debugging purposes and for comparison to NOTIFY messages (which include the PID of the notifying backend process). Note that the PID belongs to a process executing on the database server host, not the local host!

connectionNeedsPassword :: Connection -> IO Bool

Returns `True` if the connection authentication method required a password, but none was available. Returns `False` if not.

This function can be applied after a failed connection attempt to decide whether to prompt the user for a password.

connectionUsedPassword :: Connection -> IO Bool

Returns `True` if the connection authentication method used a password. Returns `False` if not.

This function can be applied after either a failed or successful connection attempt to detect whether the server demanded a password.

# Command Execution Functions

Once a connection to a database server has been successfully established, the functions described here are used to perform SQL queries and commands.

data Result

`Result` encapsulates the result of a query (or more precisely, of a single SQL command --- a query string given to `sendQuery` can contain multiple commands and thus return multiple instances of `Result`.

Instances

Eq Result |
Show Result |

exec

Arguments

:: Connection | connection
-> ByteString | statement
-> IO (Maybe Result) | result

Submits a command to the server and waits for the result.

Returns a `Result` or possibly `Nothing`. A `Result` will generally be returned except in out-of-memory conditions or serious errors such as inability to send the command to the server. If a `Nothing` is returned, it should be treated like a `FatalError` result. Use `errorMessage` to get more information about such errors.

It is allowed to include multiple SQL commands (separated by semicolons) in the command string. Multiple queries sent in a single `exec` call are processed in a single transaction, unless there are explicit BEGIN/COMMIT commands included in the query string to divide it into multiple transactions. Note however that the returned `Result` structure describes only the result of the last command executed from the string. Should one of the commands fail, processing of the string stops with it and the returned `Result` describes the error condition.

data Format

Constructors

Text |
Binary |

Instances

Enum Format |
Eq Format |
Ord Format |
Show Format |

newtype Oid

Constructors

Oid CUInt |

Instances

Eq Oid |
Ord Oid |
Read Oid |
Show Oid |
Storable Oid |

invalidOid :: Oid

execParams

Arguments

:: Connection | connection
-> ByteString | statement
-> [Maybe (Oid, ByteString, Format)] | parameters
-> Format | result format
-> IO (Maybe Result) | result

Submits a command to the server and waits for the result, with the ability to pass parameters separately from the SQL command text.

`execParams` is like `exec`, but offers additional functionality: parameter values can be specified separately from the command string proper, and query results can be requested in either text or binary format. `execParams` is supported only in protocol 3.0 and later connections; it will fail when using protocol 2.0.

The primary advantage of `execParams` over `exec` is that parameter values can be separated from the command string, thus avoiding the need for tedious and error-prone quoting and escaping.

Unlike `exec`, `execParams` allows at most one SQL command in the given string. (There can be semicolons in it, but not more than one nonempty command.) This is a limitation of the underlying protocol, but has some usefulness as an extra defense against SQL-injection attacks.

Tip: Specifying parameter types via OIDs is tedious, particularly if you prefer not to hard-wire particular OID values into your program. However, you can avoid doing so even in cases where the server by itself cannot determine the type of the parameter, or chooses a different type than you want. In the SQL command text, attach an explicit cast to the parameter symbol to show what data type you will send. For example: SELECT * FROM mytable WHERE x = $1::bigint; This forces parameter $1 to be treated as bigint, whereas by default it would be assigned the same type as x. Forcing the parameter type decision, either this way or by specifying a numeric type OID, is strongly recommended when sending parameter values in binary format, because binary format has less redundancy than text format and so there is less chance that the server will detect a type mismatch mistake for you.

prepare

Arguments

:: Connection | connection
-> ByteString | stmtName
-> ByteString | query
-> Maybe [Oid] | paramTypes
-> IO (Maybe Result) | result

Submits a request to create a prepared statement with the given parameters, and waits for completion.

`prepare` creates a prepared statement for later execution with `execPrepared`. This feature allows commands that will be used repeatedly to be parsed and planned just once, rather than each time they are executed. `prepare` is supported only in protocol 3.0 and later connections; it will fail when using protocol 2.0.

The function creates a prepared statement named stmtName from the query string, which must contain a single SQL command. stmtName can be "" to create an unnamed statement, in which case any pre-existing unnamed statement is automatically replaced; otherwise it is an error if the statement name is already defined in the current session. If any parameters are used, they are referred to in the query as $1, $2, etc. paramTypes specifies, by `Oid`, the data types to be assigned to the parameter symbols. If paramTypes is `Nothing`, or any particular element in the array is zero, the server assigns a data type to the parameter symbol in the same way it would do for an untyped literal string. Also, the query can use parameter symbols with numbers higher than the length of paramTypes; data types will be inferred for these symbols as well. (See `describePrepared` for a means to find out what data types were inferred.)

As with `exec`, the result is normally a `Result` whose contents indicate server-side success or failure. A `Nothing` result indicates out-of-memory or inability to send the command at all. Use `errorMessage` to get more information about such errors.

Prepared statements for use with `execPrepared` can also be created by executing SQL PREPARE statements. (But `prepare` is more flexible since it does not require parameter types to be pre-specified.) Also, although there is no libpq function for deleting a prepared statement, the SQL DEALLOCATE statement can be used for that purpose.

execPrepared

Arguments

:: Connection | connection
-> ByteString | stmtName
-> [Maybe (ByteString, Format)] | parameters
-> Format | result format
-> IO (Maybe Result) | result

Sends a request to execute a prepared statement with given parameters, and waits for the result.

`execPrepared` is like `execParams`, but the command to be executed is specified by naming a previously-prepared statement, instead of giving a query string. This feature allows commands that will be used repeatedly to be parsed and planned just once, rather than each time they are executed. The statement must have been prepared previously in the current session. `execPrepared` is supported only in protocol 3.0 and later connections; it will fail when using protocol 2.0.

The parameters are identical to `execParams`, except that the name of a prepared statement is given instead of a query string, and the paramTypes parameter is not present (it is not needed since the prepared statement's parameter types were determined when it was created).

describePrepared

Arguments

:: Connection |
-> ByteString | stmtName
-> IO (Maybe Result) |

Submits a request to obtain information about the specified prepared statement, and waits for completion.

`describePrepared` allows an application to obtain information about a previously prepared statement. `describePrepared` is supported only in protocol 3.0 and later connections; it will fail when using protocol 2.0.

stmtName can be empty to reference the unnamed statement, otherwise it must be the name of an existing prepared statement. On success, a `Result` with status `CommandOk` is returned. The functions `nparams` and `paramtype` can be applied to this `Result` to obtain information about the parameters of the prepared statement, and the functions `nfields`, `fname`, `ftype`, etc provide information about the result columns (if any) of the statement.

describePortal

Arguments

:: Connection |
-> ByteString | portalName
-> IO (Maybe Result) |

Submits a request to obtain information about the specified portal, and waits for completion.

`describePortal` allows an application to obtain information about a previously created portal. (libpq does not provide any direct access to portals, but you can use this function to inspect the properties of a cursor created with a DECLARE CURSOR SQL command.) `describePortal` is supported only in protocol 3.0 and later connections; it will fail when using protocol 2.0.

portalName can be empty to reference the unnamed portal, otherwise it must be the name of an existing portal. On success, a `Result` with status `CommandOk` is returned. The functions `nfields`, `fname`, `ftype`, etc can be applied to the `Result` to obtain information about the result columns (if any) of the portal.

data ExecStatus

Constructors

EmptyQuery | The string sent to the server was empty.
CommandOk | Successful completion of a command returning no data.
TuplesOk | Successful completion of a command returning data (such as a SELECT or SHOW).
CopyOut | Copy Out (from server) data transfer started.
CopyIn | Copy In (to server) data transfer started.
BadResponse | The server's response was not understood.
NonfatalError | A nonfatal error (a notice or warning) occurred.
FatalError | A fatal error occurred.

Instances

Enum ExecStatus |
Eq ExecStatus |
Show ExecStatus |

resultStatus :: Result -> IO ExecStatus

Returns the result status of the command.

resStatus :: ExecStatus -> IO ByteString

Converts the `ExecStatus` returned by `resultStatus` into a string describing the status code. The caller should not free the result.

resultErrorMessage :: Result -> IO (Maybe ByteString)

Returns the error message most recently generated by an operation on the connection.

data FieldCode

Constructors

DiagSeverity | The severity; the field contents are ERROR, FATAL, or PANIC (in an error message), or WARNING, NOTICE, DEBUG, INFO, or LOG (in a notice message), or a localized translation of one of these. Always present.
DiagSqlstate | The SQLSTATE code for the error. The SQLSTATE code identifies the type of error that has occurred; it can be used by front-end applications to perform specific operations (such as error handling) in response to a particular database error. For a list of the possible SQLSTATE codes, see Appendix A. This field is not localizable, and is always present.
DiagMessagePrimary | The primary human-readable error message (typically one line). Always present.
DiagMessageDetail | Detail: an optional secondary error message carrying more detail about the problem. Might run to multiple lines.
DiagMessageHint | Hint: an optional suggestion what to do about the problem. This is intended to differ from detail in that it offers advice (potentially inappropriate) rather than hard facts. Might run to multiple lines.
DiagStatementPosition | A string containing a decimal integer indicating an error cursor position as an index into the original statement string. The first character has index 1, and positions are measured in characters not bytes.
DiagInternalPosition | This is defined the same as the `DiagStatementPosition` field, but it is used when the cursor position refers to an internally generated command rather than the one submitted by the client. The `DiagInternalQuery` field will always appear when this field appears.
DiagInternalQuery | The text of a failed internally-generated command. This could be, for example, a SQL query issued by a PL/pgSQL function.
DiagContext | An indication of the context in which the error occurred. Presently this includes a call stack traceback of active procedural language functions and internally-generated queries. The trace is one entry per line, most recent first.
DiagSourceFile | The file name of the source-code location where the error was reported.
DiagSourceLine | The line number of the source-code location where the error was reported.
DiagSourceFunction | The name of the source-code function reporting the error.

Instances

Enum FieldCode |
Eq FieldCode |
Show FieldCode |

resultErrorField :: Result -> FieldCode -> IO (Maybe ByteString)

Returns an individual field of an error report.

fieldcode is an error field identifier; see the symbols listed below. `Nothing` is returned if the PGresult is not an error or warning result, or does not include the specified field. Field values will normally not include a trailing newline.

The client is responsible for formatting displayed information to meet its needs; in particular it should break long lines as needed. Newline characters appearing in the error message fields should be treated as paragraph breaks, not line breaks.

Errors generated internally by libpq will have severity and primary message, but typically no other fields. Errors returned by a pre-3.0-protocol server will include severity and primary message, and sometimes a detail message, but no other fields.

Note that error fields are only available from `Result` objects, not `Connection` objects; there is no errorField function.

unsafeFreeResult :: Result -> IO ()

Frees the memory associated with a result. Note that using this function correctly is especially tricky; you need to ensure that no references to the result. This means no references to a value returned by `getvalue`, no references hiding inside an unevaluated thunk, etc. Improper use of this function is likely to cause a segfault. Also, the use of this function is not strictly necessary; the memory will get freed by the garbage collector when there are no more references to the result.

# Retrieving Query Result Information

These functions are used to extract information from a `Result` that represents a successful query result (that is, one that has status `TuplesOk`). They can also be used to extract information from a successful Describe operation: a Describe's result has all the same column information that actual execution of the query would provide, but it has zero rows. For objects with other status values, these functions will act as though the result has zero rows and zero columns.

ntuples :: Result -> IO Row

Returns the number of rows (tuples) in the query result. Because it returns an integer result, large result sets might overflow the return value on 32-bit operating systems.

nfields :: Result -> IO Column

Returns the number of columns (fields) in each row of the query result.

newtype Row

Constructors

Row CInt |

Instances

Enum Row |
Eq Row |
Num Row |
Ord Row |
Show Row |

newtype Column

Constructors

Col CInt |

Instances

Enum Column |
Eq Column |
Num Column |
Ord Column |
Show Column |

toRow :: Integral a => a -> Row

toColumn :: Integral a => a -> Column

fname :: Result -> Column -> IO (Maybe ByteString)

Returns the column name associated with the given `Column` number. Column numbers start at 0.

fnumber :: Result -> ByteString -> IO (Maybe Column)

Returns the column number associated with the given column name.

ftable :: Result -> Column -> IO Oid

Returns the OID of the table from which the given column was fetched. Column numbers start at 0.

ftablecol :: Result -> Column -> IO Column

Returns the column number (within its table) of the column making up the specified query result column. Query-result column numbers start at 0, but table columns have nonzero numbers.

fformat :: Result -> Column -> IO Format

Returns the `Format` of the given column. Column numbers start at 0.

ftype :: Result -> Column -> IO Oid

Returns the data type associated with the given column number. The `Oid` returned is the internal OID number of the type. Column numbers start at 0.

You can query the system table pg_type to obtain the names and properties of the various data types. The OIDs of the built-in data types are defined in the file srcincludecatalog/pg_type.h in the source tree.

fmod :: Result -> Column -> IO Int

Returns the type modifier of the column associated with the given column number. Column numbers start at 0.

The interpretation of modifier values is type-specific; they typically indicate precision or size limits. The value -1 is used to indicate "no information available". Most data types do not use modifiers, in which case the value is always -1.

fsize :: Result -> Column -> IO Int

Returns the size in bytes of the column associated with the given column number. Column numbers start at 0.

`fsize` returns the space allocated for this column in a database row, in other words the size of the server's internal representation of the data type. (Accordingly, it is not really very useful to clients.) A negative value indicates the data type is variable-length.

getvalue :: Result -> Row -> Column -> IO (Maybe ByteString)

Returns a single field value of one row of a PGresult. Row and column numbers start at 0.

For convenience, this binding uses `getisnull` and `getlength` to help construct the result.

Note: The `ByteString` returned holds a reference to the Result. As long as ByteString is live, the Result will not be garbage collected. `getvalue'` returns a copy of the data.

getvalue' :: Result -> Row -> Column -> IO (Maybe ByteString)

Returns a copy of a single field value of one row of a PGresult. Row and column numbers start at 0.

For convenience, this binding uses `getisnull` and `getlength` to help construct the result.

getisnull :: Result -> Row -> Column -> IO Bool

Tests a field for a null value. Row and column numbers start at 0.

getlength :: Result -> Row -> Column -> IO Int

Returns the actual length of a field value in bytes. Row and column numbers start at 0.

This is the actual data length for the particular data value, that is, the size of the object pointed to by `getvalue`. For text data format this is the same as strlen(). For binary format this is essential information. Note that one should not rely on `fsize` to obtain the actual data length.

nparams :: Result -> IO Int

Returns the number of parameters of a prepared statement.

This function is only useful when inspecting the result of PQdescribePrepared. For other types of queries it will return zero.

paramtype

Arguments

:: Result |
-> Int | param_number
-> IO Oid |

Returns the data type of the indicated statement parameter. Parameter numbers start at 0.

This function is only useful when inspecting the result of `describePrepared`. For other types of queries it will return zero.

These functions are used to extract other information from PGresult objects.

cmdStatus :: Result -> IO (Maybe ByteString)

Returns the command status tag from the SQL command that generated the PGresult.

Commonly this is just the name of the command, but it might include additional data such as the number of rows processed.

cmdTuples :: Result -> IO (Maybe ByteString)

Returns the number of rows affected by the SQL command.

This function returns a string containing the number of rows affected by the SQL statement that generated the `Result`. This function can only be used following the execution of a SELECT, CREATE TABLE AS, INSERT, UPDATE, DELETE, MOVE, FETCH, or COPY statement, or an EXECUTE of a prepared query that contains an INSERT, UPDATE, or DELETE statement. If the command that generated the `Result` was anything else, `cmdTuples` returns an empty string.

# Escaping Strings for Inclusion in SQL Commands

escapeStringConn :: Connection -> ByteString -> IO (Maybe ByteString)

Escapes a string for use within an SQL command. This is useful when inserting data values as literal constants in SQL commands. Certain characters (such as quotes and backslashes) must be escaped to prevent them from being interpreted specially by the SQL parser.

# Escaping Binary Strings for Inclusion in SQL Commands

escapeByteaConn :: Connection -> ByteString -> IO (Maybe ByteString)

Escapes binary data for use within an SQL command with the type bytea. As with `escapeStringConn`, this is only used when inserting data directly into an SQL command string.

unescapeBytea :: ByteString -> IO (Maybe ByteString)

Converts a `ByteString` representation of binary data into binary data - the reverse of `PQescapeByteaConn`. This is needed when retrieving bytea data in text format, but not when retrieving it in binary format.

The parameter points to a string such as might be returned by `getvalue` when applied to a bytea column. `unescapeBytea` converts this string representation into its binary representation. It returns a `ByteString`, or `Nothing` on error.

This conversion is not exactly the inverse of `escapeByteaConn`, because the string is not expected to be "escaped" when received from `getvalue`. In particular this means there is no need for string quoting considerations, and so no need for a `Connection` parameter.

# Escaping Identifiers for Inclusion in SQL Commands

escapeIdentifier :: Connection -> ByteString -> IO (Maybe ByteString)

`escapeIdentifier` escapes a string for use as an SQL identifier, such as a table, column, or function name. This is useful when a user-supplied identifier might contain special characters that would otherwise not be interpreted as part of the identifier by the SQL parser, or when the identifier might contain upper case characters whose case should be preserved.

The return string has all special characters replaced so that it will be properly processed as an SQL identifier. The return string will also be surrounded by double quotes.

On error, `escapeIdentifier` returns `Nothing` and a suitable message is stored in the conn object.

# Using COPY

This provides support for PostgreSQL's `COPY FROM` facility.

For more information, see:

- http://www.postgresql.org/docs/current/static/sql-copy.html
- http://www.postgresql.org/docs/current/static/libpq-copy.html

data CopyInResult

Constructors

CopyInOk | The data was sent.
CopyInError | An error occurred (use `errorMessage` to retrieve details).
CopyInWouldBlock | The data was not sent because the attempt would block (this case is only possible if the connection is in nonblocking mode) Wait for write-ready (e.g. by using `threadWaitWrite` on the `socket`) and try again.

Instances

Eq CopyInResult |
Show CopyInResult |

putCopyData :: Connection -> ByteString -> IO CopyInResult

Send raw `COPY` data to the server during the `CopyIn` state.

putCopyEnd :: Connection -> Maybe ByteString -> IO CopyInResult

Send end-of-data indication to the server during the `CopyIn` state.

- `putCopyEnd conn Nothing` ends the `CopyIn` operation successfully.
- `putCopyEnd conn (Just errormsg)` forces the `COPY` to fail, with `errormsg` used as the error message.

After `putCopyEnd` returns `CopyOk`, call `getResult` to obtain the final result status of the `COPY` command. Then return to normal operation.

data CopyOutResult

Constructors

CopyOutRow !ByteString | Data representing a single row of the result
CopyOutWouldBlock | A complete row is not yet available. This case is only possible when `getCopyData` is has the async parameter set to `True`.
CopyOutDone | No more rows are available
CopyOutError | An error occurred (e.g. the connection is not in the `CopyOut` state). Call `errorMessage` for more information.

Instances

Show CopyOutResult |

getCopyData :: Connection -> Bool -> IO CopyOutResult

Receive raw `COPY` data from the server during the `CopyOut` state. The boolean parameter determines whether or not the call will block while waiting for data.

# Asynchronous Command Processing

The `exec` function is adequate for submitting commands in normal, synchronous applications. It has a couple of deficiencies, however, that can be of importance to some users:

- `exec` waits for the command to be completed. The application might have other work to do (such as maintaining a user interface), in which case it won't want to block waiting for the response.
- Since the execution of the client application is suspended while it waits for the result, it is hard for the application to decide that it would like to try to cancel the ongoing command. (It can be done from a signal handler, but not otherwise.)
- `exec` can return only one `Result`. If the submitted command string contains multiple SQL commands, all but the last `Result` are discarded by `exec`.

Applications that do not like these limitations can instead use the underlying functions that `exec` is built from: `sendQuery` and `getResult`. There are also `sendQueryParams`, `sendPrepare`, `sendQueryPrepared`, `sendDescribePrepared`, and `sendDescribePortal`, which can be used with `getResult` to duplicate the functionality of `execParams`, `prepare`, `execPrepared`, `describePrepared`, and `describePortal` respectively.

sendQuery :: Connection -> ByteString -> IO Bool

Submits a command to the server without waiting for the result(s). `True` is returned if the command was successfully dispatched and `False` if not (in which case, use `errorMessage` to get more information about the failure).

sendQueryParams :: Connection -> ByteString -> [Maybe (Oid, ByteString, Format)] -> Format -> IO Bool

Submits a command and separate parameters to the server without waiting for the result(s).

sendPrepare :: Connection -> ByteString -> ByteString -> Maybe [Oid] -> IO Bool

Sends a request to create a prepared statement with the given parameters, without waiting for completion.

sendQueryPrepared :: Connection -> ByteString -> [Maybe (ByteString, Format)] -> Format -> IO Bool

Sends a request to execute a prepared statement with given parameters, without waiting for the result(s).

sendDescribePrepared

Arguments

:: Connection |
-> ByteString | stmtName
-> IO Bool |

Submits a request to obtain information about the specified prepared statement, without waiting for completion.

This is an asynchronous version of `describePrepared`: it returns `True` if it was able to dispatch the request, and `False` if not. After a successful call, call `getResult` to obtain the results. The function's parameters are handled identically to `describePrepared`. Like `describePrepared`, it will not work on 2.0-protocol connections.

sendDescribePortal

Arguments

:: Connection |
-> ByteString | portalName
-> IO Bool |

Submits a request to obtain information about the specified portal, without waiting for completion.

This is an asynchronous version of `describePortal`: it returns `True` if it was able to dispatch the request, and `False` if not. After a successful call, call `getResult` to obtain the results. The function's parameters are handled identically to `describePortal`. Like `describePortal`, it will not work on 2.0-protocol connections.

getResult :: Connection -> IO (Maybe Result)

Waits for the next result from a prior `sendQuery`, `sendQueryParams`, `sendPrepare`, or `sendQueryPrepared` call, and returns it. A null pointer is returned when the command is complete and there will be no more results.

consumeInput :: Connection -> IO Bool

If input is available from the server, consume it.

`consumeInput` normally returns `True` indicating "no error", but returns `False` if there was some kind of trouble (in which case `errorMessage` can be consulted). Note that the result does not say whether any input data was actually collected. After calling `consumeInput`, the application can check `isBusy` and/or `notifies` to see if their state has changed.

isBusy :: Connection -> IO Bool

Returns True if a command is busy, that is, getResult would block waiting for input. A False return indicates that getResult can be called with assurance of not blocking.

`isBusy` will not itself attempt to read data from the server; therefore `consumeInput` must be invoked first, or the busy state will never end.

setnonblocking :: Connection -> Bool -> IO Bool

Sets the nonblocking status of the connection.

isnonblocking :: Connection -> IO Bool

Returns the blocking status of the database connection.

data FlushStatus

Constructors

FlushOk |
FlushFailed |
FlushWriting |

Instances

Eq FlushStatus |
Show FlushStatus |

flush :: Connection -> IO FlushStatus

Attempts to flush any queued output data to the server. Returns `FlushOk` if successful (or if the send queue is empty), `FlushFailed` if it failed for some reason, or `FlushWriting` if it was unable to send all the data in the send queue yet (this case can only occur if the connection is nonblocking).

# Cancelling Queries in Progress

A client application can request cancellation of a command that is still being processed by the server, using the functions described in this section.

data Cancel

Contains the information needed to cancel a command issued through a particular database connection.

Instances

Eq Cancel |
Show Cancel |

getCancel :: Connection -> IO (Maybe Cancel)

Creates a data structure containing the information needed to cancel a command issued through a particular database connection.

`getCancel` creates a `Cancel` object given a `Connection`. It will return `Nothing` if the given conn is an invalid connection.

cancel :: Cancel -> IO (Either ByteString ())

Requests that the server abandon processing of the current command.

The return value is 'Right ()' if the cancel request was successfully dispatched and if not, 'Left B.ByteString' containing an error message explaining why not.

Successful dispatch is no guarantee that the request will have any effect, however. If the cancellation is effective, the current command will terminate early and return an error result. If the cancellation fails (say, because the server was already done processing the command), then there will be no visible result at all.

# Asynchronous Notification

PostgreSQL offers asynchronous notification via the LISTEN and NOTIFY commands. A client session registers its interest in a particular notification channel with the LISTEN command (and can stop listening with the UNLISTEN command). All sessions listening on a particular channel will be notified asynchronously when a NOTIFY command with that channel name is executed by any session. A "payload" string can be passed to communicate additional data to the listeners.

libpq applications submit LISTEN, UNLISTEN, and NOTIFY commands as ordinary SQL commands. The arrival of NOTIFY messages can subsequently be detected by calling `notifies`.

data Notify

Constructors

Notify |
Fields notifyRelname :: !ByteString notification channel name notifyBePid :: !CPid process ID of notifying server process notifyExtra :: !ByteString notification payload string

Instances

Show Notify |
Storable Notify |

notifies :: Connection -> IO (Maybe Notify)

Returns the next notification from a list of unhandled notification messages received from the server. It returns a `Nothing` if there are no pending notifications. Once a notification is returned from notifies, it is considered handled and will be removed from the list of notifications.

# Control Functions

These functions control miscellaneous details of libpq's behavior.

clientEncoding :: Connection -> IO ByteString

Returns the client encoding.

setClientEncoding :: Connection -> ByteString -> IO Bool

Sets the client encoding.

data Verbosity

Constructors

ErrorsTerse |
ErrorsDefault |
ErrorsVerbose |

Instances

Enum Verbosity |
Eq Verbosity |
Show Verbosity |

setErrorVerbosity :: Connection -> Verbosity -> IO Verbosity

Determines the verbosity of messages returned by `errorMessage` and `resultErrorMessage`.

`setErrorVerbosity` sets the verbosity mode, returning the connection's previous setting. In `ErrorsTerse` mode, returned messages include severity, primary text, and position only; this will normally fit on a single line. The default mode produces messages that include the above plus any detail, hint, or context fields (these might span multiple lines). The `ErrorsVerbose` mode includes all available fields. Changing the verbosity does not affect the messages available from already-existing `Result` objects, only subsequently-created ones.

# Large Objects

newtype LoFd

LoFd is a Large Object (pseudo) File Descriptor. It is understood by libpq but not by operating system calls.

Constructors

LoFd CInt |

Instances

Eq LoFd |
Ord LoFd |
Show LoFd |

loCreat :: Connection -> IO (Maybe Oid)

Creates a new large object, returns the Object ID of the newly created object.

loCreate :: Connection -> Oid -> IO (Maybe Oid)

Creates a new large object with a particular Object ID. Returns `Nothing` if the requested Object ID is already in use by some other large object or other failure. If `invalidOid` is used as a parameter, then `loCreate` will assign an unused `Oid`.

loImport :: Connection -> FilePath -> IO (Maybe Oid)

Imports an operating system file as a large object. Note that the file is read by the client interface library, not by the server; so it must exist in the client file system and be readable by the client application.

loImportWithOid :: Connection -> FilePath -> Oid -> IO (Maybe Oid)

Imports an operating system file as a large object with the given Object ID. Combines the behavior of `loImport` and `loCreate`

loExport :: Connection -> Oid -> FilePath -> IO (Maybe ())

Exports a large object into a operating system file. Note that the file is written by the client interface library, not the server. Returns 'Just ()' on success, `Nothing` on failure.

loOpen :: Connection -> Oid -> IOMode -> IO (Maybe LoFd)

Opens an existing large object for reading or writing. The Oid specifies the large object to open. A large object cannot be opened before it is created. A large object descriptor is returned for later use in `loRead`, `loWrite`, `loSeek`, `loTell`, and `loClose`. The descriptor is only valid for the duration of the current transation. On failure, `Nothing` is returned.

The server currently does not distinguish between `WriteMode` and `ReadWriteMode`; write-only modes are not enforced. However there is a significant difference between `ReadMode` and the other modes: with `ReadMode` you cannot write on the descriptor, and the data read from it will reflect the contents of the large object at the time of the transaction snapshot that was active when `loOpen` was executed, regardless of later writes by this or other transactions. Reading from a descriptor opened in `WriteMode`, `ReadWriteMode`, or `AppendMode` returns data that reflects all writes of other committed transactions as well as the writes of the current transaction. This is similar to the behavior of `REPEATABLE READ` versus `READ COMMITTED` transaction modes for ordinary SQL `SELECT` commands.

loWrite :: Connection -> LoFd -> ByteString -> IO (Maybe Int)

`loWrite conn fd buf` writes the bytestring `buf` to the large object descriptor `fd`. The number of bytes actually written is returned. In the event of an error, `Nothing` is returned.

loRead :: Connection -> LoFd -> Int -> IO (Maybe ByteString)

`loRead conn fd len` reads up to `len` bytes from the large object descriptor `fd`. In the event of an error, `Nothing` is returned.

loSeek :: Connection -> LoFd -> SeekMode -> Int -> IO (Maybe Int)

Changes the current read or write location associated with a large object descriptor. The return value is the new location pointer, or `Nothing` on error.

loTell :: Connection -> LoFd -> IO (Maybe Int)

Obtains the current read or write location of a large object descriptor.

loTruncate :: Connection -> LoFd -> Int -> IO (Maybe ())

Truncates a large object to a given length. If the length is greater than the current large object, then the large object is extended with null bytes. ('\x00')

The file offest is not changed.

`loTruncate` is new as of PostgreSQL 8.3; if this function is run against an older server version, it will fail and return `Nothing`

loClose :: Connection -> LoFd -> IO (Maybe ())

Closes a large object descriptor. Any large object descriptors that remain open at the end of a transaction will be closed automatically.

loUnlink :: Connection -> Oid -> IO (Maybe ())

Removes a large object from the database.