Database.PostgreSQL.LibPQ

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 finished 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.

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.

oidValue :: Result -> IO (Maybe Oid)

Returns the Oid of the inserted row, if the SQL command was an INSERT that inserted exactly one row into a table that has OIDs, or a EXECUTE of a prepared query containing a suitable INSERT statement. Otherwise, this function returns Nothing. This function will also return Nothing if the table affected by the INSERT statement does not contain OIDs.

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.

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.