Database.PostgreSQL.LibPQ

Connection encapsulates a connection to the backend.

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 \.

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

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.

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

Test if a connection is the Null Connection.

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.

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

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.

Closes the connection to the server.

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

Returns the database name of the connection.

Returns the user name of the connection.

Returns the password of the connection.

Returns the server host name of the connection.

Returns the port of the connection.

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

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.

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.

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.

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.)

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.

Return the version of libpq that is being used.

The result of this function can be used to determine, at run time, if specific functionality is available in the currently loaded version of libpq. The function can be used, for example, to determine which connection options are available for PQconnectdb or if the hex bytea output added in PostgreSQL 9.0 is supported.

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.

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.)

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!

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.

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.

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.

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.

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.

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.

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).

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.

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.

Returns the result status of the command.

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

Returns the error message associated with the command, or an empty string if there was no error.

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.

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.

Returns the number of rows (tuples) in the query result. (Note that PGresult objects are limited to no more than INT_MAX rows, so an int result is sufficient.)

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

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

Returns the column number associated with the given column name.

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

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.

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

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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).

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

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

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

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.

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.

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.

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.

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.

Sets the nonblocking status of the connection.

Returns the blocking status of the database connection.

Select single-row mode for the currently-executing query.

This function can only be called immediately after PQsendQuery or one of its sibling functions, before any other operation on the connection such as PQconsumeInput or PQgetResult. If called at the correct time, the function activates single-row mode for the current query and returns 1. Otherwise the mode stays unchanged and the function returns 0. In any case, the mode reverts to normal after completion of the current query.

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).

Returns the current pipeline mode status of the libpq connection.

Causes a connection to enter pipeline mode if it is currently idle or already in pipeline mode.

Causes a connection to exit pipeline mode if it is currently in pipeline mode with an empty queue and no pending results.

Marks a synchronization point in a pipeline by sending a sync message and flushing the send buffer. This serves as the delimiter of an implicit transaction and an error recovery point>

Sends a request for the server to flush its output buffer.

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

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.

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.

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.

Returns the client encoding.

Sets the client encoding.

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.

Upon connection initialization, any notices received from the server are normally written to the console. Notices are akin to warnings, and are distinct from notifications. This function suppresses notices. You may later call enableNoticeReporting after calling this function.

Upon connection initialization, any notices received from the server are normally written to the console. Notices are akin to warnings, and are distinct from notifications. This function enables notices to be programmatically retreived using the getNotice function. You may later call disableNoticeReporting after calling this function.

This function retrieves any notices received from the backend. Because multiple notices can be received at a time, you will typically want to call this function in a loop until you get back a Nothing.

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

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

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.

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.

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

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.

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

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.

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

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

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

Removes a large object from the database.