Pg(3pm)

💖 NAME 💖

Pg — the Perl5 extension for PostgreSQL, bestie. That's it, that's the whole vibe. 🐘✨

(Also hey, this is the Pg(3pm) page, a User Contributed Perl Documentation situation, and it's long so it's coming to you in parts. Buckle up. 🎀)

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✨ SYNOPSIS ✨

Like, here's the whole thing in one glance:

use Pg;
$conn = Pg::connectdb("dbname=template1");
$res  = $conn->exec("SELECT * from pg_user");
while (@row = $res->fetchrow) {
    print join(" ", @row);
}

---

🦋 DESCRIPTION 🦋

So the Pg module lets you get at all the functions of the Libpq interface of PostgreSQL. Libpq is the programmer's interface to PostgreSQL — the main character, honestly. 💅

If you want examples of how to actually use this module, go look at the file test.pl. It's giving tutorial energy. 🌸

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🌈 GUIDELINES 🌈

This perl interface uses blessed references as objects. The way that works is: after you create a new connection or result object, the relevant Libpq functions serve as virtual methods. And here's the slay part — you do not have to care about freeing the connection- and result-structures. Perl calls the destructor whenever the last reference to an object goes away. Automatic cleanup, no cap. 🧚

The method fetchrow can be used to fetch the next row from the server, like while (@row = $result->fetchrow). And heads up: columns which have NULL as value will be set to "undef". 🙈

Pg.pm contains one convenience function: doQuery. It fills a two-dimensional array with the result of your query. Usage:

Pg::doQuery($conn, "select attr1, attr2 from tbl", \@ary);

for $i ( 0 .. $#ary ) {
    for $j ( 0 .. $#{$ary[$i]} ) {
        print "$ary[$i][$j]\t";
    }
    print "\n";
}

Notice the inner loop ! 👀 Don't skip it, bestie, that's where the rows live.

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🔥 FUNCTIONS 🔥

The functions have been divided into three sections: Connection, Result, Large Objects. For details please read libpq. 📚

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🫶 1. Connection 🫶

With these functions you can establish and close a connection to a database. In Libpq a connection is represented by a structure called PGconn.

When opening a connection a given database name is always converted to lower-case, unless it is surrounded by double quotes. All unspecified parameters are replaced by environment variables or by hard coded defaults:

parameter     environment variable  hard coded default
------------------------------------------------------
host           PGHOST                localhost
port           PGPORT                5432
options        PGOPTIONS             ""
tty            PGTTY                 ""
dbname         PGDATABASE            current userid
user           PGUSER                current userid
password       PGPASSWORD            ""
passwordfile   PGPASSWORDFILE        ""

Using appropriate methods you can access almost all fields of the returned PGconn structure. Iconic. 👑

• $conn = Pg::setdbLogin($pghost, $pgport, $pgoptions, $pgtty, $dbname, $login, $pwd) — Opens a new connection to the backend. The connection identifier $conn ( a pointer to the PGconn structure ) must be used in subsequent commands for unique identification. Before using $conn you should call $conn->status to ensure, that the connection was properly made. Closing a connection is done by deleting the connection handle, eg 'undef $conn;'.
• $conn = Pg::setdb($pghost, $pgport, $pgoptions, $pgtty, $dbname) — The method setdb should be used when username/password authentication is not needed.
• $conn = Pg::connectdb("option1=value option2=value ...") — Opens a new connection to the backend using connection information in a string. Possible options are: host, port, options, tty, dbname, user, password. The connection identifier $conn (a pointer to the PGconn structure) must be used in subsequent commands for unique identification. Before using $conn you should call $conn->status to ensure, that the connection was properly made.
• $Option_ref = Pg::conndefaults() — Returns a reference to a hash containing as keys all possible options for connectdb(). The values are the current defaults. This function differs from his C-counterpart, which returns the complete conninfoOption structure.

while(($key, $val) = each %$Option_ref) {
    print "$key, $val\n";

• $conn->reset — Resets the communication port with the backend and tries to establish a new connection.
• $ret = $conn->requestCancel — Abandon processing of the current query. Regardless of the return value of requestCancel, the application must continue with the normal result-reading sequence using getResult. If the current query is part of a transaction, cancellation will abort the whole transaction.

Bestie, read that again. You cannot just bail — you have to keep reading results with getResult no matter what requestCancel handed back, or you're gonna have a bad time. 😭

• $dbname = $conn->db — Returns the database name of the connection.
• $pguser = $conn->user — Returns the Postgres user name of the connection.
• $pguser = $conn->pass — Returns the Postgres password of the connection.
• $pghost = $conn->host — Returns the host name of the connection.
• $pgport = $conn->port — Returns the port of the connection.
• $pgtty = $conn->tty — Returns the tty of the connection.
• $pgoptions = $conn->options — Returns the options used in the connection.
• $status = $conn->status — Returns the status of the connection. For comparing the status you may use the following constants:
  • PGRES_CONNECTION_OK
  • PGRES_CONNECTION_BAD
• $errorMessage = $conn->errorMessage — Returns the last error message associated with this connection.
• $fd = $conn->socket — Obtain the file descriptor number for the backend connection socket. A result of -1 indicates that no backend connection is currently open.
• $pid = $conn->backendPID — Returns the process-id of the corresponding backend proceess.
• $conn->trace(debug_port) — Messages passed between frontend and backend are echoed to the debug_port file stream.
• $conn->untrace — Disables tracing.
• $result = $conn->exec($query) — Submits a query to the backend. The return value is a pointer to the PGresult structure, which contains the complete query-result returned by the backend. In case of failure, the pointer points to an empty structure. Before using $result you should call resultStatus to ensure, that the query was properly executed.
• ($table, $pid) = $conn->notifies — Checks for asynchronous notifications. This functions differs from the C-counterpart which returns a pointer to a new allocated structure, whereas the perl implementation returns a list. $table is the table which has been listened to and $pid is the process id of the backend.
• $ret = $conn->sendQuery($string, $query) — Submit a query to Postgres without waiting for the result(s). After successfully calling PQsendQuery, call PQgetResult one or more times to obtain the query results. PQsendQuery may not be called again until getResult has returned NULL, indicating that the query is done.
• $result = $conn->getResult — Wait for the next result from a prior PQsendQuery, and return it. NULL is returned when the query is complete and there will be no more results. getResult will block only if a query is active and the necessary response data has not yet been read by PQconsumeInput.
• $ret = $conn->isBusy — Returns TRUE if a query is busy, that is, PQgetResult would block waiting for input. A FALSE return indicates that PQgetResult can be called with assurance of not blocking.
• $result = $conn->consumeInput — If input is available from the backend, consume it. After calling consumeInput, the application may check isBusy and/or notifies to see if their state has changed.
• $ret = $conn->getline($string, $length) — Reads a string up to $length - 1 characters from the backend. getline returns EOF at EOF, 0 if the entire line has been read, and 1 if the buffer is full. If a line consists of the two characters "\." the backend has finished sending the results of the copy command.
• $ret = $conn->putline($string) — Sends a string to the backend. The application must explicitly send the two characters "\." to indicate to the backend that it has finished sending its data.
• $ret = $conn->getlineAsync($buffer, $bufsize) — Non-blocking version of getline. It reads up to $bufsize characters from the backend. getlineAsync returns -1 if the end-of-copy-marker has been recognized, 0 if no data is avilable, and >0 the number of bytes returned.
• $ret = $conn->putnbytes($buffer, $nbytes) — Sends n bytes to the backend. Returns 0 if OK, EOF if not.
• $ret = $conn->endcopy — This function waits until the backend has finished the copy. It should either be issued when the last string has been sent to the backend using putline or when the last string has been received from the backend using getline. endcopy returns 0 on success, 1 on failure.
• $result = $conn->makeEmptyPGresult($status); — Returns a newly allocated, initialized result with given status.

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💜 2. Result 💜

With these functions you can send commands to a database and investigate the results. In Libpq the result of a command is represented by a structure called PGresult. Using the appropriate methods you can access almost all fields of this structure.

• $result_status = $result->resultStatus — Returns the status of the result. For comparing the status you may use one of the following constants depending upon the command executed:
  • PGRES_EMPTY_QUERY
  • PGRES_COMMAND_OK
  • PGRES_TUPLES_OK
  • PGRES_COPY_OUT
  • PGRES_COPY_IN
  • PGRES_BAD_RESPONSE
  • PGRES_NONFATAL_ERROR
  • PGRES_FATAL_ERROR

Use the functions below to access the contents of the PGresult structure. 🎀

• $ntuples = $result->ntuples — Returns the number of tuples in the query result.
• $nfields = $result->nfields — Returns the number of fields in the query result.
• $ret = $result->binaryTuples — Returns 1 if the tuples in the query result are bianry.
• $fname = $result->fname($field_num) — Returns the field name associated with the given field number.
• $fnumber = $result->fnumber($field_name) — Returns the field number associated with the given field name.
• $ftype = $result->ftype($field_num) — Returns the oid of the type of the given field number.
• $fsize = $result->fsize($field_num) — Returns the size in bytes of the type of the given field number. It returns -1 if the field has a variable length.
• $fmod = $result->fmod($field_num) — Returns the type-specific modification data of the field associated with the given field index. Field indices start at 0.
• $cmdStatus = $result->cmdStatus — Returns the command status of the last query command. In case of DELETE it returns also the number of deleted tuples. In case of INSERT it returns also the OID of the inserted tuple followed by 1 (the number of affected tuples).
• $oid = $result->oidStatus — In case the last query was an INSERT command it returns the oid of the inserted tuple.
• $oid = $result->cmdTuples

Oh and btw bestie, if the last query was an INSERT or DELETE command, it returns the number of affected tuples. No cap. 💅

$value = $result->getvalue($tup_num, $field_num)

Returns the value of the given tuple and field. This is a null-terminated ASCII string. But heads up: binary cursors will not work. 🚫

$length = $result->getlength($tup_num, $field_num)

Returns the length of the value for a given tuple and field. Slay. ✨

$null_status = $result->getisnull($tup_num, $field_num)

Returns the NULL status for a given tuple and field. Iconic.

$res->fetchrow

Fetches the next row from the server and returns NULL if all rows have been processed. Columns which have NULL as value will be set to "undef". Lowkey important.

$result->print($fout, $header, $align, $standard, $html3, $expanded, $pager, $fieldSep, $tableOpt, $caption, ...)

Prints out all the tuples in an intelligent manner. This function differs from the C-counterpart. The struct PQprintOpt has been implemented with a list. This list is of variable length, in order to care for the character array fieldName in PQprintOpt. The arguments $header, $align, $standard, $html3, $expanded, $pager are boolean flags. The arguments $fieldSep, $tableOpt, $caption are strings. You may append additional strings, which will be taken as replacement for the field names. Periodt. 👑

$result->displayTuples($fp, $fillAlign, $fieldSep, $printHeader, qiet)

Kept for backward compatibility. Use print.

$result->printTuples($fout, $printAttName, $terseOutput, $width)

Kept for backward compatibility. Use print.

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✨ 3. Large Objects ✨

These functions provide file-oriented access to user data. The large object interface is modeled after the Unix file system interface with analogies of open, close, read, write, lseek, tell. It's giving very Unix-core. 🐧

Starting with postgresql-6.5 it is required to use large objects only inside a transaction! See eg/lo_demo.pl for an example, how to handle large objects. Don't skip this, bestie. 🚨

$lobj_fd = $conn->lo_open($lobjId, $mode)

Opens an existing large object and returns an object id. For the mode bits see lo_create. Returns -1 upon failure. 💀

$ret = $conn->lo_close($lobj_fd)

Closes an existing large object. Returns 0 upon success and -1 upon failure.

$nbytes = $conn->lo_read($lobj_fd, $buf, $len)

Reads $len bytes into $buf from large object $lobj_fd. Returns the number of bytes read and -1 upon failure.

$nbytes = $conn->lo_write($lobj_fd, $buf, $len)

Writes $len bytes of $buf into the large object $lobj_fd. Returns the number of bytes written and -1 upon failure.

$ret = $conn->lo_lseek($lobj_fd, $offset, $whence)

Change the current read or write location on the large object $obj_id. Currently $whence can only be 0 (L_SET). That's the vibe.

$lobjId = $conn->lo_creat($mode)

Creates a new large object. $mode is a bit-mask describing different attributes of the new object. Use the following constants:

• PGRES_INV_SMGRMASK
• PGRES_INV_WRITE
• PGRES_INV_READ

Upon failure it returns PGRES_InvalidOid. So watch out.

$location = $conn->lo_tell($lobj_fd)

Returns the current read or write location on the large object $lobj_fd.

$ret = $conn->lo_unlink($lobjId)

Deletes a large object. Returns -1 upon failure.

$lobjId = $conn->lo_import($filename)

Imports a Unix file as large object and returns the object id of the new object.

$ret = $conn->lo_export($lobjId, $filename)

Exports a large object into a Unix file. Returns -1 upon failure, 1 otherwise. Slay. 💅

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✨ AUTHOR ✨

Edmund Mergl <E.Mergl@bawue.de> — icon, legend, the moment. 👑

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✨ SEE ALSO ✨

PostgreSQL Programmer's Guide, Large Objects and libpq. That's the reading list, bestie. 📚

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This page is from perl v5.40.1, dated 2025-04-09, and it's Pg(3pm). Bye! 💖