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
PGconn object, which
is obtained from the function PQconnectdb,
PQconnectdbParams, or
PQsetdbLogin. Note that these functions will always
return a non-null object pointer, unless perhaps there is too
little memory even to allocate the PGconn object.
The PQstatus function should be called to check
the return value for a successful connection before queries are sent
via the connection object.
If untrusted users have access to a database that has not adopted a
secure schema usage pattern,
begin each session by removing publicly-writable schemas from
search_path. One can set parameter key
word options to
value -csearch_path=. Alternately, one can
issue PQexec( after
connecting. This consideration is not specific
to libpq; it applies to every interface for
executing arbitrary SQL commands.
conn, "SELECT
pg_catalog.set_config('search_path', '', false)")
On Unix, forking a process with open libpq connections can lead to
unpredictable results because the parent and child processes share
the same sockets and operating system resources. For this reason,
such usage is not recommended, though doing an exec from
the child process to load a new executable is safe.
PQconnectdbParams #Makes a new connection to the database server.
PGconn *PQconnectdbParams(const char * const *keywords,
const char * const *values,
int expand_dbname);
This function opens a new database connection using the parameters taken
from two NULL-terminated arrays. The first,
keywords, is defined as an array of strings, each one
being a key word. The second, values, gives the value
for each key word. Unlike PQsetdbLogin below, the parameter
set can be extended without changing the function signature, so use of
this function (or its nonblocking analogs PQconnectStartParams
and PQconnectPoll) is preferred for new application
programming.
The currently recognized parameter key words are listed in Section 34.1.2.
The passed arrays can be empty to use all default parameters, or can
contain one or more parameter settings. They must be matched in length.
Processing will stop at the first NULL entry
in the keywords array.
Also, if the values entry associated with a
non-NULL keywords entry is
NULL or an empty string, that entry is ignored and
processing continues with the next pair of array entries.
When expand_dbname is non-zero, the value for
the first dbname key word is checked to see
if it is a connection string. If so, it
is “expanded” into the individual connection
parameters extracted from the string. The value is considered to
be a connection string, rather than just a database name, if it
contains an equal sign (=) or it begins with a
URI scheme designator. (More details on connection string formats
appear in Section 34.1.1.) Only the first
occurrence of dbname is treated in this way;
any subsequent dbname parameter is processed
as a plain database name.
In general the parameter arrays are processed from start to end.
If any key word is repeated, the last value (that is
not NULL or empty) is used. This rule applies in
particular when a key word found in a connection string conflicts
with one appearing in the keywords array. Thus,
the programmer may determine whether array entries can override or
be overridden by values taken from a connection string. Array
entries appearing before an expanded dbname
entry can be overridden by fields of the connection string, and in
turn those fields are overridden by array entries appearing
after dbname (but, again, only if those
entries supply non-empty values).
After processing all the array entries and any expanded connection string, any connection parameters that remain unset are filled with default values. If an unset parameter's corresponding environment variable (see Section 34.15) is set, its value is used. If the environment variable is not set either, then the parameter's built-in default value is used.
PQconnectdb #Makes a new connection to the database server.
PGconn *PQconnectdb(const char *conninfo);
This function opens a new database connection using the parameters taken
from the string conninfo.
The passed string can be empty to use all default parameters, or it can contain one or more parameter settings separated by whitespace, or it can contain a URI. See Section 34.1.1 for details.
PQsetdbLogin #Makes a new connection to the database server.
PGconn *PQsetdbLogin(const char *pghost,
const char *pgport,
const char *pgoptions,
const char *pgtty,
const char *dbName,
const char *login,
const char *pwd);
This is the predecessor of PQconnectdb with a fixed
set of parameters. It has the same functionality except that the
missing parameters will always take on default values. Write NULL or an
empty string for any one of the fixed parameters that is to be defaulted.
If the dbName contains
an = sign or has a valid connection URI prefix, it
is taken as a conninfo string in exactly the same way as
if it had been passed to PQconnectdb, and the remaining
parameters are then applied as specified for PQconnectdbParams.
pgtty is no longer used and any value passed will
be ignored.
PQsetdb #Makes a new connection to the database server.
PGconn *PQsetdb(char *pghost,
char *pgport,
char *pgoptions,
char *pgtty,
char *dbName);
This is a macro that calls PQsetdbLogin with null pointers
for the login and pwd parameters. It is provided
for backward compatibility with very old programs.
PQconnectStartParamsPQconnectStartPQconnectPoll #Make a connection to the database server in a nonblocking manner.
PGconn *PQconnectStartParams(const char * const *keywords,
const char * const *values,
int expand_dbname);
PGconn *PQconnectStart(const char *conninfo);
PostgresPollingStatusType PQconnectPoll(PGconn *conn);
These three functions are used to open a connection to a database server such
that your application's thread of execution is not blocked on remote I/O
whilst doing so. The point of this approach is that the waits for I/O to
complete can occur in the application's main loop, rather than down inside
PQconnectdbParams or PQconnectdb, and so the
application can manage this operation in parallel with other activities.
With PQconnectStartParams, the database connection is made
using the parameters taken from the keywords and
values arrays, and controlled by expand_dbname,
as described above for PQconnectdbParams.
With PQconnectStart, the database connection is made
using the parameters taken from the string conninfo as
described above for PQconnectdb.
Neither PQconnectStartParams nor PQconnectStart
nor PQconnectPoll will block, so long as a number of
restrictions are met:
The hostaddr parameter must be used appropriately
to prevent DNS queries from being made. See the documentation of
this parameter in Section 34.1.2 for details.
If you call PQtrace, ensure that the stream object
into which you trace will not block.
You must ensure that the socket is in the appropriate state
before calling PQconnectPoll, as described below.
To begin a nonblocking connection request,
call PQconnectStart
or PQconnectStartParams. If the result is null,
then libpq has been unable to allocate a
new PGconn structure. Otherwise, a
valid PGconn pointer is returned (though not
yet representing a valid connection to the database). Next
call PQstatus(conn). If the result
is CONNECTION_BAD, the connection attempt has already
failed, typically because of invalid connection parameters.
If PQconnectStart
or PQconnectStartParams succeeds, the next stage
is to poll libpq so that it can proceed with
the connection sequence.
Use PQsocket(conn) to obtain the descriptor of the
socket underlying the database connection.
(Caution: do not assume that the socket remains the same
across PQconnectPoll calls.)
Loop thus: If PQconnectPoll(conn) last returned
PGRES_POLLING_READING, wait until the socket is ready to
read (as indicated by select(), poll(), or
similar system function).
Then call PQconnectPoll(conn) again.
Conversely, if PQconnectPoll(conn) last returned
PGRES_POLLING_WRITING, wait until the socket is ready
to write, then call PQconnectPoll(conn) again.
On the first iteration, i.e., if you have yet to call
PQconnectPoll, behave as if it last returned
PGRES_POLLING_WRITING. Continue this loop until
PQconnectPoll(conn) returns
PGRES_POLLING_FAILED, indicating the connection procedure
has failed, or PGRES_POLLING_OK, indicating the connection
has been successfully made.
At any time during connection, the status of the connection can be
checked by calling PQstatus. If this call returns CONNECTION_BAD, then the
connection procedure has failed; if the call returns CONNECTION_OK, then the
connection is ready. Both of these states are equally detectable
from the return value of PQconnectPoll, described above. Other states might also occur
during (and only during) an asynchronous connection procedure. These
indicate the current stage of the connection procedure and might be useful
to provide feedback to the user for example. These statuses are:
CONNECTION_STARTED #Waiting for connection to be made.
CONNECTION_MADE #Connection OK; waiting to send.
CONNECTION_AWAITING_RESPONSE #Waiting for a response from the server.
CONNECTION_AUTH_OK #Received authentication; waiting for backend start-up to finish.
CONNECTION_SSL_STARTUP #Negotiating SSL encryption.
CONNECTION_SETENV #Negotiating environment-driven parameter settings.
CONNECTION_CHECK_WRITABLE #Checking if connection is able to handle write transactions.
CONNECTION_CONSUME #Consuming any remaining response messages on connection.
Note that, although these constants will remain (in order to maintain compatibility), an application should never rely upon these occurring in a particular order, or at all, or on the status always being one of these documented values. An application might do something like this:
switch(PQstatus(conn))
{
case CONNECTION_STARTED:
feedback = "Connecting...";
break;
case CONNECTION_MADE:
feedback = "Connected to server...";
break;
.
.
.
default:
feedback = "Connecting...";
}
The connect_timeout connection parameter is ignored
when using PQconnectPoll; it is the application's
responsibility to decide whether an excessive amount of time has elapsed.
Otherwise, PQconnectStart followed by a
PQconnectPoll loop is equivalent to
PQconnectdb.
Note that when PQconnectStart
or PQconnectStartParams returns a non-null
pointer, you must call PQfinish when you are
finished with it, in order to dispose of the structure and any
associated memory blocks. This must be done even if the connection
attempt fails or is abandoned.
PQconndefaults #Returns the default connection options.
PQconninfoOption *PQconndefaults(void);
typedef struct
{
char *keyword; /* The keyword of the option */
char *envvar; /* Fallback environment variable name */
char *compiled; /* Fallback compiled in default value */
char *val; /* Option's current value, or NULL */
char *label; /* Label for field in connect dialog */
char *dispchar; /* Indicates how to display this field
in a connect dialog. Values are:
"" Display entered value as is
"*" Password field - hide value
"D" Debug option - don't show by default */
int dispsize; /* Field size in characters for dialog */
} PQconninfoOption;
Returns a connection options array. This can be used to determine
all possible PQconnectdb options and their
current default values. The return value points to an array of
PQconninfoOption structures, which ends
with an entry having a null keyword pointer. The
null pointer is returned if memory could not be allocated. Note that
the current default values (val fields)
will depend on environment variables and other context. A
missing or invalid service file will be silently ignored. Callers
must treat the connection options data as read-only.
After processing the options array, free it by passing it to
PQconninfoFree. If this is not done, a small amount of memory
is leaked for each call to PQconndefaults.
PQconninfo #Returns the connection options used by a live connection.
PQconninfoOption *PQconninfo(PGconn *conn);
Returns a connection options array. This can be used to determine
all possible PQconnectdb options and the
values that were used to connect to the server. The return
value points to an array of PQconninfoOption
structures, which ends with an entry having a null keyword
pointer. All notes above for PQconndefaults also
apply to the result of PQconninfo.
PQconninfoParse #Returns parsed connection options from the provided connection string.
PQconninfoOption *PQconninfoParse(const char *conninfo, char **errmsg);
Parses a connection string and returns the resulting options as an
array; or returns NULL if there is a problem with the connection
string. This function can be used to extract
the PQconnectdb options in the provided
connection string. The return value points to an array of
PQconninfoOption structures, which ends
with an entry having a null keyword pointer.
All legal options will be present in the result array, but the
PQconninfoOption for any option not present
in the connection string will have val set to
NULL; default values are not inserted.
If errmsg is not NULL, then *errmsg is set
to NULL on success, else to a malloc'd error string explaining
the problem. (It is also possible for *errmsg to be
set to NULL and the function to return NULL;
this indicates an out-of-memory condition.)
After processing the options array, free it by passing it to
PQconninfoFree. If this is not done, some memory
is leaked for each call to PQconninfoParse.
Conversely, if an error occurs and errmsg is not NULL,
be sure to free the error string using PQfreemem.
PQfinish #
Closes the connection to the server. Also frees
memory used by the PGconn object.
void PQfinish(PGconn *conn);
Note that even if the server connection attempt fails (as
indicated by PQstatus), the application should call PQfinish
to free the memory used by the PGconn object.
The PGconn pointer must not be used again after
PQfinish has been called.
PQreset #Resets the communication channel to the server.
void PQreset(PGconn *conn);
This function will close the connection to the server and attempt to establish a new connection, using all the same parameters previously used. This might be useful for error recovery if a working connection is lost.
PQresetStartPQresetPoll #Reset the communication channel to the server, in a nonblocking manner.
int PQresetStart(PGconn *conn); PostgresPollingStatusType PQresetPoll(PGconn *conn);
These functions will close the connection to the server and attempt to
establish a new connection, using all the same
parameters previously used. This can be useful for error recovery if a
working connection is lost. They differ from PQreset (above) in that they
act in a nonblocking manner. These functions suffer from the same
restrictions as PQconnectStartParams, PQconnectStart
and PQconnectPoll.
To initiate a connection reset, call
PQresetStart. If it returns 0, the reset has
failed. If it returns 1, poll the reset using
PQresetPoll in exactly the same way as you
would create the connection using PQconnectPoll.
PQpingParams #
PQpingParams reports the status of the
server. It accepts connection parameters identical to those of
PQconnectdbParams, described above. It is not
necessary to supply correct user name, password, or database name
values to obtain the server status; however, if incorrect values
are provided, the server will log a failed connection attempt.
PGPing PQpingParams(const char * const *keywords,
const char * const *values,
int expand_dbname);
The function returns one of the following values:
PQPING_OK #The server is running and appears to be accepting connections.
PQPING_REJECT #The server is running but is in a state that disallows connections (startup, shutdown, or crash recovery).
PQPING_NO_RESPONSE #The server could not be contacted. This might indicate that the server is not running, or that there is something wrong with the given connection parameters (for example, wrong port number), or that there is a network connectivity problem (for example, a firewall blocking the connection request).
PQPING_NO_ATTEMPT #No attempt was made to contact the server, because the supplied parameters were obviously incorrect or there was some client-side problem (for example, out of memory).
PQping #
PQping reports the status of the
server. It accepts connection parameters identical to those of
PQconnectdb, described above. It is not
necessary to supply correct user name, password, or database name
values to obtain the server status; however, if incorrect values
are provided, the server will log a failed connection attempt.
PGPing PQping(const char *conninfo);
The return values are the same as for PQpingParams.
PQsetSSLKeyPassHook_OpenSSL #
PQsetSSLKeyPassHook_OpenSSL lets an application override
libpq's default
handling of encrypted client certificate key files using
sslpassword or interactive prompting.
void PQsetSSLKeyPassHook_OpenSSL(PQsslKeyPassHook_OpenSSL_type hook);
The application passes a pointer to a callback function with signature:
int callback_fn(char *buf, int size, PGconn *conn);
which libpq will then call
instead of its default
PQdefaultSSLKeyPassHook_OpenSSL handler. The
callback should determine the password for the key and copy it to
result-buffer buf of size
size. The string in buf
must be null-terminated. The callback must return the length of the
password stored in buf excluding the null
terminator. On failure, the callback should set
buf[0] = '\0' and return 0. See
PQdefaultSSLKeyPassHook_OpenSSL in
libpq's source code for an example.
If the user specified an explicit key location,
its path will be in conn->sslkey when the callback
is invoked. This will be empty if the default key path is being used.
For keys that are engine specifiers, it is up to engine implementations
whether they use the OpenSSL password
callback or define their own handling.
The app callback may choose to delegate unhandled cases to
PQdefaultSSLKeyPassHook_OpenSSL,
or call it first and try something else if it returns 0, or completely override it.
The callback must not escape normal flow control with exceptions,
longjmp(...), etc. It must return normally.
PQgetSSLKeyPassHook_OpenSSL #
PQgetSSLKeyPassHook_OpenSSL returns the current
client certificate key password hook, or NULL
if none has been set.
PQsslKeyPassHook_OpenSSL_type PQgetSSLKeyPassHook_OpenSSL(void);
Several libpq functions parse a user-specified string to obtain connection parameters. There are two accepted formats for these strings: plain keyword/value strings and URIs. URIs generally follow RFC 3986, except that multi-host connection strings are allowed as further described below.
In the keyword/value format, each parameter setting is in the form
keyword =
value, with space(s) between settings.
Spaces around a setting's equal sign are
optional. To write an empty value, or a value containing spaces, surround it
with single quotes, for example keyword = 'a value'.
Single quotes and backslashes within
a value must be escaped with a backslash, i.e., \' and
\\.
Example:
host=localhost port=5432 dbname=mydb connect_timeout=10
The recognized parameter key words are listed in Section 34.1.2.
The general form for a connection URI is:
postgresql://[userspec@][hostspec][/dbname][?paramspec] whereuserspecis:user[:password] andhostspecis: [host][:port][,...] andparamspecis:name=value[&...]
The URI scheme designator can be either
postgresql:// or postgres://. Each
of the remaining URI parts is optional. The
following examples illustrate valid URI syntax:
postgresql:// postgresql://localhost postgresql://localhost:5433 postgresql://localhost/mydb postgresql://user@localhost postgresql://user:secret@localhost postgresql://other@localhost/otherdb?connect_timeout=10&application_name=myapp postgresql://host1:123,host2:456/somedb?target_session_attrs=any&application_name=myapp
Values that would normally appear in the hierarchical part of the URI can alternatively be given as named parameters. For example:
postgresql:///mydb?host=localhost&port=5433
All named parameters must match key words listed in
Section 34.1.2, except that for compatibility
with JDBC connection URIs, instances
of ssl=true are translated into
sslmode=require.
The connection URI needs to be encoded with percent-encoding
if it includes symbols with special meaning in any of its parts. Here is
an example where the equal sign (=) is replaced with
%3D and the space character with
%20:
postgresql://user@localhost:5433/mydb?options=-c%20synchronous_commit%3Doff
The host part may be either a host name or an IP address. To specify an IPv6 address, enclose it in square brackets:
postgresql://[2001:db8::1234]/database
The host part is interpreted as described for the parameter host. In particular, a Unix-domain socket connection is chosen if the host part is either empty or looks like an absolute path name, otherwise a TCP/IP connection is initiated. Note, however, that the slash is a reserved character in the hierarchical part of the URI. So, to specify a non-standard Unix-domain socket directory, either omit the host part of the URI and specify the host as a named parameter, or percent-encode the path in the host part of the URI:
postgresql:///dbname?host=/var/lib/postgresql postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
It is possible to specify multiple host components, each with an optional
port component, in a single URI. A URI of the form
postgresql://host1:port1,host2:port2,host3:port3/
is equivalent to a connection string of the form
host=host1,host2,host3 port=port1,port2,port3.
As further described below, each
host will be tried in turn until a connection is successfully established.
It is possible to specify multiple hosts to connect to, so that they are
tried in the given order. In the Keyword/Value format, the host,
hostaddr, and port options accept comma-separated
lists of values. The same number of elements must be given in each
option that is specified, such
that e.g., the first hostaddr corresponds to the first host name,
the second hostaddr corresponds to the second host name, and so
forth. As an exception, if only one port is specified, it
applies to all the hosts.
In the connection URI format, you can list multiple host:port pairs
separated by commas in the host component of the URI.
In either format, a single host name can translate to multiple network addresses. A common example of this is a host that has both an IPv4 and an IPv6 address.
When multiple hosts are specified, or when a single host name is translated to multiple addresses, all the hosts and addresses will be tried in order, until one succeeds. If none of the hosts can be reached, the connection fails. If a connection is established successfully, but authentication fails, the remaining hosts in the list are not tried.
If a password file is used, you can have different passwords for different hosts. All the other connection options are the same for every host in the list; it is not possible to e.g., specify different usernames for different hosts.
The currently recognized parameter key words are:
host #
Name of host to connect to. If a host name looks like an absolute path
name, it specifies Unix-domain communication rather than TCP/IP
communication; the value is the name of the directory in which the
socket file is stored. (On Unix, an absolute path name begins with a
slash. On Windows, paths starting with drive letters are also
recognized.) If the host name starts with @, it is
taken as a Unix-domain socket in the abstract namespace (currently
supported on Linux and Windows).
The default behavior when host is not
specified, or is empty, is to connect to a Unix-domain
socket in
/tmp (or whatever socket directory was specified
when PostgreSQL was built). On Windows,
the default is to connect to localhost.
A comma-separated list of host names is also accepted, in which case each host name in the list is tried in order; an empty item in the list selects the default behavior as explained above. See Section 34.1.1.3 for details.
hostaddr #
Numeric IP address of host to connect to. This should be in the
standard IPv4 address format, e.g., 172.28.40.9. If
your machine supports IPv6, you can also use those addresses.
TCP/IP communication is
always used when a nonempty string is specified for this parameter.
If this parameter is not specified, the value of host
will be looked up to find the corresponding IP address — or, if
host specifies an IP address, that value will be
used directly.
Using hostaddr allows the
application to avoid a host name look-up, which might be important
in applications with time constraints. However, a host name is
required for GSSAPI or SSPI authentication
methods, as well as for verify-full SSL
certificate verification. The following rules are used:
If host is specified
without hostaddr, a host name lookup occurs.
(When using PQconnectPoll, the lookup occurs
when PQconnectPoll first considers this host
name, and it may cause PQconnectPoll to block
for a significant amount of time.)
If hostaddr is specified without host,
the value for hostaddr gives the server network address.
The connection attempt will fail if the authentication
method requires a host name.
If both host and hostaddr are specified,
the value for hostaddr gives the server network address.
The value for host is ignored unless the
authentication method requires it, in which case it will be
used as the host name.
Note that authentication is likely to fail if host
is not the name of the server at network address hostaddr.
Also, when both host and hostaddr
are specified, host
is used to identify the connection in a password file (see
Section 34.16).
A comma-separated list of hostaddr values is also
accepted, in which case each host in the list is tried in order.
An empty item in the list causes the corresponding host name to be
used, or the default host name if that is empty as well. See
Section 34.1.1.3 for details.
Without either a host name or host address,
libpq will connect using a local
Unix-domain socket; or on Windows, it will attempt to connect to
localhost.
port #
Port number to connect to at the server host, or socket file
name extension for Unix-domain
connections.
If multiple hosts were given in the host or
hostaddr parameters, this parameter may specify a
comma-separated list of ports of the same length as the host list, or
it may specify a single port number to be used for all hosts.
An empty string, or an empty item in a comma-separated list,
specifies the default port number established
when PostgreSQL was built.
dbname #The database name. Defaults to be the same as the user name. In certain contexts, the value is checked for extended formats; see Section 34.1.1 for more details on those.
user #PostgreSQL user name to connect as. Defaults to be the same as the operating system name of the user running the application.
password #Password to be used if the server demands password authentication.
passfile #
Specifies the name of the file used to store passwords
(see Section 34.16).
Defaults to ~/.pgpass, or
%APPDATA%\postgresql\pgpass.conf on Microsoft Windows.
(No error is reported if this file does not exist.)
require_auth #Specifies the authentication method that the client requires from the server. If the server does not use the required method to authenticate the client, or if the authentication handshake is not fully completed by the server, the connection will fail. A comma-separated list of methods may also be provided, of which the server must use exactly one in order for the connection to succeed. By default, any authentication method is accepted, and the server is free to skip authentication altogether.
Methods may be negated with the addition of a !
prefix, in which case the server must not attempt
the listed method; any other method is accepted, and the server is free
not to authenticate the client at all. If a comma-separated list is
provided, the server may not attempt any of the
listed negated methods. Negated and non-negated forms may not be
combined in the same setting.
As a final special case, the none method requires the
server not to use an authentication challenge. (It may also be negated,
to require some form of authentication.)
The following methods may be specified:
passwordThe server must request plaintext password authentication.
md5The server must request MD5 hashed password authentication.
gssThe server must either request a Kerberos handshake via GSSAPI or establish a GSS-encrypted channel (see also gssencmode).
sspiThe server must request Windows SSPI authentication.
scram-sha-256The server must successfully complete a SCRAM-SHA-256 authentication exchange with the client.
noneThe server must not prompt the client for an authentication exchange. (This does not prohibit client certificate authentication via TLS, nor GSS authentication via its encrypted transport.)
channel_binding #
This option controls the client's use of channel binding. A setting
of require means that the connection must employ
channel binding, prefer means that the client will
choose channel binding if available, and disable
prevents the use of channel binding. The default
is prefer if
PostgreSQL is compiled with SSL support;
otherwise the default is disable.
Channel binding is a method for the server to authenticate itself to
the client. It is only supported over SSL connections
with PostgreSQL 11 or later servers using
the SCRAM authentication method.
connect_timeout #
Maximum time to wait while connecting, in seconds (write as a decimal integer,
e.g., 10). Zero, negative, or not specified means
wait indefinitely. The minimum allowed timeout is 2 seconds, therefore
a value of 1 is interpreted as 2.
This timeout applies separately to each host name or IP address.
For example, if you specify two hosts and connect_timeout
is 5, each host will time out if no connection is made within 5
seconds, so the total time spent waiting for a connection might be
up to 10 seconds.
client_encoding #
This sets the client_encoding
configuration parameter for this connection. In addition to
the values accepted by the corresponding server option, you
can use auto to determine the right
encoding from the current locale in the client
(LC_CTYPE environment variable on Unix
systems).
options #
Specifies command-line options to send to the server at connection
start. For example, setting this to -c geqo=off sets the
session's value of the geqo parameter to
off. Spaces within this string are considered to
separate command-line arguments, unless escaped with a backslash
(\); write \\ to represent a literal
backslash. For a detailed discussion of the available
options, consult Chapter 20.
application_name #Specifies a value for the application_name configuration parameter.
fallback_application_name #
Specifies a fallback value for the application_name configuration parameter.
This value will be used if no value has been given for
application_name via a connection parameter or the
PGAPPNAME environment variable. Specifying
a fallback name is useful in generic utility programs that
wish to set a default application name but allow it to be
overridden by the user.
keepalives #Controls whether client-side TCP keepalives are used. The default value is 1, meaning on, but you can change this to 0, meaning off, if keepalives are not wanted. This parameter is ignored for connections made via a Unix-domain socket.
keepalives_idle #
Controls the number of seconds of inactivity after which TCP should
send a keepalive message to the server. A value of zero uses the
system default. This parameter is ignored for connections made via a
Unix-domain socket, or if keepalives are disabled.
It is only supported on systems where TCP_KEEPIDLE or
an equivalent socket option is available, and on Windows; on other
systems, it has no effect.
keepalives_interval #
Controls the number of seconds after which a TCP keepalive message
that is not acknowledged by the server should be retransmitted. A
value of zero uses the system default. This parameter is ignored for
connections made via a Unix-domain socket, or if keepalives are disabled.
It is only supported on systems where TCP_KEEPINTVL or
an equivalent socket option is available, and on Windows; on other
systems, it has no effect.
keepalives_count #
Controls the number of TCP keepalives that can be lost before the
client's connection to the server is considered dead. A value of
zero uses the system default. This parameter is ignored for
connections made via a Unix-domain socket, or if keepalives are disabled.
It is only supported on systems where TCP_KEEPCNT or
an equivalent socket option is available; on other systems, it has no
effect.
tcp_user_timeout #
Controls the number of milliseconds that transmitted data may
remain unacknowledged before a connection is forcibly closed.
A value of zero uses the system default. This parameter is
ignored for connections made via a Unix-domain socket.
It is only supported on systems where TCP_USER_TIMEOUT
is available; on other systems, it has no effect.
replication #This option determines whether the connection should use the replication protocol instead of the normal protocol. This is what PostgreSQL replication connections as well as tools such as pg_basebackup use internally, but it can also be used by third-party applications. For a description of the replication protocol, consult Section 55.4.
The following values, which are case-insensitive, are supported:
true, on,
yes, 1
The connection goes into physical replication mode.
database
The connection goes into logical replication mode, connecting to
the database specified in the dbname parameter.
false, off,
no, 0
The connection is a regular one, which is the default behavior.
In physical or logical replication mode, only the simple query protocol can be used.
gssencmode #This option determines whether or with what priority a secure GSS TCP/IP connection will be negotiated with the server. There are three modes:
disableonly try a non-GSSAPI-encrypted connection
prefer (default)if there are GSSAPI credentials present (i.e., in a credentials cache), first try a GSSAPI-encrypted connection; if that fails or there are no credentials, try a non-GSSAPI-encrypted connection. This is the default when PostgreSQL has been compiled with GSSAPI support.
requireonly try a GSSAPI-encrypted connection
gssencmode is ignored for Unix domain socket
communication. If PostgreSQL is compiled
without GSSAPI support, using the require option
will cause an error, while prefer will be accepted
but libpq will not actually attempt
a GSSAPI-encrypted
connection.
sslmode #This option determines whether or with what priority a secure SSL TCP/IP connection will be negotiated with the server. There are six modes:
disableonly try a non-SSL connection
allowfirst try a non-SSL connection; if that fails, try an SSL connection
prefer (default)first try an SSL connection; if that fails, try a non-SSL connection
require
only try an SSL connection. If a root CA
file is present, verify the certificate in the same way as
if verify-ca was specified
verify-caonly try an SSL connection, and verify that the server certificate is issued by a trusted certificate authority (CA)
verify-fullonly try an SSL connection, verify that the server certificate is issued by a trusted CA and that the requested server host name matches that in the certificate
See Section 34.19 for a detailed description of how these options work.
sslmode is ignored for Unix domain socket
communication.
If PostgreSQL is compiled without SSL support,
using options require, verify-ca, or
verify-full will cause an error, while
options allow and prefer will be
accepted but libpq will not actually attempt
an SSL
connection.
Note that if GSSAPI encryption is possible,
that will be used in preference to SSL
encryption, regardless of the value of sslmode.
To force use of SSL encryption in an
environment that has working GSSAPI
infrastructure (such as a Kerberos server), also
set gssencmode to disable.
requiressl #
This option is deprecated in favor of the sslmode
setting.
If set to 1, an SSL connection to the server
is required (this is equivalent to sslmode
require). libpq will then refuse
to connect if the server does not accept an
SSL connection. If set to 0 (default),
libpq will negotiate the connection type with
the server (equivalent to sslmode
prefer). This option is only available if
PostgreSQL is compiled with SSL support.
sslcompression #If set to 1, data sent over SSL connections will be compressed. If set to 0, compression will be disabled. The default is 0. This parameter is ignored if a connection without SSL is made.
SSL compression is nowadays considered insecure and its use is no longer recommended. OpenSSL 1.1.0 disables compression by default, and many operating system distributions disable it in prior versions as well, so setting this parameter to on will not have any effect if the server does not accept compression. PostgreSQL 14 disables compression completely in the backend.
If security is not a primary concern, compression can improve throughput if the network is the bottleneck. Disabling compression can improve response time and throughput if CPU performance is the limiting factor.
sslcert #
This parameter specifies the file name of the client SSL
certificate, replacing the default
~/.postgresql/postgresql.crt.
This parameter is ignored if an SSL connection is not made.
sslkey #
This parameter specifies the location for the secret key used for
the client certificate. It can either specify a file name that will
be used instead of the default
~/.postgresql/postgresql.key, or it can specify a key
obtained from an external “engine” (engines are
OpenSSL loadable modules). An external engine
specification should consist of a colon-separated engine name and
an engine-specific key identifier. This parameter is ignored if an
SSL connection is not made.
sslpassword #
This parameter specifies the password for the secret key specified in
sslkey, allowing client certificate private keys
to be stored in encrypted form on disk even when interactive passphrase
input is not practical.
Specifying this parameter with any non-empty value suppresses the
Enter PEM pass phrase:
prompt that OpenSSL will emit by default
when an encrypted client certificate key is provided to
libpq.
If the key is not encrypted this parameter is ignored. The parameter has no effect on keys specified by OpenSSL engines unless the engine uses the OpenSSL password callback mechanism for prompts.
There is no environment variable equivalent to this option, and no
facility for looking it up in .pgpass. It can be
used in a service file connection definition. Users with
more sophisticated uses should consider using OpenSSL engines and
tools like PKCS#11 or USB crypto offload devices.
sslcertmode #This option determines whether a client certificate may be sent to the server, and whether the server is required to request one. There are three modes:
disableA client certificate is never sent, even if one is available (default location or provided via sslcert).
allow (default)A certificate may be sent, if the server requests one and the client has one to send.
requireThe server must request a certificate. The connection will fail if the client does not send a certificate and the server successfully authenticates the client anyway.
sslcertmode=require doesn't add any additional
security, since there is no guarantee that the server is validating
the certificate correctly; PostgreSQL servers generally request TLS
certificates from clients whether they validate them or not. The
option may be useful when troubleshooting more complicated TLS
setups.
sslrootcert #
This parameter specifies the name of a file containing SSL
certificate authority (CA) certificate(s).
If the file exists, the server's certificate will be verified
to be signed by one of these authorities. The default is
~/.postgresql/root.crt.
The special value system may be specified instead, in
which case the system's trusted CA roots will be loaded. The exact
locations of these root certificates differ by SSL implementation and
platform. For OpenSSL in particular, the
locations may be further modified by the SSL_CERT_DIR
and SSL_CERT_FILE environment variables.
When using sslrootcert=system, the default
sslmode is changed to verify-full,
and any weaker setting will result in an error. In most cases it is
trivial for anyone to obtain a certificate trusted by the system for a
hostname they control, rendering verify-ca and all
weaker modes useless.
The magic system value will take precedence over a
local certificate file with the same name. If for some reason you find
yourself in this situation, use an alternative path like
sslrootcert=./system instead.
sslcrl #
This parameter specifies the file name of the SSL server certificate
revocation list (CRL). Certificates listed in this file, if it
exists, will be rejected while attempting to authenticate the
server's certificate. If neither
sslcrl nor
sslcrldir is set, this setting is
taken as
~/.postgresql/root.crl.
sslcrldir #This parameter specifies the directory name of the SSL server certificate revocation list (CRL). Certificates listed in the files in this directory, if it exists, will be rejected while attempting to authenticate the server's certificate.
The directory needs to be prepared with the
OpenSSL command
openssl rehash or c_rehash. See
its documentation for details.
Both sslcrl and sslcrldir can be
specified together.
sslsni #If set to 1 (default), libpq sets the TLS extension “Server Name Indication” (SNI) on SSL-enabled connections. By setting this parameter to 0, this is turned off.
The Server Name Indication can be used by SSL-aware proxies to route connections without having to decrypt the SSL stream. (Note that this requires a proxy that is aware of the PostgreSQL protocol handshake, not just any SSL proxy.) However, SNI makes the destination host name appear in cleartext in the network traffic, so it might be undesirable in some cases.
requirepeer #
This parameter specifies the operating-system user name of the
server, for example requirepeer=postgres.
When making a Unix-domain socket connection, if this
parameter is set, the client checks at the beginning of the
connection that the server process is running under the specified
user name; if it is not, the connection is aborted with an error.
This parameter can be used to provide server authentication similar
to that available with SSL certificates on TCP/IP connections.
(Note that if the Unix-domain socket is in
/tmp or another publicly writable location,
any user could start a server listening there. Use this parameter
to ensure that you are connected to a server run by a trusted user.)
This option is only supported on platforms for which the
peer authentication method is implemented; see
Section 21.9.
ssl_min_protocol_version #
This parameter specifies the minimum SSL/TLS protocol version to allow
for the connection. Valid values are TLSv1,
TLSv1.1, TLSv1.2 and
TLSv1.3. The supported protocols depend on the
version of OpenSSL used, older versions
not supporting the most modern protocol versions. If not specified,
the default is TLSv1.2, which satisfies industry
best practices as of this writing.
ssl_max_protocol_version #
This parameter specifies the maximum SSL/TLS protocol version to allow
for the connection. Valid values are TLSv1,
TLSv1.1, TLSv1.2 and
TLSv1.3. The supported protocols depend on the
version of OpenSSL used, older versions
not supporting the most modern protocol versions. If not set, this
parameter is ignored and the connection will use the maximum bound
defined by the backend, if set. Setting the maximum protocol version
is mainly useful for testing or if some component has issues working
with a newer protocol.
krbsrvname #
Kerberos service name to use when authenticating with GSSAPI.
This must match the service name specified in the server
configuration for Kerberos authentication to succeed. (See also
Section 21.6.)
The default value is normally postgres,
but that can be changed when
building PostgreSQL via
the --with-krb-srvnam option
of configure.
In most environments, this parameter never needs to be changed.
Some Kerberos implementations might require a different service name,
such as Microsoft Active Directory which requires the service name
to be in upper case (POSTGRES).
gsslib #
GSS library to use for GSSAPI authentication.
Currently this is disregarded except on Windows builds that include
both GSSAPI and SSPI support. In that case, set
this to gssapi to cause libpq to use the GSSAPI
library for authentication instead of the default SSPI.
gssdelegation #
Forward (delegate) GSS credentials to the server. The default is
0 which means credentials will not be forwarded
to the server. Set this to 1 to have credentials
forwarded when possible.
service #
Service name to use for additional parameters. It specifies a service
name in pg_service.conf that holds additional connection parameters.
This allows applications to specify only a service name so connection parameters
can be centrally maintained. See Section 34.17.
target_session_attrs #This option determines whether the session must have certain properties to be acceptable. It's typically used in combination with multiple host names to select the first acceptable alternative among several hosts. There are six modes:
any (default)any successful connection is acceptable
read-write
session must accept read-write transactions by default (that
is, the server must not be in hot standby mode and
the default_transaction_read_only parameter
must be off)
read-onlysession must not accept read-write transactions by default (the converse)
primaryserver must not be in hot standby mode
standbyserver must be in hot standby mode
prefer-standby
first try to find a standby server, but if none of the listed
hosts is a standby server, try again in any
mode
load_balance_hosts #Controls the order in which the client tries to connect to the available hosts and addresses. Once a connection attempt is successful no other hosts and addresses will be tried. This parameter is typically used in combination with multiple host names or a DNS record that returns multiple IPs. This parameter can be used in combination with target_session_attrs to, for example, load balance over standby servers only. Once successfully connected, subsequent queries on the returned connection will all be sent to the same server. There are currently two modes:
disable (default)No load balancing across hosts is performed. Hosts are tried in the order in which they are provided and addresses are tried in the order they are received from DNS or a hosts file.
randomHosts and addresses are tried in random order. This value is mostly useful when opening multiple connections at the same time, possibly from different machines. This way connections can be load balanced across multiple PostgreSQL servers.
While random load balancing, due to its random nature, will almost never result in a completely uniform distribution, it statistically gets quite close. One important aspect here is that this algorithm uses two levels of random choices: First the hosts will be resolved in random order. Then secondly, before resolving the next host, all resolved addresses for the current host will be tried in random order. This behaviour can skew the amount of connections each node gets greatly in certain cases, for instance when some hosts resolve to more addresses than others. But such a skew can also be used on purpose, e.g. to increase the number of connections a larger server gets by providing its hostname multiple times in the host string.
When using this value it's recommended to also configure a reasonable value for connect_timeout. Because then, if one of the nodes that are used for load balancing is not responding, a new node will be tried.