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PostgreSQL 13.0 Documentation > 33.1. Database Connection Control Functions

33.1. Database Connection Control Functions

33.1. Database Connection Control Functions
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33.1.1. Connection Strings
33.1.2. Parameter Key Words

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.

Warning

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( conn , "SELECT pg_catalog.set_config('search_path', '', false)") after connecting. This consideration is not specific to libpq ; it applies to every interface for executing arbitrary SQL commands.

Warning

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.

Note

On Windows, there is a way to improve performance if a single database connection is repeatedly started and shutdown. Internally, libpq calls WSAStartup() and WSACleanup() for connection startup and shutdown, respectively. WSAStartup() increments an internal Windows library reference count which is decremented by WSACleanup() . When the reference count is just one, calling WSACleanup() frees all resources and all DLLs are unloaded. This is an expensive operation. To avoid this, an application can manually call WSAStartup() so resources will not be freed when the last database connection is closed.

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 33.1.2 .

When expand_dbname is non-zero, the dbname key word value is allowed to be recognized as a connection string. Only the first occurrence of dbname is expanded this way, any subsequent dbname value is processed as plain database name. More details on the possible connection string formats appear in Section 33.1.1 .

The passed arrays can be empty to use all default parameters, or can contain one or more parameter settings. They should be matched in length. Processing will stop at the first NULL element in the keywords array.

If any parameter is NULL or an empty string, the corresponding environment variable (see Section 33.14 ) is checked. If the environment variable is not set either, then the indicated built-in defaults are used.

In general key words are processed from the beginning of these arrays in index order. The effect of this is that when key words are repeated, the last processed value is retained. Therefore, through careful placement of the dbname key word, it is possible to determine what may be overridden by a conninfo string, and what may not.

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 33.1.1 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 .

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.

PQconnectStartParams PQconnectStart PQconnectPoll

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 33.1.2 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 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.

PQresetStart PQresetPoll

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 reestablish a new connection to the same server, 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);

33.1.1. Connection Strings

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.

33.1.1.1. Keyword/Value Connection Strings

In the first format, 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 \\ .

Example:

host=localhost port=5432 dbname=mydb connect_timeout=10

The recognized parameter key words are listed in Section 33.1.2 .

33.1.1.2. Connection URIs

The general form for a connection URI is:

postgresql://[user[:password]@][netloc][:port][,...][/dbname][?param1=value1&...]

The URI scheme designator can be either postgresql:// or postgres:// . Each of the URI parts is optional. The following examples illustrate valid URI syntax uses:

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

Components of the hierarchical part of the URI can also be given as parameters. For example:

postgresql:///mydb?host=localhost&port=5433

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

Any connection parameters not corresponding to key words listed in Section 33.1.2 are ignored and a warning message about them is sent to stderr .

For improved compatibility with JDBC connection URI s, instances of parameter ssl=true are translated into sslmode=require .

The host part may be either host name or an IP address. To specify an IPv6 host address, enclose it in square brackets:

postgresql://[2001:db8::1234]/database

The host component 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 specification in the URI and specify the host as a parameter, or percent-encode the path in the host component 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 . Each host will be tried in turn until a connection is successfully established.

33.1.1.3. Specifying Multiple Hosts

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 a comma-separated list 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.

33.1.2. Parameter Key Words

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.) 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 and on machines without Unix-domain sockets, 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 33.1.1.3 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 33.15 ).

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 33.1.1.3 for details.

Without either a host name or host address, libpq will connect using a local Unix-domain socket; or on Windows and on machines without Unix-domain sockets, 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 33.1.1 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 33.15 ). Defaults to ~/.pgpass , or %APPDATA%\postgresql\pgpass.conf on Microsoft Windows. (No error is reported if this file does not exist.)

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 19 .

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.

tty

Ignored (formerly, this specified where to send server debug output).

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 52.4 .

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:

disable: only 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.
require: only 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:

disable: only try a non- SSL connection
allow: first 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-ca: only try an SSL connection, and verify that the server certificate is issued by a trusted certificate authority ( CA )
verify-full: only 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 33.18 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.

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.

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.

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 .

sslcrl

This parameter specifies the file name of the SSL certificate revocation list (CRL). Certificates listed in this file, if it exists, will be rejected while attempting to authenticate the server's certificate. The default is ~/.postgresql/root.crl .

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 20.9 .

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 20.6 .)

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.

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 33.16 .

target_session_attrs

If this parameter is set to read-write , only a connection in which read-write transactions are accepted by default is considered acceptable. The query SHOW transaction_read_only will be sent upon any successful connection; if it returns on , the connection will be closed. If multiple hosts were specified in the connection string, any remaining servers will be tried just as if the connection attempt had failed. The default value of this parameter, any , regards all connections as acceptable.

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