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max_protocol_version

Reading PostgreSQL 18.6.

Description

Specifies the protocol version to request from the server. The default is to use version 3.0 of the PostgreSQL protocol, unless the connection string specifies a feature that relies on a higher protocol version, in which case the latest version supported by libpq is used. If the server does not support the protocol version requested by the client, the connection is automatically downgraded to a lower minor protocol version that the server supports. After the connection attempt has completed you can use PQfullProtocolVersion to find out which exact protocol version was negotiated.

Client library: libpq 18.6
Manual definition: Documented
Source environment fallback: PGMAXPROTOCOLVERSION
Compiled fallback expression: NULL

Usage

max_protocol_version

Default resolution and service-file precedence

The following environment variables can be used to select default connection parameter values, which will be used by PQconnectdb , PQsetdbLogin and PQsetdb if no value is directly specified by the calling code. These are useful to avoid hard-coding database connection information into simple client applications, for example.

Service names can be defined in either a per-user service file or a system-wide file. If the same service name exists in both the user and the system file, the user file takes precedence. By default, the per-user service file is named ~/.pg_service.conf . On Microsoft Windows, it is named %APPDATA%\postgresql\.pg_service.conf (where %APPDATA% refers to the Application Data subdirectory in the user's profile). A different file name can be specified by setting the environment variable PGSERVICEFILE . The system-wide file is named pg_service.conf . By default it is sought in the etc directory of the PostgreSQL installation (use pg_config --sysconfdir to identify this directory precisely). Another directory, but not a different file name, can be specified by setting the environment variable PGSYSCONFDIR .

Connection parameters obtained from a service file are combined with parameters obtained from other sources. A service file setting overrides the corresponding environment variable, and in turn can be overridden by a value given directly in the connection string. For example, using the above service file, a connection string service=mydb port=5434 will use host somehost , port 5434 , user admin , and other parameters as set by environment variables or built-in defaults.

Environment variable evidence

PGMAXPROTOCOLVERSION behaves the same as the max_protocol_version connection parameter.

Environment fallback

Variable	Documented behavior
PGMAXPROTOCOLVERSION	PGMAXPROTOCOLVERSION behaves the same as the max_protocol_version connection parameter.

Manual definition

max_protocol_version

Specifies the protocol version to request from the server. The default is to use version 3.0 of the PostgreSQL protocol, unless the connection string specifies a feature that relies on a higher protocol version, in which case the latest version supported by libpq is used. If the server does not support the protocol version requested by the client, the connection is automatically downgraded to a lower minor protocol version that the server supports. After the connection attempt has completed you can use PQfullProtocolVersion to find out which exact protocol version was negotiated.

The current supported values are 3.0, 3.2, and latest. The latest value is equivalent to the latest protocol version supported by the libpq version being used, which is currently 3.2.

Related entries

Documentation and source

Source build

Version: 18.6
Build: https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2
Source fingerprint: ee8d1a3612338fd9adf250730cb640fcc5233b5491337cc00a316a44e3a0b9f8

Compare versions

PostgreSQL 17 → 18: added.

--- PostgreSQL 17
+++ PostgreSQL 18
@@ -1 +1,10 @@
-Not recorded in this version
+{
+  "compiled_default_expression": "NULL",
+  "default_evidence": [
+    "Specifies the protocol version to request from the server. The default is to use version 3.0 of the PostgreSQL protocol, unless the connection string specifies a feature that relies on a higher protocol version, in which case the latest version supported by libpq is used. If the server does not support the protocol version requested by the client, the connection is automatically downgraded to a lower minor protocol version that the server supports. After the connection attempt has completed you can use PQfullProtocolVersion to find out which exact protocol version was negotiated."
+  ],
+  "definition": "Specifies the protocol version to request from the server. The default is to use version 3.0 of the PostgreSQL protocol, unless the connection string specifies a feature that relies on a higher protocol version, in which case the latest version supported by libpq is used. If the server does not support the protocol version requested by the client, the connection is automatically downgraded to a lower minor protocol version that the server supports. After the connection attempt has completed you can use PQfullProtocolVersion to find out which exact protocol version was negotiated. The current supported values are 3.0 , 3.2 , and latest . The latest value is equivalent to the latest protocol version supported by the libpq version being used, which is currently 3.2 .",
+  "documented": true,
+  "environment": "PGMAXPROTOCOLVERSION",
+  "keyword": "max_protocol_version"
+}

Compares recorded interfaces and attributes. Source fingerprints and build metadata are excluded; an absent sample is not proof of the introduction or removal release.

Related entries

Export JSON · Back to Connection Parameters · Recorded in PostgreSQL 18 through 20; the first sample is not necessarily its introduction.