From 35f2a0c8971d31ec63b7752292b011b90b1ca004 Mon Sep 17 00:00:00 2001
From: Jim Jones <jim.jones@uni-muenster.de>
Date: Thu, 30 Oct 2025 09:27:24 +0100
Subject: [PATCH v6] Add %i prompt escape to indicate server read-only status

This patch introduces a new prompt escape %i for psql, which shows
whether the connected server is operating in read-only or read/write
mode. It expands to "read-only" if any of the following are true:

  - The server is in hot standby mode (in_hot_standby = on)
  - The session's default transaction mode is read-only
    (default_transaction_read_only = on)
  - The current transaction is explicitly set to read-only via
    SET TRANSACTION READ ONLY (transaction_read_only = on)

Otherwise, it displays "read/write".

To minimize overhead, the implementation checks session-level parameters
(in_hot_standby and default_transaction_read_only) first via
PQparameterStatus(). If both indicate read/write mode, it queries the
server for the transaction-level transaction_read_only setting.
This means no extra queries are issued on hot standby systems or sessions
with default_transaction_read_only = on.

This is useful for distinguishing read-only sessions (e.g. connected
to a standby, using a default read-only transaction mode, or in an
explicit read-only transaction) from "read/write" ones at a glance,
especially when working with multiple connections in replicated or
restricted environments.
---
 doc/src/sgml/ref/psql-ref.sgml | 16 ++++++++++++++++
 src/bin/psql/prompt.c          | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+)

diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index 1a339600bc..3b45514f34 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -5044,6 +5044,22 @@ testdb=&gt; <userinput>INSERT INTO my_table VALUES (:'content');</userinput>
         </listitem>
       </varlistentry>
 
+      <varlistentry id="app-psql-prompting-i">
+        <term><literal>%i</literal></term>
+        <listitem>
+          <para>
+          Displays the session's read-only status as <literal>read-only</literal>
+          if the current transaction is read-only (<literal>transaction_read_only</literal>
+          is <literal>on</literal>), the server is in hot standby
+          (<literal>in_hot_standby</literal> is <literal>on</literal>), or the
+          default transaction mode is read-only (<literal>default_transaction_read_only</literal>
+          is <literal>on</literal>); otherwise displays <literal>read-write</literal>.
+          Useful for identifying sessions that cannot perform writes, such as in
+          replication setups or when explicit read-only transactions are in use.
+          </para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry id="app-psql-prompting-x">
         <term><literal>%x</literal></term>
         <listitem>
diff --git a/src/bin/psql/prompt.c b/src/bin/psql/prompt.c
index b08d7328fb..5d43d5bbcb 100644
--- a/src/bin/psql/prompt.c
+++ b/src/bin/psql/prompt.c
@@ -43,6 +43,8 @@
  *			or a ! if session is not connected to a database;
  *		in prompt2 -, *, ', or ";
  *		in prompt3 nothing
+ * %i - displays "read-only" if in hot standby, or if default_transaction_read_only
+ *     	or transaction_read_only are on, "read/write" otherwise.
  * %x - transaction status: empty, *, !, ? (unknown or no connection)
  * %l - The line number inside the current statement, starting from 1.
  * %? - the error code of the last query (not yet implemented)
@@ -247,7 +249,39 @@ get_prompt(promptStatus_t status, ConditionalStack cstack)
 							break;
 					}
 					break;
+				case 'i':
+					if (pset.db)
+					{
+						const char *hs = PQparameterStatus(pset.db, "in_hot_standby");
+						const char *ro = PQparameterStatus(pset.db, "default_transaction_read_only");
+
+						if (!hs || !ro)
+							strlcpy(buf, _("unknown"), sizeof(buf));
+						else if (strcmp(hs, "on") == 0 || strcmp(ro, "on") == 0)
+							strlcpy(buf, _("read-only"), sizeof(buf));
+						else
+						{
+							const char *tr = NULL;
+							PGresult   *res;
+
+							res = PQexec(pset.db, "SHOW transaction_read_only");
+							if (PQresultStatus(res) == PGRES_TUPLES_OK &&
+								PQntuples(res) == 1)
+								tr = PQgetvalue(res, 0, 0);
+
+							if (!tr)
+								strlcpy(buf, _("unknown"), sizeof(buf));
+							else if (strcmp(tr, "on") == 0)
+								strlcpy(buf, _("read-only"), sizeof(buf));
+							else
+								strlcpy(buf, _("read/write"), sizeof(buf));
 
+							PQclear(res);
+						}
+					}
+					else
+						buf[0] = '\0';
+					break;
 				case 'x':
 					if (!pset.db)
 						buf[0] = '?';
-- 
2.43.0