Hello Pavel,

new update - rebase, changed message

I did yet another rebase of your patch after Tom alphabetically ordered backslash commands. Here is the result.

--
Fabien.
diff --git a/doc/src/sgml/ref/psql-ref.sgml b/doc/src/sgml/ref/psql-ref.sgml
index e6eba21..833460e 100644
--- a/doc/src/sgml/ref/psql-ref.sgml
+++ b/doc/src/sgml/ref/psql-ref.sgml
@@ -1943,6 +1943,15 @@ Tue Oct 26 21:40:57 CEST 1999
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\gdesc</literal></term>
+        <listitem>
+        <para>
+        Show the description of the result of the current query buffer without
+        actually executing it, by considering it a prepared statement.
+        </para>
+        </listitem>
+      </varlistentry>
 
       <varlistentry>
         <term><literal>\gexec</literal></term>
diff --git a/src/bin/psql/command.c b/src/bin/psql/command.c
index b3263a9..a1c8e1d 100644
--- a/src/bin/psql/command.c
+++ b/src/bin/psql/command.c
@@ -88,6 +88,7 @@ static backslashResult exec_command_errverbose(PsqlScanState scan_state, bool ac
 static backslashResult exec_command_f(PsqlScanState scan_state, bool active_branch);
 static backslashResult exec_command_g(PsqlScanState scan_state, bool active_branch,
 			   const char *cmd);
+static backslashResult exec_command_gdesc(PsqlScanState scan_state, bool active_branch);
 static backslashResult exec_command_gexec(PsqlScanState scan_state, bool active_branch);
 static backslashResult exec_command_gset(PsqlScanState scan_state, bool active_branch);
 static backslashResult exec_command_help(PsqlScanState scan_state, bool active_branch);
@@ -337,6 +338,8 @@ exec_command(const char *cmd,
 		status = exec_command_f(scan_state, active_branch);
 	else if (strcmp(cmd, "g") == 0 || strcmp(cmd, "gx") == 0)
 		status = exec_command_g(scan_state, active_branch, cmd);
+	else if (strcmp(cmd, "gdesc") == 0)
+		status = exec_command_gdesc(scan_state, active_branch);
 	else if (strcmp(cmd, "gexec") == 0)
 		status = exec_command_gexec(scan_state, active_branch);
 	else if (strcmp(cmd, "gset") == 0)
@@ -1328,6 +1331,25 @@ exec_command_g(PsqlScanState scan_state, bool active_branch, const char *cmd)
 }
 
 /*
+ * \gdesc -- describe query result
+ */
+static backslashResult
+exec_command_gdesc(PsqlScanState scan_state, bool active_branch)
+{
+	backslashResult status = PSQL_CMD_SKIP_LINE;
+
+	if (active_branch)
+	{
+		pset.gdesc_flag = true;
+		status = PSQL_CMD_SEND;
+	}
+	else
+		ignore_slash_filepipe(scan_state);
+
+	return status;
+}
+
+/*
  * \gexec -- send query and execute each field of result
  */
 static backslashResult
diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index a2f1259..5baf372 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -1323,7 +1323,88 @@ SendQuery(const char *query)
 		}
 	}
 
-	if (pset.fetch_count <= 0 || pset.gexec_flag ||
+	if (pset.gdesc_flag)
+	{
+		/*
+		 * Unnamed prepared statement is used. Is not possible to
+		 * create any unnamed prepared statement from psql user space,
+		 * so there should not be any conflict. In this moment is not
+		 * possible to deallocate this prepared statement, so it should
+		 * to live to end of session or to another \gdesc call.
+		 */
+		results = PQprepare(pset.db, "", query, 0, NULL);
+		if (PQresultStatus(results) != PGRES_COMMAND_OK)
+		{
+			psql_error("%s", PQerrorMessage(pset.db));
+			ClearOrSaveResult(results);
+			ResetCancelConn();
+			goto sendquery_cleanup;
+		}
+		PQclear(results);
+
+		results = PQdescribePrepared(pset.db, "");
+		OK = ProcessResult(&results);
+		if (OK && results)
+		{
+			if (PQnfields(results) > 0)
+			{
+				PQExpBufferData		buf;
+				int		i;
+
+				initPQExpBuffer(&buf);
+
+				printfPQExpBuffer(&buf,
+					  "SELECT name AS \"%s\", pg_catalog.format_type(tp, tpm) AS \"%s\"\n"
+					  "FROM (VALUES",
+					  gettext_noop("Name"),
+					  gettext_noop("Type"));
+
+				for(i = 0; i< PQnfields(results); i++)
+				{
+					char	*name;
+					char	*escname;
+					size_t		name_length;
+
+					if (i > 0)
+						appendPQExpBufferStr(&buf, ",");
+
+					name = PQfname(results, i);
+					name_length = strlen(name);
+					escname = PQescapeLiteral(pset.db, name, name_length);
+
+					if (escname == NULL)
+					{
+						psql_error("%s", PQerrorMessage(pset.db));
+						PQclear(results);
+						termPQExpBuffer(&buf);
+						goto sendquery_cleanup;
+					}
+
+					appendPQExpBuffer(&buf, "(%s, %d, %d)",
+									  escname, PQftype(results,i), PQfmod(results,i));
+					PQfreemem(escname);
+				}
+
+				appendPQExpBuffer(&buf,") s (name, tp, tpm)");
+				PQclear(results);
+
+				results = PQexec(pset.db, buf.data);
+				OK = ProcessResult(&results);
+
+				if (OK && results)
+					OK = PrintQueryResults(results);
+
+				termPQExpBuffer(&buf);
+			}
+			else
+				fprintf(pset.queryFout, _("The result has no columns or the command has no result.\n"));
+		}
+
+		ClearOrSaveResult(results);
+		ResetCancelConn();
+		results = NULL;			/* PQclear(NULL) does nothing */
+	}
+	else if (pset.fetch_count <= 0 || pset.gexec_flag ||
 		pset.crosstab_flag || !is_select_command(query))
 	{
 		/* Default fetch-it-all-and-print mode */
@@ -1478,6 +1559,9 @@ sendquery_cleanup:
 		pset.ctv_args[i] = NULL;
 	}
 
+	/* reset \gdesc trigger */
+	pset.gdesc_flag = false;
+
 	return OK;
 }
 
diff --git a/src/bin/psql/help.c b/src/bin/psql/help.c
index f097b06..dffced5 100644
--- a/src/bin/psql/help.c
+++ b/src/bin/psql/help.c
@@ -167,13 +167,14 @@ slashUsage(unsigned short int pager)
 	 * Use "psql --help=commands | wc" to count correctly.  It's okay to count
 	 * the USE_READLINE line even in builds without that.
 	 */
-	output = PageOutput(122, pager ? &(pset.popt.topt) : NULL);
+	output = PageOutput(123, pager ? &(pset.popt.topt) : NULL);
 
 	fprintf(output, _("General\n"));
 	fprintf(output, _("  \\copyright             show PostgreSQL usage and distribution terms\n"));
 	fprintf(output, _("  \\crosstabview [COLUMNS] execute query and display results in crosstab\n"));
 	fprintf(output, _("  \\errverbose            show most recent error message at maximum verbosity\n"));
 	fprintf(output, _("  \\g [FILE] or ;         execute query (and send results to file or |pipe)\n"));
+	fprintf(output, _("  \\gdesc                 describe result of query without executing it\n"));
 	fprintf(output, _("  \\gexec                 execute query, then execute each value in its result\n"));
 	fprintf(output, _("  \\gset [PREFIX]         execute query and store results in psql variables\n"));
 	fprintf(output, _("  \\gx [FILE]             as \\g, but forces expanded output mode\n"));
diff --git a/src/bin/psql/settings.h b/src/bin/psql/settings.h
index 70ff181..a175912 100644
--- a/src/bin/psql/settings.h
+++ b/src/bin/psql/settings.h
@@ -95,6 +95,7 @@ typedef struct _psqlSettings
 	char	   *gset_prefix;	/* one-shot prefix argument for \gset */
 	bool		gexec_flag;		/* one-shot flag to execute query's results */
 	bool		crosstab_flag;	/* one-shot request to crosstab results */
+	bool		gdesc_flag;		/* one-shot request to describe query */
 	char	   *ctv_args[4];	/* \crosstabview arguments */
 
 	bool		notty;			/* stdin or stdout is not a tty (as determined
diff --git a/src/bin/psql/tab-complete.c b/src/bin/psql/tab-complete.c
index 92cc8aa..76c79ca 100644
--- a/src/bin/psql/tab-complete.c
+++ b/src/bin/psql/tab-complete.c
@@ -1433,7 +1433,7 @@ psql_completion(const char *text, int start, int end)
 		"\\e", "\\echo", "\\ef", "\\elif", "\\else", "\\encoding",
 		"\\endif", "\\errverbose", "\\ev",
 		"\\f",
-		"\\g", "\\gexec", "\\gset", "\\gx",
+		"\\g", "\\gdesc", "\\gexec", "\\gset", "\\gx",
 		"\\h", "\\help", "\\H",
 		"\\i", "\\if", "\\ir",
 		"\\l", "\\lo_import", "\\lo_export", "\\lo_list", "\\lo_unlink",
diff --git a/src/test/regress/expected/psql.out b/src/test/regress/expected/psql.out
index d602aee..394e4f4 100644
--- a/src/test/regress/expected/psql.out
+++ b/src/test/regress/expected/psql.out
@@ -2964,3 +2964,73 @@ SELECT 3
 UNION SELECT 4 
 UNION SELECT 5
 ORDER BY 1;
+-- gdesc tests
+SELECT
+    NULL AS zero,
+    1 AS one,
+    2.0 AS two,
+    'three' AS three,
+    $1 AS four,
+    sin($2) as five,
+    CURRENT_DATE AS now
+\gdesc
+ Name  |       Type       
+-------+------------------
+ zero  | text
+ one   | integer
+ two   | numeric
+ three | text
+ four  | text
+ five  | double precision
+ now   | date
+(7 rows)
+
+PREPARE test AS SELECT 1 AS first;
+EXECUTE test \gdesc
+ Name  |  Type   
+-------+---------
+ first | integer
+(1 row)
+
+-- should fail - syntax error
+SELECT 1 + \gdesc
+ERROR:  syntax error at end of input
+LINE 1: SELECT 1 + 
+                   ^
+-- empty results
+SELECT \gdesc
+The result has no columns or the command has no result.
+CREATE TABLE bububu(a int)\gdesc
+The result has no columns or the command has no result.
+-- one line
+SELECT 1 AS x, 'Hello', 2 AS y, true AS "dirty name" \gdesc \g
+    Name    |  Type   
+------------+---------
+ x          | integer
+ ?column?   | text
+ y          | integer
+ dirty name | boolean
+(4 rows)
+
+ x | ?column? | y | dirty name 
+---+----------+---+------------
+ 1 | Hello    | 2 | t
+(1 row)
+
+-- query buffer should not be broken
+SELECT 1 AS x, 'Hello', 2 AS y, true AS "dirty name"
+\gdesc
+    Name    |  Type   
+------------+---------
+ x          | integer
+ ?column?   | text
+ y          | integer
+ dirty name | boolean
+(4 rows)
+
+\g
+ x | ?column? | y | dirty name 
+---+----------+---+------------
+ 1 | Hello    | 2 | t
+(1 row)
+
diff --git a/src/test/regress/sql/psql.sql b/src/test/regress/sql/psql.sql
index b56a05f..13dd999 100644
--- a/src/test/regress/sql/psql.sql
+++ b/src/test/regress/sql/psql.sql
@@ -560,3 +560,32 @@ UNION SELECT 5
 ORDER BY 1;
 \r
 \p
+
+-- gdesc tests
+SELECT
+    NULL AS zero,
+    1 AS one,
+    2.0 AS two,
+    'three' AS three,
+    $1 AS four,
+    sin($2) as five,
+    CURRENT_DATE AS now
+\gdesc
+
+PREPARE test AS SELECT 1 AS first;
+EXECUTE test \gdesc
+
+-- should fail - syntax error
+SELECT 1 + \gdesc
+
+-- empty results
+SELECT \gdesc
+CREATE TABLE bububu(a int)\gdesc
+
+-- one line
+SELECT 1 AS x, 'Hello', 2 AS y, true AS "dirty name" \gdesc \g
+
+-- query buffer should not be broken
+SELECT 1 AS x, 'Hello', 2 AS y, true AS "dirty name"
+\gdesc
+\g
-- 
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