Improve error messages about misuse of SELECT INTO.

Improve two places in plpgsql, and one in spi.c, where an error
message would confusingly tell you that you couldn't use a SELECT
query, when what you had written *was* a SELECT query.  The actual
problem is that you can't use SELECT ... INTO in these contexts,
but the messages failed to make that apparent.  Special-case
SELECT INTO to make these errors more helpful.

Also, fix the same spots in plpgsql, as well as several messages
in exec_eval_expr(), to not quote the entire complained-of query or
expression in the primary error message.  That behavior very easily
led to violating our message style guideline about keeping the primary
error message short and single-line.  Also, since the important part
of the message was after the inserted text, it could make the real
problem very hard to see.  We can report the query or expression as
the first line of errcontext instead.

Per complaint from Roger Mason.  Back-patch to v14, since (a) some
of these messages are new in v14 and (b) v14's translatable strings
are still somewhat in flux.  The problem's older than that of
course, but I'm hesitant to change the behavior further back.

Discussion: https://postgr.es/m/1914708.1629474624@sss.pgh.pa.us

src/backend/executor/spi.c

@@ -1489,16 +1489,22 @@ SPI_cursor_open_internal(const char *name, SPIPlanPtr plan,
 	if (!SPI_is_cursor_plan(plan))
 	{
 		/* try to give a good error message */
+		const char *cmdtag;
+
 		if (list_length(plan->plancache_list) != 1)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_CURSOR_DEFINITION),
 					 errmsg("cannot open multi-query plan as cursor")));
 		plansource = (CachedPlanSource *) linitial(plan->plancache_list);
+		/* A SELECT that fails SPI_is_cursor_plan() must be SELECT INTO */
+		if (plansource->commandTag == CMDTAG_SELECT)
+			cmdtag = "SELECT INTO";
+		else
+			cmdtag = GetCommandTagName(plansource->commandTag);
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_CURSOR_DEFINITION),
 		/* translator: %s is name of a SQL command, eg INSERT */
-				 errmsg("cannot open %s query as cursor",
+				 errmsg("cannot open %s query as cursor", cmdtag)));
-						GetCommandTagName(plansource->commandTag))));
 	}
 
 	Assert(list_length(plan->plancache_list) == 1);

src/pl/plpgsql/src/expected/plpgsql_array.out

@@ -73,8 +73,9 @@ PL/pgSQL function inline_code_block line 2 at assignment
 insert into onecol values(array[11]);
 do $$ declare a int[];
 begin a := f1 from onecol; raise notice 'a = %', a; end$$;
-ERROR:  query "a := f1 from onecol" returned more than one row
+ERROR:  query returned more than one row
-CONTEXT:  PL/pgSQL function inline_code_block line 2 at assignment
+CONTEXT:  query: a := f1 from onecol
+PL/pgSQL function inline_code_block line 2 at assignment
 do $$ declare a int[];
 begin a := f1 from onecol limit 1; raise notice 'a = %', a; end$$;
 NOTICE:  a = {1,2}

src/pl/plpgsql/src/pl_exec.c

@@ -3557,9 +3557,22 @@ exec_stmt_return_query(PLpgSQL_execstate *estate,
 
 		rc = SPI_execute_plan_extended(expr->plan, &options);
 		if (rc != SPI_OK_SELECT)
+		{
+			/*
+			 * SELECT INTO deserves a special error message, because "query is
+			 * not a SELECT" is not very helpful in that case.
+			 */
+			if (rc == SPI_OK_SELINTO)
 				ereport(ERROR,
 						(errcode(ERRCODE_SYNTAX_ERROR),
-					 errmsg("query \"%s\" is not a SELECT", expr->query)));
+						 errmsg("query is SELECT INTO, but it should be plain SELECT"),
+						 errcontext("query: %s", expr->query)));
+			else
+				ereport(ERROR,
+						(errcode(ERRCODE_SYNTAX_ERROR),
+						 errmsg("query is not a SELECT"),
+						 errcontext("query: %s", expr->query)));
+		}
 	}
 	else
 	{
@@ -5644,7 +5657,8 @@ exec_eval_expr(PLpgSQL_execstate *estate,
 	if (rc != SPI_OK_SELECT)
 		ereport(ERROR,
 				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
-				 errmsg("query \"%s\" did not return data", expr->query)));
+				 errmsg("query did not return data"),
+				 errcontext("query: %s", expr->query)));
 
 	/*
 	 * Check that the expression returns exactly one column...
@@ -5652,11 +5666,11 @@ exec_eval_expr(PLpgSQL_execstate *estate,
 	if (estate->eval_tuptable->tupdesc->natts != 1)
 		ereport(ERROR,
 				(errcode(ERRCODE_SYNTAX_ERROR),
-				 errmsg_plural("query \"%s\" returned %d column",
+				 errmsg_plural("query returned %d column",
-							   "query \"%s\" returned %d columns",
+							   "query returned %d columns",
 							   estate->eval_tuptable->tupdesc->natts,
-							   expr->query,
+							   estate->eval_tuptable->tupdesc->natts),
-							   estate->eval_tuptable->tupdesc->natts)));
+				 errcontext("query: %s", expr->query)));
 
 	/*
 	 * ... and get the column's datatype.
@@ -5680,8 +5694,8 @@ exec_eval_expr(PLpgSQL_execstate *estate,
 	if (estate->eval_processed != 1)
 		ereport(ERROR,
 				(errcode(ERRCODE_CARDINALITY_VIOLATION),
-				 errmsg("query \"%s\" returned more than one row",
+				 errmsg("query returned more than one row"),
-						expr->query)));
+				 errcontext("query: %s", expr->query)));
 
 	/*
 	 * Return the single result Datum.
@@ -5748,9 +5762,22 @@ exec_run_select(PLpgSQL_execstate *estate,
 	rc = SPI_execute_plan_with_paramlist(expr->plan, paramLI,
 										 estate->readonly_func, maxtuples);
 	if (rc != SPI_OK_SELECT)
+	{
+		/*
+		 * SELECT INTO deserves a special error message, because "query is not
+		 * a SELECT" is not very helpful in that case.
+		 */
+		if (rc == SPI_OK_SELINTO)
 			ereport(ERROR,
 					(errcode(ERRCODE_SYNTAX_ERROR),
-				 errmsg("query \"%s\" is not a SELECT", expr->query)));
+					 errmsg("query is SELECT INTO, but it should be plain SELECT"),
+					 errcontext("query: %s", expr->query)));
+		else
+			ereport(ERROR,
+					(errcode(ERRCODE_SYNTAX_ERROR),
+					 errmsg("query is not a SELECT"),
+					 errcontext("query: %s", expr->query)));
+	}
 
 	/* Save query results for eventual cleanup */
 	Assert(estate->eval_tuptable == NULL);