sql: fix error when DISTINCT applied to MAP/ARRAY

After this patch, when DISTINCT is used with MAP or ARRAY, an error will
be thrown instead of a segmentation fault.

Part of #6668
Needed for #6773

NO_DOC=Will be added in another commit.
NO_CHANGELOG=Will be added in another commit.

src/box/sql.c

@@ -381,7 +381,16 @@ sql_ephemeral_space_new(const struct sql_space_info *info)
 		parts[i].exclude_null = false;
 		parts[i].sort_order = SORT_ORDER_ASC;
 		parts[i].path = NULL;
-		parts[i].type = info->types[j];
+		enum field_type type = info->types[j];
+		parts[i].type = type;
+		if (!field_type1_contains_type2(FIELD_TYPE_SCALAR, type)) {
+			const char *err =
+				tt_sprintf("field type '%s' is not comparable",
+					   field_type_strs[type]);
+			diag_set(ClientError, ER_SQL_EXECUTE, err);
+			return NULL;
+		}
+
 		parts[i].coll_id = info->coll_ids[j];
 	}
 

test/sql-luatest/gh_6668_problems_with_map_array_test.lua

+local server = require('test.luatest_helpers.server')
+local t = require('luatest')
+local g = t.group()
+
+g.before_all(function()
+    g.server = server:new({alias = 'map_array_problems'})
+    g.server:start()
+end)
+
+g.after_all(function()
+    g.server:stop()
+end)
+
+-- Make sure that DISTINCT cannot be used with ARRAY or MAP.
+g.test_distinct = function()
+    g.server:exec(function()
+        local t = require('luatest')
+        box.execute([[CREATE TABLE ta(i INT PRIMARY KEY, a ARRAY);]])
+        box.execute([[INSERT INTO ta VALUES(1, [1]), (2, [2]);]])
+        local sql = [[SELECT DISTINCT a FROM ta;]]
+        local res = [[Failed to execute SQL statement: ]]..
+                    [[field type 'array' is not comparable]]
+        local _, err = box.execute(sql)
+        t.assert_equals(err.message, res)
+        box.execute([[DROP TABLE ta;]])
+
+        box.execute([[CREATE TABLE tm(i INT PRIMARY KEY, m map);]])
+        sql = [[SELECT DISTINCT m FROM tm;]]
+        res = [[Failed to execute SQL statement: ]]..
+              [[field type 'map' is not comparable]]
+        _, err = box.execute(sql)
+        t.assert_equals(err.message, res)
+        box.execute([[DROP TABLE tm;]])
+    end)
+end