MDEV-26996 Reverse-ordered indexes: improve print-out

When printing a range into optimizer trace, print DESC for columns
that are reverse-ordered, for example:

   "(4) <= (key1 DESC) <= (2)"

mysql-test/main/desc_index_range.result

@@ -13,9 +13,9 @@ json_detailed(json_extract(trace, '$**.range_access_plan.ranges'))
 [
     
     [
-        "(6) <= (a) <= (6)",
-        "(4) <= (a) <= (4)",
-        "(2) <= (a) <= (2)"
+        "(6) <= (a DESC) <= (6)",
+        "(4) <= (a DESC) <= (4)",
+        "(2) <= (a DESC) <= (2)"
     ]
 ]
 set optimizer_trace=default;
@@ -57,7 +57,7 @@ json_detailed(json_extract(trace, '$**.range_access_plan.ranges'))
 [
     
     [
-        "(8,50) <= (a,b)"
+        "(8,50) <= (a,b DESC)"
     ]
 ]
 select * from t1 force index(ab) where a>=8 and b<=50;
@@ -104,7 +104,7 @@ json_detailed(json_extract(trace, '$**.range_access_plan.ranges'))
 [
     
     [
-        "(2,80) <= (a,b) <= (4,50)"
+        "(2,80) <= (a,b DESC) <= (4,50)"
     ]
 ]
 select * from t1 where a between 2 and 4 and b between 50 and 80;
@@ -138,7 +138,7 @@ json_detailed(json_extract(trace, '$**.range_access_plan.ranges'))
 [
     
     [
-        "(4) <= (a) <= (2)"
+        "(4) <= (a DESC) <= (2)"
     ]
 ]
 explain
@@ -151,7 +151,7 @@ json_detailed(json_extract(trace, '$**.range_access_plan.ranges'))
 [
     
     [
-        "(4,80) <= (a,b) <= (2,50)"
+        "(4,80) <= (a DESC,b DESC) <= (2,50)"
     ]
 ]
 drop table t2;

sql/opt_range.cc

@@ -16637,6 +16637,8 @@ void print_keyparts_name(String *out, const KEY_PART_INFO *key_part,
       else
         out->append(STRING_WITH_LEN(","));
       out->append(key_part->field->field_name);
+      if (key_part->key_part_flag & HA_REVERSE_SORT)
+        out->append(STRING_WITH_LEN(" DESC"));
     }
     else
       break;