MDEV-26247 Clean up spider_fields

Spider gbh query rewrite should get table for fields in a simple way.
Add a method spider_fields::find_table that searches its table holders
to find table for a given field. This way we will be able to get rid
of the first pass during the gbh creation where field_chains and
field_holders are created.

We also check that the field belongs to a spider table while walking
through the query, so we could remove
all_query_fields_are_query_table_members(). However, this requires an
earlier creation of the table_holder so that tables are added before
checking. We do that, and in doing so, also decouple table_holder and
spider_fields

Remove unused methods and fields. Add comments.

storage/spider/spd_db_conn.cc

@@ -9167,19 +9167,20 @@ int spider_db_open_item_ident(
       } else {
         if (str)
         {
-          SPIDER_FIELD_CHAIN *field_chain = fields->get_next_field_chain();
-          SPIDER_FIELD_HOLDER *field_holder = field_chain->field_holder;
-          spider = field_holder->spider;
+          SPIDER_TABLE_HOLDER *table= fields->find_table(field);
+          /* If table or table->spider is NULL the GBH creation
+          would have been skipped the first pass (see below). */
+          spider = table->spider;
           share = spider->share;
           if ((error_num = share->dbton_share[dbton_id]->
             append_column_name_with_alias(str, field->field_index,
-            field_holder->alias->ptr(), field_holder->alias->length())))
+            table->alias->ptr(), table->alias->length())))
             DBUG_RETURN(error_num);
-        } else {
-          if ((error_num = fields->add_field(field)))
+        } else
         {
-            DBUG_RETURN(error_num);
-          }
+          SPIDER_TABLE_HOLDER *table= fields->find_table(field);
+          if (!table || !table->spider)
+            DBUG_RETURN(ER_SPIDER_COND_SKIP_NUM);
         }
       }
     }
@@ -9298,21 +9299,20 @@ int spider_db_open_item_field(
         } else {
           if (str)
           {
-            SPIDER_FIELD_CHAIN *field_chain = fields->get_next_field_chain();
-            SPIDER_FIELD_HOLDER *field_holder = field_chain->field_holder;
-            spider = field_holder->spider;
+            SPIDER_TABLE_HOLDER *table= fields->find_table(field);
+            /* If table or table->spider is NULL the GBH creation
+            would have been skipped the first pass (see below). */
+            spider = table->spider;
             share = spider->share;
-            field = spider->field_exchange(field);
-            DBUG_ASSERT(field);
             if ((error_num = share->dbton_share[dbton_id]->
               append_column_name_with_alias(str, field->field_index,
-              field_holder->alias->ptr(), field_holder->alias->length())))
+              table->alias->ptr(), table->alias->length())))
               DBUG_RETURN(error_num);
-          } else {
-            if ((error_num = fields->add_field(field)))
+          } else
           {
-              DBUG_RETURN(error_num);
-            }
+            SPIDER_TABLE_HOLDER *table= fields->find_table(field);
+            if (!table || !table->spider)
+              DBUG_RETURN(ER_SPIDER_COND_SKIP_NUM);
           }
           DBUG_RETURN(0);
         }

storage/spider/spd_db_include.h

@@ -604,34 +604,26 @@ typedef struct spider_conn_holder
   spider_conn_holder *next;
 } SPIDER_CONN_HOLDER;
 
+/* Record information of a local (spider) table, for use of the spider
+group by handler. */
 typedef struct spider_table_holder
 {
   TABLE *table;
   ha_spider *spider;
+  /* alias of the table, in the form of tk, where k is the index of
+  the table from `query->from' indexed by next_local. */
   spider_string *alias;
 } SPIDER_TABLE_HOLDER;
 
-typedef struct spider_field_holder
-{
-  Field *field;
-  ha_spider *spider;
-  spider_string *alias;
-  spider_field_holder *next;
-} SPIDER_FIELD_HOLDER;
-
-typedef struct spider_field_chain
-{
-  spider_field_holder *field_holder;
-  spider_field_chain *next;
-} SPIDER_FIELD_CHAIN;
-
+/* For use of the spider group by handler. */
 class spider_fields
 {
   uint dbton_count;
   uint current_dbton_num;
   uint dbton_ids[SPIDER_DBTON_SIZE];
+  /* Number of tables in `query->from'. */
   uint table_count;
-  uint current_table_num;
+  /* All tables in `query->from', in the same order by next_local. */
   SPIDER_TABLE_HOLDER *table_holder;
   SPIDER_LINK_IDX_CHAIN *first_link_idx_chain;
   SPIDER_LINK_IDX_CHAIN *last_link_idx_chain;
@@ -640,13 +632,6 @@ class spider_fields
   SPIDER_CONN_HOLDER *first_conn_holder;
   SPIDER_CONN_HOLDER *last_conn_holder;
   SPIDER_CONN_HOLDER *current_conn_holder;
-  SPIDER_FIELD_HOLDER *first_field_holder;
-  SPIDER_FIELD_HOLDER *last_field_holder;
-  SPIDER_FIELD_HOLDER *current_field_holder;
-  SPIDER_FIELD_CHAIN *first_field_chain;
-  SPIDER_FIELD_CHAIN *last_field_chain;
-  SPIDER_FIELD_CHAIN *current_field_chain;
-  Field **first_field_ptr;
   Field **current_field_ptr;
 public:
   spider_fields();
@@ -702,24 +687,14 @@ class spider_fields
   void free_conn_holder(
     SPIDER_CONN_HOLDER *conn_holder_arg
   );
-  SPIDER_TABLE_HOLDER *add_table(
-    ha_spider *spider_arg
-  );
-  bool all_query_fields_are_query_table_members();
-  int create_table_holder(
+  SPIDER_TABLE_HOLDER *find_table(Field *field);
+  void set_table_holder(
+    SPIDER_TABLE_HOLDER *table_holder_arg,
     uint table_count_arg
   );
-  void set_pos_to_first_table_holder();
-  SPIDER_TABLE_HOLDER *get_next_table_holder();
+  SPIDER_TABLE_HOLDER *get_first_table_holder();
   SPIDER_TABLE_HOLDER *get_table_holder(TABLE *table);
   uint get_table_count();
-  int add_field(Field *field_arg);
-  SPIDER_FIELD_HOLDER *create_field_holder();
-  void set_pos_to_first_field_holder();
-  SPIDER_FIELD_HOLDER *get_next_field_holder();
-  SPIDER_FIELD_CHAIN *create_field_chain();
-  void set_pos_to_first_field_chain();
-  SPIDER_FIELD_CHAIN *get_next_field_chain();
   void set_field_ptr(Field **field_arg);
   Field **get_next_field_ptr();
   int ping_table_mon_from_table(

storage/spider/spd_db_mysql.cc

@@ -15639,8 +15639,7 @@ int spider_mbase_handler::append_from_and_tables_part(
     default:
       DBUG_RETURN(0);
   }
-  fields->set_pos_to_first_table_holder();
-  table_holder = fields->get_next_table_holder();
+  table_holder = fields->get_first_table_holder();
   table_list = table_holder->table->pos_in_table_list;
   error_num = spider_db_mbase_utility->append_from_and_tables(
     table_holder->spider, fields, str,

storage/spider/spd_db_mysql.h

@@ -676,8 +676,11 @@ class spider_mbase_share: public spider_db_share
   spider_string      *show_table_status;
   spider_string      *show_records;
   spider_string      *show_index;
+  /* The remote table names */
   spider_string      *table_names_str;
+  /* The remote db names */
   spider_string      *db_names_str;
+  /* fixme: this field looks useless */
   spider_string      *db_table_str;
 #ifdef SPIDER_HAS_HASH_VALUE_TYPE
   my_hash_value_type *db_table_str_hash_value;