We use spider as an example. The test spider/bugfix.mdev_26345 fails with --view-protocol.

A simpler testcase, without using view protocol, could be:

# ... (setup)

create table t2 (a int, b int, PRIMARY KEY (a, b));

create table t1 (a int, b int, PRIMARY KEY (a, b)) ENGINE=Spider

COMMENT='WRAPPER "mysql", srv "srv",TABLE "t2"';

insert into t1 VALUES (1,4), (1,2), (2,11);

create view v as SELECT MIN(b), a FROM t1 WHERE a=1;

select * from v;

# ... (cleanup)

The expected output is

MIN(b)	a

2	1

but the actual one is

MIN(b)	a

NULL	1

The reason is that an Item_temptable_field is created for MIN(b), but the field itself is not added to the actual temp table because it is constant.

More specifically, when a group by handler is successfully created (in JOIN::make_aggr_tables_info), we first call create_tmp_table() to create a temp table for the GBH, but we skip adding the field to the table (in the table->field linked list):

// Create_tmp_table::add_fields > create_tmp_table > JOIN::make_aggr_tables_info:

      if (item->const_item() &&

          param->hidden_field_count < (fieldnr + 1))

        continue; // We don't have to store this

We then call change_to_use_tmp_fields(), in which we create an Item_temptable_field for each of the select field (later become the join->fields Item list after the call join->set_items_ref_array(join->items1) in do_select()):

// change_to_use_tmp_fields > JOIN::make_aggr_tables_info:

    else if ((field= item->get_tmp_table_field()))

    {

      if (item->type() == Item::SUM_FUNC_ITEM && field->table->group)

        item_field= ((Item_sum*) item)->result_item(thd, field);

      else

        item_field= (Item *) new (thd->mem_root) Item_temptable_field(thd, field);

Therefore GBH has no way to store the query result to the (skipped) table field, even if it has the result. Later when writing results from join->fields to the record (select_unit::fill_record) then from the record to a tmp row (select_unit::write_record) that will be sent as results (select_send::send_data), the underlying Field has no value thus NULL.

Why does it work with view but without GBH? Without GBH, no temp table is created for the GBH to store results to, and no Item_temptable_fields is created either. So the mismatch between the temp table fields and join->fields does not happen.

Why does it work without view but with GBH? Without view, TMP_TABLE_ALL_COLUMNS is not set (see the code block below), causing item->get_tmp_table_field() to return NULL, so no Item_temptable_field will be created for MIN(b), and the item will remain the Item_sum_min, whose constant value was computed during optimization, so it will not be NULL without GBH storing any results in the (skipped) field.

// mysql_derived_prepare:

  if (!(derived->table) &&

      derived->derived_result->create_result_table(thd, &unit->types, FALSE,

                                                   (first_select->options |

                                                   thd->variables.option_bits |

                                                   TMP_TABLE_ALL_COLUMNS),

                                                   &derived->alias,

                                                   FALSE, FALSE, FALSE,

                                                   0))

This is a general issue that could happen to any GBH. However, in the mariadb codebase, only spider and the sequence engine use GBH, and I am not sure if a case can be produced for the sequence engine due to restrictions of sequence tables.