MDEV-35673 Item_subselect::used_tables_cache: outer-reference name resolution and query-merge maintenance

[mariadb-RexJohnston]

Split from MDEV-32294, discovered while inspecting how
Item_subselect::used_tables_cache is recalculated across the 1st and 2nd
executions of a prepared statement.

Core name-resolution / used_tables rework:
 - Maintain SELECT_LEX::outer_references_resolved_here, a statement-memory
   list of the outer references resolved in each select_lex (relies on
   MDEV-30073 so these are not freed at end of PS execution), and rewrite
   Item_subselect::recalc_used_tables() to compute used_tables_cache from
   it (Item_belongs_to + Field_fixer).
 - Preserve Item_field::depended_from across executions and use it in
   fix_fields/fix_outer_field instead of re-running fix_outer_field, so
   2nd-execution resolution is stable.
 - Maintain nest_level/nest_level_base and merged_into during derived and
   semi-join merges, and update outer_references_resolved_here when a
   subquery is merged into its parent.

find_field_in_tables: wrap a HAVING outer reference in an Item_ref during
name resolution (previously done only in Item_field::fix_outer_field),
fixing a marked_for_read() assertion on queries such as
  SELECT 1 FROM (SELECT a FROM t1) b HAVING (SELECT b.a)=1

create_view_field: resolve view-field substitutions against
current_select and allocate them on statement memory.

Ban EXPLAIN EXTENDED under the mtr --ps-protocol (warning output differs
because some select_transformers do not run during PREPARE).

Tests: introduce main.outer_reference with ~90 labelled cases and the
execute_various_ways.inc harness, which runs each query six ways (direct,
prepare+execute twice, derived table, view, CTE, stored procedure) and,
via include/evw_capture.inc, cross-checks that all six return identical
rows.

[gemini-code-assist]

Code Review

This pull request refactors how outer references are tracked and resolved across subquery merges and prepared statement executions, introducing a list of resolved outer references on SELECT_LEX and updating the recalc_used_tables logic. The review feedback highlights several safety and robustness improvements, including adding critical NULL pointer checks in Field_fixer::visit_field and SELECT_LEX::merge_subquery to prevent server crashes, avoiding unsafe downcasting of real_item() in Item_subselect::recalc_used_tables, addressing style guide violations regarding spacing after if statements, and ensuring consistent use of nullptr for pointer initializations.

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[gemini-code-assist]

In SELECT_LEX::merge_subquery, the order_list and group_list are walked. However, order->item or *order->item can be NULL in some edge cases (e.g., if resolution failed or was cleared). Dereferencing them without a NULL check can lead to a server crash. We should add defensive NULL checks, similar to how where and having are checked.

  for (order= subq_select->order_list.first ; order; order= order->next)
  {
    if (order->item && *order->item)
      (*order->item)->walk(&Item::select_update_base_processor, TRUE,
                           (void*) &update);
  }

  for (order= subq_select->group_list.first ; order; order= order->next)
  {
    if (order->item && *order->item)
      (*order->item)->walk(&Item::select_update_base_processor, TRUE,
                           (void*) &update);
  }

[gemini-code-assist]

Missing space after if keyword. According to the project's coding standards, there should be a space between if and the opening parenthesis.

    if (parent_select)                                    // can happen with SP

References

1. Follow the project's specific coding standards (e.g., CODING_STANDARDS.md) for code formatting and spacing, such as spacing around assignment operators, rather than generic style guides like the Google C++ Style Guide. ^(link)

[gemini-code-assist]

Missing space after if keyword. According to the project's coding standards, there should be a space between if and the opening parenthesis.

  if (!fixed())

References

1. Follow the project's specific coding standards (e.g., CODING_STANDARDS.md) for code formatting and spacing, such as spacing around assignment operators, rather than generic style guides like the Google C++ Style Guide. ^(link)

[gemini-code-assist]

Inconsistent pointer initialization. order->view_ref is initialized to 0 while other pointers in the same block are initialized to nullptr. We should use nullptr consistently for all pointer initializations in modern C++ code.

  order->view_ref= nullptr;   // may be read in find_order_in_list() before being set
  order->resolution= NOT_RESOLVED;
  order->select_item= nullptr;
  order->from_field= nullptr;