TODO: st_select_lex_unit::optimize is modified about mysql-server-mysql-8.0.30 HOT 3 OPEN

bool Query_block::optimize(THD *thd) {
DBUG_TRACE;

assert(join == nullptr);
JOIN const join_local = new (thd->mem_root) JOIN(thd, this);
if (!join_local) return true; / purecov: inspected */

/*
Updating Query_block::join requires acquiring THD::LOCK_query_plan
to avoid races when EXPLAIN FOR CONNECTION is used.
*/
thd->lock_query_plan();
join = join_local;
thd->unlock_query_plan();

if (join->optimize()) return true;

if (join->zero_result_cause && !is_implicitly_grouped()) return false;

for (Query_expression *query_expression = first_inner_query_expression();
query_expression;
query_expression = query_expression->next_query_expression()) {
// Derived tables and const subqueries are already optimized
if (!query_expression->is_optimized() &&
query_expression->optimize(thd, /materialize_destination=/nullptr,
/create_iterators=/false))
return true;
}

return false;
}

from mysql-server-mysql-8.0.30.

the definition of query_expression:

/**
This class represents a query expression (one query block or
several query blocks combined with UNION).
/
class Query_expression {
/*
Intrusive double-linked list of all query expressions
immediately contained within the same query block.
*/
Query_expression *next;
Query_expression **prev;

Query_block:
/**
This class represents a query block, aka a query specification, which is
a query consisting of a SELECT keyword, followed by a table list,
optionally followed by a WHERE clause, a GROUP BY, etc.
/
class Query_block {
public:
/*
@note the group_by and order_by lists below will probably be added to the
constructor when the parser is converted into a true bottom-up design.

      //SQL_I_LIST<ORDER> *group_by, SQL_I_LIST<ORDER> order_by

*/
Query_block(MEM_ROOT *mem_root, Item *where, Item *having);

from mysql-server-mysql-8.0.30.

it's similar to st_select_lex_unit and st_select_lex.

the definition:
/**
This class represents a query block, aka a query specification, which is
a query consisting of a SELECT keyword, followed by a table list,
optionally followed by a WHERE clause, a GROUP BY, etc.
*/
class st_select_lex: public Sql_alloc

/**
This class represents a query expression (one query block or
several query blocks combined with UNION).
*/
class st_select_lex_unit: public Sql_alloc
{

the Query_expression = st_select_lex_unit, and Query_block = st_select_lex.

from mysql-server-mysql-8.0.30.