在Optimize_table_order::Optimize_table_order函数中，完成对于表的join顺序的优化。 首先，要设置相应的我们优化的搜索空间表的深度问题； 然后由choose_table_order来
获取相应的表之间的顺序关系；
recalculate_lateral_deps 重新计算相关依赖的关系；
在choose_table_order的过程中，我们会选择greedy_search进行搜索空间的处理，
#0 Optimize_table_order::best_extension_by_limited_search (this=0x7f0744577c60, remaining_tables=31, idx=0, current_search_depth=62)
at /home/leehao/mysql-server/sql/sql_planner.cc:2701
#1 0x00000000038603c7 in Optimize_table_order::greedy_search (this=0x7f0744577c60, remaining_tables=31)
at /home/leehao/mysql-server/sql/sql_planner.cc:2323
#2 0x000000000385fb6f in Optimize_table_order::choose_table_order (this=0x7f0744577c60) at /home/leehao/mysql-server/sql/sql_planner.cc:1999
#3 0x000000000381df54 in JOIN::make_join_plan (this=0x7f068cad91d8) at /home/leehao/mysql-server/sql/sql_optimizer.cc:5159
#4 0x000000000381154f in JOIN::optimize (this=0x7f068cad91d8) at /home/leehao/mysql-server/sql/sql_optimizer.cc:591
#5 0x00000000038c05c3 in Query_block::optimize (this=0x7f068c15f938, thd=0x7f068c001040) at /home/leehao/mysql-server/sql/sql_select.cc:1805
#6 0x00000000039647af in Query_expression::optimize (this=0x7f068c15f1d8, thd=0x7f068c001040, materialize_destination=0x0,
create_iterators=true) at /home/leehao/mysql-server/sql/sql_union.cc:678
#7 0x00000000038be156 in Sql_cmd_dml::execute_inner (this=0x7f068cad2b48, thd=0x7f068c001040) at /home/leehao/mysql-server/sql/sql_select.cc:759
#8 0x00000000038bd766 in Sql_cmd_dml::execute (this=0x7f068cad2b48, thd=0x7f068c001040) at /home/leehao/mysql-server/sql/sql_select.cc:574
#9 0x00000000038433ee in mysql_execute_command (thd=0x7f068c001040, first_level=true) at /home/leehao/mysql-server/sql/sql_parse.cc:4436
#10 0x0000000003845396 in dispatch_sql_command (thd=0x7f068c001040, parser_state=0x7f0744579b60)
at /home/leehao/mysql-server/sql/sql_parse.cc:5033
#11 0x000000000383b915 in dispatch_command (thd=0x7f068c001040, com_data=0x7f074457ac20, command=COM_QUERY)
at /home/leehao/mysql-server/sql/sql_parse.cc:1863
#12 0x0000000003839d51 in do_command (thd=0x7f068c001040) at /home/leehao/mysql-server/sql/sql_parse.cc:1342
在Optimize_table_order::advance_sj_state函数中，我们处理了semi-join optimization，当我们把新的表添加到相应的join prefix中的时候；
get_best_combination 完成依据之前的best_position中的内容来设置相应的join order内容；
在JOIN::create_root_access_path_for_join 函数中，我们依据qep来创建相应的访问路径； 例如： filesort来创建 FileSort； windows类型
则创建 NewWindowingAccessPath这样的类型；以此类推； SingleMaterializeQueryBlock。
如果需要创建相应的parallel访问的时候也需要在这里进行添加相应的access path。
Query_expression::optimize函数中，由CreateIteratorFromAccessPath函数依据我们的access path来创解相应的（access的path->type）iterator。
接下来就是执行阶段： 经过重构后的mysql 查询引擎，越来越像PG查询引擎的架构，代码比较清晰了。
#0 Query_expression::execute (this=0x7f068c15f1d8, thd=0x7f068c001040) at /home/leehao/mysql-server/sql/sql_union.cc:1270
#1 0x00000000038be1e4 in Sql_cmd_dml::execute_inner (this=0x7f068cad2b48, thd=0x7f068c001040) at /home/leehao/mysql-server/sql/sql_select.cc:774
#2 0x00000000038bd766 in Sql_cmd_dml::execute (this=0x7f068cad2b48, thd=0x7f068c001040) at /home/leehao/mysql-server/sql/sql_select.cc:574
#3 0x00000000038433ee in mysql_execute_command (thd=0x7f068c001040, first_level=true) at /home/leehao/mysql-server/sql/sql_parse.cc:4436
#4 0x0000000003845396 in dispatch_sql_command (thd=0x7f068c001040, parser_state=0x7f0744579b60)
at /home/leehao/mysql-server/sql/sql_parse.cc:5033
#5 0x000000000383b915 in dispatch_command (thd=0x7f068c001040, com_data=0x7f074457ac20, command=COM_QUERY)
at /home/leehao/mysql-server/sql/sql_parse.cc:1863
由Query_expression中的execute函数来完成相应的对于执行计划的执行操作； 由具体的函数 Query_expression::ExecuteIteratorQuery来进行执行；
首先，会使用query_result->send_result_set_metadata 来发送结果集的元数据信息，即：都有哪些列，列的类型，大小等等信息；
具体执行有Query_result_send::send_result_set_metadata来完成。 会调用thd的 THD::send_result_metadata 函数来完成对于结果集的元数据信息
的发送；
#0 SortingIterator::Init (this=0x7f068cceb4a0) at /home/leehao/mysql-server/sql/sorting_iterator.cc:439
#1 0x000000000396653d in Query_expression::ExecuteIteratorQuery (this=0x7f068c15f1d8, thd=0x7f068c001040)
at /home/leehao/mysql-server/sql/sql_union.cc:1224
#2 0x00000000039668cb in Query_expression::execute (this=0x7f068c15f1d8, thd=0x7f068c001040) at /home/leehao/mysql-server/sql/sql_union.cc:1284
#3 0x00000000038be1e4 in Sql_cmd_dml::execute_inner (this=0x7f068cad2b48, thd=0x7f068c001040) at /home/leehao/mysql-server/sql/sql_select.cc:774
#4 0x00000000038bd766 in Sql_cmd_dml::execute (this=0x7f068cad2b48, thd=0x7f068c001040) at /home/leehao/mysql-server/sql/sql_select.cc:574
#5 0x00000000038433ee in mysql_execute_command (thd=0x7f068c001040, first_level=true) at /home/leehao/mysql-server/sql/sql_parse.cc:4436
#6 0x0000000003845396 in dispatch_sql_command (thd=0x7f068c001040, parser_state=0x7f0744579b60)
at /home/leehao/mysql-server/sql/sql_parse.cc:5033
从下面的调用栈，我们可以看到，首先执行的是一个sortIterator, 因为我们语句最外使用的是order by语句。
set debug=”+d,info,error,query,enter,general,where:O,/tmp/mysqld.trace”