调试源码，分析函数的具体执行过程，在客户端，执行select to_char(‘test’) from dual。
跟踪堆栈：pthread_start→handle_one_connection→do_handle_one_connect→do_command→dispatch_command，确定SQL函数的执行入口为dispatch_command
调试跟踪SQL内部执行过程为：
2.SQL函数执行过程
分析堆栈信息，确定SQL函数主要执行过程为：
- SQL_PARSE 语法解析
- SQL_RESOLVER prepare准备执行
- SQL_EXCUTOR 具体执行函数
SQL_PARSE堆栈：
1 To_char_instantiator::instantiate(To_char_instantiator * const this, THD * thd, PT_item_list * args) (/home/bob/work/percona-server/sql/item_create.cc:785)
2 (anonymous namespace)::Function_factory<To_char_instantiator>::create_func((anonymous namespace)::Function_factory<To_char_instantiator> * const this, THD * thd, LEX_STRING function_name, PT_item_list * item_list) (/home/bob/work/percona-server/sql/item_create.cc:1203)
3 PTI_function_call_generic_ident_sys::itemize(PTI_function_call_generic_ident_sys * const this, Parse_context * pc, Item ** res) (/home/bob/work/percona-server/sql/parse_tree_items.cc:259)
4 PTI_expr_with_alias::itemize(PTI_expr_with_alias * const this, Parse_context * pc, Item ** res) (/home/bob/work/percona-server/sql/parse_tree_items.cc:337)
5 PT_item_list::contextualize(PT_item_list * const this, Parse_context * pc) (/home/bob/work/percona-server/sql/parse_tree_helpers.h:112)
6 PT_select_item_list::contextualize(PT_select_item_list * const this, Parse_context * pc) (/home/bob/work/percona-server/sql/parse_tree_nodes.cc:3813)
7 PT_query_specification::contextualize(PT_query_specification * const this, Parse_context * pc) (/home/bob/work/percona-server/sql/parse_tree_nodes.cc:1551)
8 PT_query_expression::contextualize(PT_query_expression * const this, Parse_context * pc) (/home/bob/work/percona-server/sql/parse_tree_nodes.cc:4178)
9 PT_select_stmt::make_cmd(PT_select_stmt * const this, THD * thd) (/home/bob/work/percona-server/sql/parse_tree_nodes.cc:648)
10 LEX::make_sql_cmd(LEX * const this, Parse_tree_root * parse_tree) (/home/bob/work/percona-server/sql/sql_lex.cc:5237)
11 THD::sql_parser(THD * const this) (/home/bob/work/percona-server/sql/sql_class.cc:2978)
12 parse_sql(THD * thd, Parser_state * parser_state, Object_creation_ctx * creation_ctx) (/home/bob/work/percona-server/sql/sql_parse.cc:7333)
13 dispatch_sql_command(THD * thd, Parser_state * parser_state, bool update_userstat) (/home/bob/work/percona-server/sql/sql_parse.cc:5237)
14 dispatch_command(THD * thd, const COM_DATA * com_data, enum_server_command command) (/home/bob/work/percona-server/sql/sql_parse.cc:1978)
15 do_command(THD * thd) (/home/bob/work/percona-server/sql/sql_parse.cc:1426)
16 handle_connection(void * arg) (/home/bob/work/percona-server/sql/conn_handler/connection_handler_per_thread.cc:307)
17 pfs_spawn_thread(void * arg) (/home/bob/work/percona-server/storage/perfschema/pfs.cc:2899)
18 libpthread.so.0!start_thread(void * arg) (/build/glibc-eX1tMB/glibc-2.31/nptl/pthread_create.c:477)
19 libc.so.6!clone() (/build/glibc-eX1tMB/glibc-2.31/sysdeps/unix/sysv/linux/x86_64/clone.S:95)SQL_RESOLVER堆栈：
1 Item_func_to_char::resolve_type(Item_func_to_char * const this, THD * thd) (/home/bob/work/percona-server/sql/item_timefunc.cc:3821)
2 Item_func::fix_fields(Item_func * const this, THD * thd) (/home/bob/work/percona-server/sql/item_func.cc:309)
3 Item_str_func::fix_fields(Item_str_func * const this, THD * thd, Item ** ref) (/home/bob/work/percona-server/sql/item_strfunc.cc:161)
4 setup_fields(THD * thd, ulong want_privilege, bool allow_sum_func, bool split_sum_funcs, bool column_update, const mem_root_deque<Item*> * typed_items, mem_root_deque<Item*> * fields, Ref_item_array ref_item_array) (/home/bob/work/percona-server/sql/sql_base.cc:9216)
5 Query_block::prepare(Query_block * const this, THD * thd, mem_root_deque<Item*> * insert_field_list) (/home/bo6 b/work/percona-server/sql/sql_resolver.cc:275)
7 Sql_cmd_select::prepare_inner(Sql_cmd_select * const this, THD * thd) (/home/bob/work/percona-server/sql/sql_select.cc:467)
8 Sql_cmd_dml::prepare(Sql_cmd_dml * const this, THD * thd) (/home/bob/work/percona-server/sql/sql_select.cc:389)
9 Sql_cmd_dml::execute(Sql_cmd_dml * const this, THD * thd) (/home/bob/work/percona-server/sql/sql_select.cc:522)
10 mysql_execute_command(THD * thd, bool first_level) (/home/bob/work/percona-server/sql/sql_parse.cc:4740)
11 dispatch_sql_command(THD * thd, Parser_state * parser_state, bool update_userstat) (/home/bob/work/percona-server/sql/sql_parse.cc:5337)
12 dispatch_command(THD * thd, const COM_DATA * com_data, enum_server_command command) (/home/bob/work/percona-server/sql/sql_parse.cc:1978)
13 do_command(THD * thd) (/home/bob/work/percona-server/sql/sql_parse.cc:1426)
14 handle_connection(void * arg) (/home/bob/work/percona-server/sql/conn_handler/connection_handler_per_thread.cc:307)
15 pfs_spawn_thread(void * arg) (/home/bob/work/percona-server/storage/perfschema/pfs.cc:2899)
16 libpthread.so.0!start_thread(void * arg) (/build/glibc-eX1tMB/glibc-2.31/nptl/pthread_create.c:477)
17 libc.so.6!clone() (/build/glibc-eX1tMB/glibc-2.31/sysdeps/unix/sysv/linux/x86_64/clone.S:95)SQL_EXCUTOR 堆栈：
1 Item_func_to_char::val_str(Item_func_to_char * const this, String * str) (/home/bob/work/percona-server/sql/item_timefunc.cc:3915)
2 Item::send(Item * const this, Protocol * protocol, String * buffer) (/home/bob/work/percona-server/sql/item.cc:7025)
3 THD::send_result_set_row(THD * const this, const mem_root_deque<Item*> & row_items) (/home/bob/work/percona-server/sql/sql_class.cc:2793)
4 Query_result_send::send_data(Query_result_send * const this, THD * thd, const mem_root_deque<Item*> & items) (/home/bob/work/percona-server/sql/query_result.cc:100)
5 Query_expression::ExecuteIteratorQuery(Query_expression * const this, THD * thd) (/home/bob/work/percona-server/sql/sql_union.cc:1249)
6 Query_expression::execute(Query_expression * const this, THD * thd) (/home/bob/work/percona-server/sql/sql_union.cc:1287)
7 Sql_cmd_dml::execute_inner(Sql_cmd_dml * const this, THD * thd) (/home/bob/work/percona-server/sql/sql_select.cc:791)
8 Sql_cmd_dml::execute(Sql_cmd_dml * const this, THD * thd) (/home/bob/work/percona-server/sql/sql_select.cc:575)
9 mysql_execute_command(THD * thd, bool first_level) (/home/bob/work/percona-server/sql/sql_parse.cc:4740)
10 dispatch_sql_command(THD * thd, Parser_state * parser_state, bool update_userstat) (/home/bob/work/percona-server/sql/sql_parse.cc:5337)
11 dispatch_command(THD * thd, const COM_DATA * com_data, enum_server_command command) (/home/bob/work/percona-server/sql/sql_parse.cc:1978)
12 do_command(THD * thd) (/home/bob/work/percona-server/sql/sql_parse.cc:1426)
13 handle_connection(void * arg) (/home/bob/work/percona-server/sql/conn_handler/connection_handler_per_thread.cc:307)
14 pfs_spawn_thread(void * arg) (/home/bob/work/percona-server/storage/perfschema/pfs.cc:2899)
15 libpthread.so.0!start_thread(void * arg) (/build/glibc-eX1tMB/glibc-2.31/nptl/pthread_create.c:477)
16 libc.so.6!clone() (/build/glibc-eX1tMB/glibc-2.31/sysdeps/unix/sysv/linux/x86_64/clone.S:95)SQL_PARSE： SQL解析生成AST语法的语法树，to_char函数，实际中已解析为Item_func_to_char的语法树节点。
SQL_RESOLVER： 准备阶段prepare，初始化赋值，如Item_func_to_char::resolve_type设定函数转换后的数据类型。
SQL_EXCUTOR： 执行阶段，执行to_char函数功能，通过Item_func_to_char::val_str，实际功能处理过程。
