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Finally, the parsing begins. This is the heart of what goes on inside the server when it processes a query. The parser code is located in a couple of places (like so much of the rest of the system). It isn t that hard to follow if you realize that while being highly organized, the code is not structured to match the architecture. The function you re examining now is the mysql_parse() function (located in /sql/ sql_parse.cc). Its job is to check the query cache for the results of a previously executed query that has the same result set, then pass control to the lexical parser, and finally route the command to the query optimizer. Listing 3-7 shows a condensed view of the mysql_parse() function.
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CHAPTER 3 A TOUR OF THE MYSQL SOURCE CODE
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Listing 3-7. The mysql_parse() Function void mysql_parse(THD *thd, char *inBuf, uint length) { ... if (query_cache_send_result_to_client(thd, inBuf, length) <= 0) { LEX *lex= thd->lex; ... if (!yyparse((void *)thd) && ! thd->is_fatal_error) { ... mysql_execute_command(thd); query_cache_end_of_result(thd); ... } ... } The first thing to notice is the call to the query cache. The query cache stores all of the most frequently requested queries complete with the results. If the query is already in the query cache, you re done! All that is left is to return the results to the client. No parsing, optimizing, or even executing is necessary. How cool is that For the sake of our exploration, let s assume the query cache does not contain a copy of the example query. In this case, the function creates a new LEX structure to contain the internal representation of the query. This structure is filled out by the Lex/YACC parser, shown in Listing 3-8. Listing 3-8. The SELECT Lex/YACC Parsing Code Excerpt select: select_init { LEX *lex= Lex; lex->sql_command= SQLCOM_SELECT; } ;
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CHAPTER 3 A TOUR OF THE MYSQL SOURCE CODE
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/* Need select_init2 for subselects. */ select_init: SELECT_SYM select_init2 | '(' select_paren ')' union_opt; select_paren: SELECT_SYM select_part2 { LEX *lex= Lex; SELECT_LEX * sel= lex->current_select; if (sel->set_braces(1)) { yyerror(ER(ER_SYNTAX_ERROR)); YYABORT; } if (sel->linkage == UNION_TYPE && !sel->master_unit()->first_select()->braces) { yyerror(ER(ER_SYNTAX_ERROR)); YYABORT; } /* select in braces, can't contain global parameters */ if (sel->master_unit()->fake_select_lex) sel->master_unit()->global_parameters= sel->master_unit()->fake_select_lex; } | '(' select_paren ')'; select_init2: select_part2 { LEX *lex= Lex; SELECT_LEX * sel= lex->current_select; if (lex->current_select->set_braces(0)) { yyerror(ER(ER_SYNTAX_ERROR)); YYABORT; } if (sel->linkage == UNION_TYPE && sel->master_unit()->first_select()->braces) { yyerror(ER(ER_SYNTAX_ERROR)); YYABORT; } }
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CHAPTER 3 A TOUR OF THE MYSQL SOURCE CODE
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union_clause ; select_part2: { LEX *lex= Lex; SELECT_LEX *sel= lex->current_select; if (sel->linkage != UNION_TYPE) mysql_init_select(lex); lex->current_select->parsing_place= SELECT_LIST; } select_options select_item_list { Select->parsing_place= NO_MATTER; } select_into select_lock_type; select_into: opt_order_clause opt_limit_clause {} | into | select_from | into select_from | select_from into; select_from: FROM join_table_list where_clause group_clause having_clause opt_order_clause opt_limit_clause procedure_clause | FROM DUAL_SYM where_clause opt_limit_clause /* oracle compatibility: oracle always requires FROM clause, and DUAL is system table without fields. Is "SELECT 1 FROM DUAL" any better than "SELECT 1" Hmmm :) */ ; select_options: /* empty*/ | select_option_list { if (Select->options & SELECT_DISTINCT && Select->options & SELECT_ALL) { my_error(ER_WRONG_USAGE, MYF(0), "ALL", "DISTINCT"); YYABORT; } } ;
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CHAPTER 3 A TOUR OF THE MYSQL SOURCE CODE
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select_option_list: select_option_list select_option | select_option; select_option: STRAIGHT_JOIN { Select->options|= SELECT_STRAIGHT_JOIN; } | HIGH_PRIORITY { if (check_simple_select()) YYABORT; Lex->lock_option= TL_READ_HIGH_PRIORITY; } | DISTINCT { Select->options|= SELECT_DISTINCT; } | SQL_SMALL_RESULT { Select->options|= SELECT_SMALL_RESULT; } | SQL_BIG_RESULT { Select->options|= SELECT_BIG_RESULT; } | SQL_BUFFER_RESULT { if (check_simple_select()) YYABORT; Select->options|= OPTION_BUFFER_RESULT; } | SQL_CALC_FOUND_ROWS { if (check_simple_select()) YYABORT; Select->options|= OPTION_FOUND_ROWS; } | SQL_NO_CACHE_SYM { Lex->safe_to_cache_query=0; } | SQL_CACHE_SYM { Lex->select_lex.options|= OPTION_TO_QUERY_CACHE; } | ALL { Select->options|= SELECT_ALL; } ; select_lock_type: /* empty */ | FOR_SYM UPDATE_SYM { LEX *lex=Lex; lex->current_select->set_lock_for_tables(TL_WRITE); lex->safe_to_cache_query=0; }
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CHAPTER 3 A TOUR OF THE MYSQL SOURCE CODE
| LOCK_SYM IN_SYM SHARE_SYM MODE_SYM { LEX *lex=Lex; lex->current_select-> set_lock_for_tables(TL_READ_WITH_SHARED_LOCKS); lex->safe_to_cache_query=0; } ; select_item_list: select_item_list ',' select_item | select_item | '*' { THD *thd= YYTHD; if (add_item_to_list(thd, new Item_field(&thd->lex->current_select-> context, NULL, NULL, "*"))) YYABORT; (thd->lex->current_select->with_wild)++; };
select_item: remember_name select_item2 remember_end select_alias { if (add_item_to_list(YYTHD, $2)) YYABORT; if ($4.str) { $2->set_name($4.str, $4.length, system_charset_info); $2->is_autogenerated_name= FALSE; } else if (!$2->name) { char *str = $1; if (str[-1] == '`') str--; $2->set_name(str,(uint) ($3 - str), YYTHD->charset()); } }; I have included an excerpt from the Lex/YACC parser that shows how the SELECT token is identified and passed through the YACC code to be parsed. The way you should read this code
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