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CHAPTER 3 A TOUR OF THE MYSQL SOURCE CODE
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/* str_values's main purpose is to be used to cache the value in save_in_field */ String str_value; my_string name; /* Name from select */ /* Original item name (if it was renamed)*/ my_string orig_name; Item *next; uint32 max_length; uint name_length; /* Length of name */ uint8 marker, decimals; my_bool maybe_null; /* If item may be null */ my_bool null_value; /* if item is null */ my_bool unsigned_flag; my_bool with_sum_func; my_bool fixed; /* If item fixed with fix_fields */ my_bool is_autogenerated_name; /* indicate was name of this Item autogenerated or set by user */ DTCollation collation; // alloc & destruct is done as start of select using sql_alloc Item(); /* Constructor used by Item_field, Item_ref & aggregate (sum) functions. Used for duplicating lists in processing queries with temporary tables Also it used for Item_cond_and/Item_cond_or for creating top AND/OR structure of WHERE clause to protect it of optimisation changes in prepared statements */ Item(THD *thd, Item *item); virtual ~Item() { #ifdef EXTRA_DEBUG name=0; #endif } /*lint -e1509 */ void set_name(const char *str, uint length, CHARSET_INFO *cs); void rename(char *new_name); void init_make_field(Send_field *tmp_field,enum enum_field_types type); virtual void cleanup(); virtual void make_field(Send_field *field); Field *make_string_field(TABLE *table); ... };
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CHAPTER 3 A TOUR OF THE MYSQL SOURCE CODE
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The LEX Structure
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The LEX structure is responsible for being the internal representation (in-memory storage) of a query and its parts. It is more than that, though. The LEX structure is used to store all of the parts of a query in an organized manner. There are lists for fields, tables, expressions, and all of the parts that make up any query. The LEX structure is filled in by the parser as it discovers the parts of the query. Thus, when the parser is done the LEX structure contains everything needed to optimize and execute the query. Listing 3-16 shows a condensed view of the LEX structure. The structure is defined in the /sql/lex.h source file. Listing 3-16. The LEX Structure typedef struct st_lex { uint yylineno,yytoklen; /* Simulate lex */ LEX_YYSTYPE yylval; SELECT_LEX_UNIT unit; /* most upper unit */ SELECT_LEX select_lex; /* first SELECT_LEX */ /* current SELECT_LEX in parsing */ SELECT_LEX *current_select; /* list of all SELECT_LEX */ SELECT_LEX *all_selects_list; const uchar *buf; /* The beginning of string, used by SPs */ const uchar *ptr,*tok_start,*tok_end,*end_of_query; /* The values of tok_start/tok_end as they were one call of yylex before */ const uchar *tok_start_prev, *tok_end_prev; char *length,*dec,*change,*name; char *help_arg; char *backup_dir; /* For RESTORE/BACKUP */ char* to_log; /* For PURGE MASTER LOGS TO */ char* x509_subject,*x509_issuer,*ssl_cipher; char* found_semicolon; /* For multi queries - next query */ String *wild; sql_exchange *exchange; select_result *result; Item *default_value, *on_update_value; LEX_STRING comment, ident; LEX_USER *grant_user; XID *xid; gptr yacc_yyss,yacc_yyvs; THD *thd; CHARSET_INFO *charset; TABLE_LIST *query_tables; /* global list of all tables in this query */ ... } LEX;
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CHAPTER 3 A TOUR OF THE MYSQL SOURCE CODE
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The NET Structure
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The NET structure is responsible for storing all of the information concerning communication to and from a client. Listing 3-17 shows a condensed view of the NET structure. The buff member variable is used to store the raw communication packets (that when combined form the SQL statement). As you will see in later chapters, there are helper functions that fill in, read, and transmit the data packets to and from the client. Two examples are my_net_write(),which writes the data packets to the network protocol from the NET structure my_net_read(), which reads the data packets from the network protocol into the NET structure You can find the complete set of network communication functions in /include/mysql_com.h. Listing 3-17. The NET Structure typedef struct st_net { #if !defined(CHECK_EMBEDDED_DIFFERENCES) || !defined(EMBEDDED_LIBRARY) Vio* vio; unsigned char *buff,*buff_end,*write_pos,*read_pos; my_socket fd; /* For Perl DBI/dbd */ unsigned long max_packet,max_packet_size; unsigned int pkt_nr,compress_pkt_nr; unsigned int write_timeout, read_timeout, retry_count; int fcntl; my_bool compress; /* The following variable is set if we are doing several queries in one command ( as in LOAD TABLE ... FROM MASTER ), and do not want to confuse the client with OK at the wrong time */ unsigned long remain_in_buf,length, buf_length, where_b; unsigned int *return_status; unsigned char reading_or_writing; char save_char; my_bool no_send_ok; /* For SPs and other things that do multiple stmts */ my_bool no_send_eof; /* For SPs' first version read-only cursors */ /* Set if OK packet is already sent, and we do not need to send error messages */ my_bool no_send_error; /* Pointer to query object in query cache, do not equal NULL (0) for queries in cache that have not stored its results yet */ #endif
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