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CHAPTER 3 A TOUR OF THE MYSQL SOURCE CODE
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Table 3-3. The MySQL Parser
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/sql/lex.h /sql/lex_symbol.h /sql/lex_hash.h /sql/sql_lex.h /sql/sql_lex.cc /sql/sql_yacc.yy /sql/sql_parse.cc
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The symbol table for all of the keywords and tokens supported by the parser Type definitions for the symbol table Mapping of symbols to functions used in the parser Definition of LEX structure Definition of Lex class The Lex/YACC parser code Contains the majority of the query routing and parsing functions except for the lexical parser
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Caution Do not edit the files sql_yacc.cc, sql_yacc.h, or lex_hash.h. These files are generated
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by other utilities. See 8 for more details.
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Preparing the Query for Optimization
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Although the boundary of where the parser ends and the optimizer begins is not clear from the MySQL documentation (there are contradictions), it is clear from the definition of the optimizer that the routing and control parts of the source code can be considered part of the optimizer. To avoid confusion, I am going to call the next set of functions the preparatory stage of the optimizer. The first of these preparatory functions is the mysql_execute_command() function (located in /sql/sql_parse.cc). The name leads you to believe you are actually executing the query, but that isn t the case. This function performs much of the setup steps necessary to optimize the query. The LEX structure is copied and several variables are set to help the query optimization and later execution. You can see some of these operations in a condensed view of the function shown in Listing 3-9. Listing 3-9. The mysql_execute_command() Function bool mysql_execute_command(THD *thd) { bool res= FALSE; int result= 0; LEX *lex= thd->lex; /* first SELECT_LEX (have special meaning for many of non-SELECTcommands) */ SELECT_LEX *select_lex= &lex->select_lex; /* first table of first SELECT_LEX */ TABLE_LIST *first_table= (TABLE_LIST*) select_lex->table_list.first;
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CHAPTER 3 A TOUR OF THE MYSQL SOURCE CODE
/* list of all tables in query */ TABLE_LIST *all_tables; /* most outer SELECT_LEX_UNIT of query */ SELECT_LEX_UNIT *unit= &lex->unit; /* Saved variable value */ DBUG_ENTER("mysql_execute_command"); thd->net.no_send_error= 0; ... switch (lex->sql_command) { case SQLCOM_SELECT: { ... select_result *result=lex->result; ... res= check_access(thd, lex->exchange SELECT_ACL | FILE_ACL : SELECT_ACL, any_db, 0, 0, 0, 0); ... if (!(res= open_and_lock_tables(thd, all_tables))) { if (lex->describe) { /* We always use select_send for EXPLAIN, even if it's an EXPLAIN for SELECT ... INTO OUTFILE: a user application should be able to prepend EXPLAIN to any query and receive output for it, even if the query itself redirects the output. */ ... query_cache_store_query(thd, all_tables); res= handle_select(thd, lex, result, 0); ... } There are a number of interesting things happening in this function. You will notice another switch statement that has as its cases the SQLCOM keywords. In the case of the example query, you saw the parser set the lex->sql_command member variable to SQLCOM_SELECT. I have included a
CHAPTER 3 A TOUR OF THE MYSQL SOURCE CODE
condensed view of that case statement for you in Listing 3-9. What I did not include is the many other SQLCOM case statements. This function is a very large function. Since it is the central routing function for query processing, it contains a case for every possible command. Consequently, the source code is tens of pages long. Let s see what this case statement does. Notice the statement select_result *result= lex->result. This statement creates a result class that will be used to hold the results of the query for later transmission to the client. If you scan down, you will see the check_table_ access() function. This function is called to check the access control list for the resources used by the query. If access is granted, the function calls the open_and_lock_tables() function, which opens and locks the tables for the query. I left part of the code concerning the DESCRIBE (EXPLAIN) command for you to examine.
Note Once when I was modifying the code I needed to find all of the locations of the EXPLAIN calls so that I could alter them for a specific need. I looked everywhere until I found them in the parser. There in the middle of the Lex/YACC code was a comment that said something to the effect that DESCRIBE was left over from an earlier Oracle compatibility issue and that the correct term was EXPLAIN. Comments are useful. . . if you can find them.
The next function call is a call to the query cache. The query_cache_store_query() function stores the SQL statement in the query. As you will see later, when the results are ready they too are stored in the query cache. Finally you see that the function calls another function called handle_select(). You may be thinking, Didn t we just do the handle thing The handle_select() is a wrapper for another function named mysql_select(). Listing 3-10 shows the complete code for the handle_select() function. Near the top of the listing is the select_lex->next_select() operation, which is checking for the UNION command that appends multiple SELECT results into a single set of results. Other than that, the code just calls the next function in the chain, mysql_select(). It is at this point that you are finally close enough to transition to the query optimizer subsystem. Table 3-4 lists the source files associated with the query optimizer.
Note This is perhaps the part of the code that suffers most from ill-defined subsystems. While the code
is still very organized, the boundaries of the subsystems are fuzzy at this point in the source code.
Listing 3-10. The handle_select() Function bool handle_select(THD *thd, LEX *lex, select_result *result, ulong setup_tables_done_option) { bool res; register SELECT_LEX *select_lex = &lex->select_lex; DBUG_ENTER("handle_select");
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