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CHAPTER 10 INTERNAL QUERY REPRESENTATION
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Listing 10-13. The Query Tree Class /* Query_tree.cc DESCRIPTION This file contains the Query_tree class. It is responsible for containing the internal representation of the query to be executed. It provides methods for optimizing and forming and inspecting the query tree. This class is the very heart of the DBXP query capability! It also provides the ability to store a binary "compiled" form of the query. NOTES The data structure is a binary tree that can have 0, 1, or 2 children. Only Join operations can have 2 children. All other operations have 0 or 1 children. Each node in the tree is an operation and the links to children are the pipeline. SEE ALSO query_tree.h */ #include "query_tree.h" Query_tree::query_node::query_node() { where_expr = NULL; join_expr = NULL; child = false; join_cond = Query_tree::jcUN; join_type = Query_tree::jnUNKNOWN; left = NULL; right = NULL; nodeid = -1; node_type = Query_tree::qntUndefined; sub_query = false; parent_nodeid = -1; } Query_tree::query_node::~query_node() { if(left) delete left; if(right) delete right; }
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Query_tree::~Query_tree(void) { if(root) delete root; }
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Building the Query Tree from the MySQL Structure
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What we need next is the code to perform the transformation from the MySQL internal structure to the query tree. Let s use a helper source file rather than adding the code to the sql_parse.cc file. In fact, many of the commands represented by the case statements (in the sql_parse.cc file) are done this way. Create a new file named sql_dbxp_parse.cc. Create a new function in that file named build_query_tree as shown in Listing 10-14. The code is a basic transformation method. Take a moment to look through the code as you type it in (or download and copy and paste it into the file). Listing 10-14. The DBXP Parser Helper File /* sql_dbxp_parse.cc DESCRIPTION This file contains methods to execute the DBXP SELECT query statements. SEE ALSO query_tree.cc */ #include "query_tree.h" /* Build Query Tree SYNOPSIS build_query_tree() THD *thd LEX *lex TABLE_LIST *tables
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IN the current thread IN the pointer to the current parsed structure IN the list of tables identified in the query
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DESCRIPTION This method returns a converted MySQL internal representation (IR) of a query as a query_tree. RETURN VALUE Success = Query_tree * -- the root of the new query tree. Failed = NULL */
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CHAPTER 10 INTERNAL QUERY REPRESENTATION
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Query_tree *build_query_tree(THD *thd, LEX *lex, TABLE_LIST *tables) { DBUG_ENTER("build_query_tree"); Query_tree *qt = new Query_tree(); Query_tree::query_node *qn = (Query_tree::query_node *)my_malloc(sizeof(Query_tree::query_node), MYF(MY_ZEROFILL | MY_WME)); TABLE_LIST *table; int i = 0; int num_tables = 0; /* Create a new restrict node. */ qn->parent_nodeid = -1; qn->child = false; /* Set the query type to unknown because we're creating a project node. */ qn->join_type = (Query_tree::type_join) jnUNKNOWN; qn->nodeid = 0; qn->node_type = (Query_tree::query_node_type) qntProject; qn->left = 0; qn->right = 0; if(lex->select_lex.options & SELECT_DISTINCT) { //qt->set_distinct(true); /* placeholder for future modifications */ } /* Get the tables (relations) */ i = 0; for(table = tables; table; table = table->next_local) { num_tables++; qn->relations[i] = table; i++; } /* Populate attributes */ qn->fields = &lex->select_lex.item_list; /* Process joins */ if (num_tables > 0) //indicates more than 1 table processed for(table = tables; table; table = table->next_local) if ((table->on_expr != 0) && (qn->join_expr == 0)) qn->join_expr = table->on_expr; qn->where_expr = lex->select_lex.where; qt->root = qn; DBUG_RETURN(qt); }
CHAPTER 10 INTERNAL QUERY REPRESENTATION
Notice that the build_query_tree code begins with creating a new query node, identifies the tables used in the query, populates the fields list, and captures the join and where expressions. These are all of the basic items needed to execute the most basic of queries.
Stubbing the Query Tree Execution
Now let s consider what it takes to create a query tree in code. Create a new function named DBXP_select_command and copy the code from Listing 10-15. Place this function in the sql_DBXP_parse.cc file. This function will be called from the case statement in sql_parse.cc. Listing 10-15. Handling the SELECT DBXP Command /* Perform Select Command SYNOPSIS DBXP_select_command() THD *thd IN the current thread DESCRIPTION This method executes the command using the query tree and optimizer. RETURN VALUE Success = 0 Failed = 1
/* Note: The use of 0 as success is a MySQL coding rule. */
*/ int DBXP_select_command(THD *thd) { DBUG_ENTER("dbxp_select_command"); Query_tree *qt = build_query_tree(thd, thd->lex, (TABLE_LIST*) thd->lex->select_lex.table_list.first); List<Item> field_list; Protocol *protocol= thd->protocol; field_list.push_back(new Item_empty_string("Database Experiment Project (DBXP)",40)); if (protocol->send_fields(&field_list, Protocol::SEND_NUM_ROWS | Protocol::SEND_EOF)) DBUG_RETURN(TRUE); protocol->prepare_for_resend(); protocol->store("Query tree was built.", system_charset_info); if (protocol->write()) DBUG_RETURN(TRUE); send_eof(thd); delete qt; DBUG_RETURN(0); }
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