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CHAPTER 10 INTERNAL QUERY REPRESENTATION
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Listing 10-26. The DBXP EXPLAIN Command Source Code /* Perform EXPLAIN command. SYNOPSIS DBXP_explain_select_command() THD *thd IN the current thread DESCRIPTION This method executes the EXPLAIN SELECT command. RETURN VALUE Success = 0 Failed = 1 */ int DBXP_explain_select_command(THD *thd) { DBUG_ENTER("dbxp_explain_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("Execution Path",NAME_LEN)); if (protocol->send_fields(&field_list, Protocol::SEND_NUM_ROWS | Protocol::SEND_EOF)) DBUG_RETURN(TRUE); protocol->prepare_for_resend(); show_plan(protocol, qt->root, qt->root, false); send_eof(thd); delete qt; DBUG_RETURN(0); } Now, let s compile the server and give it a go with the test file.
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Testing the DBXP EXPLAIN Command
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As with the previous tests, you can either use the test described in an earlier section or enter a valid SQL command in the MySQL command-line client. Listing 10-27 shows an example of what the query execution path would look like. It should be stated at this point that the query is not optimized and will appear as a single node. Once you add the optimizer (see 11), the query execution path will reflect the appropriate execution for the query statement entered.
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CHAPTER 10 INTERNAL QUERY REPRESENTATION
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Listing 10-27. Results of the DBXP EXPLAIN Test mysql> EXPLAIN SELECT DBXP * FROM customer; +--------------------------+ | Execution Path | +--------------------------+ | test.customer | | | | | | | | | | | V | | ------------------- | | | PROJECT | | | ------------------- | | | Access Method: | | | | iterator | | | ------------------- | | | | | | | | V | | Result Set | +--------------------------+ 15 rows in set (0.00 sec) mysql> This is much more interesting than a dull listing of facts. Adding the EXPLAIN command at this stage of the DBXP project allows you to witness and diagnose how the optimizer is forming the query tree. You ll find this very helpful when you begin your own experiments. If you haven t been doing so thus far, you should run the complete test that tests all three portions of the code presented in this chapter.
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I presented in this chapter some of the more complex database internal technologies. You learned how queries are represented internally within the MySQL server as they are parsed and processed via the big switch. More importantly, you discovered how MySQL can be used to conduct your own database internals experiments with the query tree class. The knowledge of these technologies should provide you with a greater understanding of why and how the MySQL internal components are built. In the next chapter, I ll show you more about internal query representation through an example implementation of a query tree optimization strategy. If you ve ever wondered what it takes to build an optimizer for a relational database system, the next chapter will show you an example of a heuristic query optimizer using the query tree class.
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CHAPTER 10 INTERNAL QUERY REPRESENTATION
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The following lists several areas for further exploration. They represent the types of activities you might want to conduct as experiments (or as a class assignment) to explore relational database technologies. 1. The query in Figure 10-1 exposes a design flaw in one of the tables. What is it Does the flaw violate any of the normal forms If so, which one 2. Explore the TABLE structure and change the SELECT DBXP stub to return information about the table and its fields. 3. Change the EXPLAIN SELECT DBXP command to produce an output similar to the MySQL EXPLAIN SELECT command. 4. Modify the build_query_tree function to identify and process the LIMIT clause. 5. How can the query tree query_node structure be changed to accommodate HAVING, GROUP BY, and ORDER clauses
Query Optimization
he query tree class shown in 10 forms the starting point for building the experimental query optimization and execution engine for DBXP. In this chapter, I ll show you how to add the optimizer to the query tree class. I ll begin by explaining the rationale for the heuristics (or rules) used in the optimizer, and then jump into writing the code. Because the code for some of the functions is quite lengthy, the code examples shown in this chapter are excerpts. If you are following along by coding the examples, I recommend you download the source code for this chapter instead of typing in the code from scratch.
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