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The following questions are from 11.
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Question 1. Complete the code for the balance_joins() method. Hint: You will need to create an algorithm that can move conjunctive joins around so that the join that is most restrictive is executed first (is lowest in the tree).
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This exercise is all about how to move joins around in the tree to push the most restrictive joins down. The tricky part is using the statistics of the tables to determine which joins will produce the fewest results. Look to the handler and table classes for information about accessing this data. Beyond that, you will need helper methods to traverse the tree and get information about the tables. This is necessary because it is possible (and likely) that the joins will be higher in the tree and may not contain direct reference to the table.
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Question 2. Complete the code for the cost_optimization() method. Hint: You will need to walk the tree and indicate nodes that can use indexes.
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This exercise requires you to interrogate the handler and table classes to determine which tables have indexes and what those columns are.
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Question 3. Examine the code for the heuristic optimizer. Does it cover all possible queries If not, are there any other rules that can be used to complete the coverage
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You should discover that there are many such heuristics and that this optimizer covers only the most effective of the heuristics. For example, you could implement heuristics that take into account the GROUP BY and HAVING operations, creating methods similar to how I implemented the heuristics for project and restrict.
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Question 4. Examine the code for the query tree and heuristic optimizer. How can you implement the distinct node type as listed in the query tree class Hint: See the code that follows the prune_tree() method in the heuristic_optimization() method.
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Most of the hints for this exercise are in the sample code. The excerpt that follows shows how you can identify when a DISTINCT option is specified on the query.
Question 5. How can you change the code to recognize invalid queries What are the conditions that determine a query is invalid and how would you test for them
Part of the solution for this exercise is done for you. For example, a query statement that is syntactically incorrect will be detected by the parser and an appropriate error displayed. However, for those queries that are syntactically correct but semantically meaningless, you will need to add additional error handling code to detect any anomalies. For example, try a query that is syntactically correct but references the wrong fields for the table. Create tests of this nature and trace (or debug) the code. You should see places in the code where additional error handling can be placed. Lastly, you could also create a method in the Query_tree class that validates the query tree itself. This could be particularly handy if you attempt to create additional node types or implement other heuristic methods.
Question 6. (advanced) MySQL does not currently support the intersect operation (as defined by Date). Change the MySQL parser to recognize the new keyword and process queries like SELECT * FROM A INTERSECT B. Are there any limitations of this operation and are they reflected in the optimizer
What sounds like a very difficult problem has a very straightforward solution. Consider adding a new node type named intersect that has two children. The operation merely returns those rows that are in both tables. Hint: Use one of the many merge sort variants to accomplish this.
Question 7. (advanced) How would you implement the GROUP BY, ORDER BY, and HAVING clauses Make the changes to the optimizer to enable these clauses.
There are many ways to accomplish this. In keeping with the design of the Query_tree class, each of these operations can be represented as another node type. You can build a method to handle each of these, just as we did with restrict, project, and join. Note, however, that the HAVING clause is used with the GROUP BY clause and the ORDER BY clause is usually processed last.
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