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CHAPTER 12 QUERY EXECUTION
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/* Check to see if either buffer needs to be rewound to allow us to process many rows on one side to one row on the other. The results of this rewind must be saved because there was no match and we may have to reuse one or more of the rows. */ check_rewind(left_record_buffer_ptr, lprev, right_record_buffer_ptr, rprev); /* If the left buffer has been changed and if the buffer is not at the end, set the buffer to the next row and copy the data into the record buffer/ */ if (left != left_record_buffer_ptr) { if (left_record_buffer_ptr != NULL) { memcpy((byte *)ltable->record[0], (byte *)left_record_buffer_ptr->record->rec_buf, ltable->s->rec_buff_length); lbuff = left_record_buffer_ptr->record; } } /* If the right buffer has been changed and if the buffer is not at the end, set the buffer to the next row and copy the data into the record buffer/ */ if (right_record_buffer_ptr != NULL) if ((right_record_buffer_ptr->next == NULL) && (right_record_buffer_ptr->prev == NULL)) lbuff = NULL; if (right != right_record_buffer_ptr) { if (right_record_buffer_ptr != NULL) { memcpy((byte *)rtable->record[0], (byte *)right_record_buffer_ptr->record->rec_buf, rtable->s->rec_buff_length); rbuff = right_record_buffer_ptr->record; } }
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CHAPTER 12 QUERY EXECUTION
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/* Now check for end of file and save results in eof array */ if (left_record_buffer_ptr == NULL) qn->eof[2] = true; else qn->eof[2] = false; if (right_record_buffer_ptr == NULL) qn->eof[3] = true; else qn->eof[3] = false; } } else next_tup = NULL; /* at end, return null */ break; } /* placeholder for exercise... */ case (jnCROSSPRODUCT) : { break; } /* placeholder for exercises... Union and intersect are mirrors of each other -- same code will work for both except the dupe elimination/inclusion part (see below) */ case (jnUNION) : case (jnINTERSECT) : { break; } } DBUG_RETURN(next_tup); } Notice in the code that under any condition other than a match, the record returned from the code is set to NULL. This allows the loop in the get_next() method to repeatedly call the do_join() method until a match is returned. This is similar to the way the do_restrict() method call is made. You may note that I have not implemented the code for any of the other join operations. The main reason is that it allows you to experiment with the code (see the exercises at end of this chapter). Fortunately, you should find that the code can be modified with a few simple alterations to allow the processing of the outer joins. Adding code for the cross-product, union, and intersect operations can be accomplished by implementing the theoretical algorithm described in the first part of this chapter. After you have studied the pseudocode for the method, you should find reading the code easier. The most complex part of this code is the check_rewind() method. This is implemented
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CHAPTER 12 QUERY EXECUTION
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as a function in the class to make the code less complex and easier to read. There are several other helper methods, which are described in more detail in the following section.
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Several helper methods make up the DBXP execution engine. Table 12-1 lists the new methods and their uses. The more complex methods are described in more detail in the text that follows.
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Table 12-1. The DBXP Execution Engine Helper Methods
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Class::Method
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Query_tree::get_next() Query_tree::insertion_sort() Query_tree::Eof() Query_tree::check_rewind() send_data() Expression::evaluate() Expression::compare_join() Handler::rnd_init() Handler::rnd_next()
Description
Retrieves next tuple from child node. Creates an ordered buffer of READ_RECORD pointers. Used in the join operations for ordering the incoming tuples. Checks for the end-of-file condition for the storage engine or temporary buffers. Checks to see if the record buffers need to be adjusted to reread tuples for multiple matches. Sends data to the client. See sql_dbxp_parse.cc. Evaluates the WHERE clause for a restrict operation. Evaluates the join condition for a join operation. Initializes read from storage engine (see 7). Reads the next tuple from storage engine (see 7).
The get_next() Method The get_next() method is the heart of the query execution flow in DBXP. It is responsible for calling the do_... methods that implement the query operations. It is called once from the while loop in the DBXP_select_command() method. Once this method is initiated the first time, it performs the operation for the current node, calling the children nodes to get their result. The process is repeated in a recursive fashion until all the children in the current node have returned a single tuple. Listing 12-24 shows the code for the get_next() method. Listing 12-24. The get_next() Method /* Get the next tuple (row) in the result set. SYNOPSIS Eof() query_node *qn IN the operational node in the query tree.
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