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Listing 12-26. The check_rewind() Method /* Adjusts pointers to record buffers for join. SYNOPSIS check_rewind() record_buff *cur_left IN the left record buffer record_buff *cur_left_prev IN the left record buffer previous record_buff *cur_right IN the left record buffer record_buff *cur_right_prev IN the left record buffer previous DESCRIPTION This method is used to push a tuple back into the buffer during a join operation that preempts the pipeline. NOTES Now, here's where we have to check the next tuple in each relation to see if they are the same. If one of them is the same and the other isn't, push one of them back. need to rewind if one The next record in R2 The next record in R1 The next record in R1 different (or EOF) 4. The next record in R2 different (or EOF) We 1. 2. 3. of the following is true: has the same join value as R1 has the same join value as R2 has the same join value and R2 is has the same join value and R1 is

RETURN VALUE Success = int index number Failed = -1 */ int Query_tree::check_rewind(record_buff *cur_left, record_buff *curr_left_prev, record_buff *cur_right, record_buff *curr_right_prev) { record_buff *left_rcd_ptr = cur_left; record_buff *right_rcd_ptr = cur_right; int i; DBUG_ENTER("check_rewind"); /* If the next tuple in right record is the same as the present tuple AND the next tuple in right record is different, rewind until it is the same

else Push left record back. */ /* if both buffers are at EOF, return -- nothing to do */ if ((left_rcd_ptr == NULL) && (right_rcd_ptr == NULL)) DBUG_RETURN(0); /* if the currently processed record is null, get the one before it */ if (cur_right == NULL) right_rcd_ptr = curr_right_prev; /* if left buffer is not at end, check to see if we need to rewind right buffer */ if (left_rcd_ptr != NULL) { /* compare the join conditions to check order */ i = memcmp(left_rcd_ptr->field_ptr, right_rcd_ptr->field_ptr, left_rcd_ptr->field_length < right_rcd_ptr->field_length left_rcd_ptr->field_length : right_rcd_ptr->field_length); /* i == 0 means the rows are the same. In this case, we need to check to see if we need to advance or rewind the right buffer. */ if (i == 0) { /* If there is a next row in the right buffer, check to see if it matches the left row. If the right row is greater than the left row, rewind the right buffer to one previous to the current row or until we hit the start. */ if (right_rcd_ptr->next != NULL) { right_rcd_ptr = right_rcd_ptr->next; i = memcmp(left_rcd_ptr->field_ptr, right_rcd_ptr->field_ptr, left_rcd_ptr->field_length < right_rcd_ptr->field_length left_rcd_ptr->field_length : right_rcd_ptr->field_length); if (i > 0) { do {

if (right_rcd_ptr->prev != NULL) { right_rcd_ptr = right_rcd_ptr->prev; i = memcmp(left_rcd_ptr->field_ptr, right_rcd_ptr->field_ptr, left_rcd_ptr->field_length < right_rcd_ptr->field_length left_rcd_ptr->field_length : right_rcd_ptr->field_length); } } while ((i == 0) && (right_rcd_ptr->prev != NULL)); /* now advance one more to set pointer to correct location */ if (right_rcd_ptr->next != NULL) right_rcd_ptr = right_rcd_ptr->next; } /* if no next right row, rewind to previous row */ else right_rcd_ptr = right_rcd_ptr->prev; } /* If there is a next row in the left buffer, check to see if it matches the right row. If there is a match and the right buffer is not at start, rewind the right buffer to one previous to the current row. */ else if (left_rcd_ptr->next != NULL) { if (right_rcd_ptr->prev != NULL) { i = memcmp(left_rcd_ptr->field_ptr, right_rcd_ptr->prev->field_ptr, left_rcd_ptr->field_length < right_rcd_ptr->prev->field_length left_rcd_ptr->field_length : right_rcd_ptr->prev->field_length); } if ((i == 0) && (right_rcd_ptr->prev != NULL)) right_rcd_ptr = right_rcd_ptr->prev; } } /* if the left row is less than right row, rewind right buffer */ else if (i < 0) { if (right_rcd_ptr->prev != NULL) right_rcd_ptr = right_rcd_ptr->prev; } /* if the right row is less than the left row, advance right row */