tuplelist.c

关系型数据库 Postgresql 6.5.2

/* Module:          tuplelist.c
 *
 * Description:     This module contains functions for creating a manual result set
 *                  (the TupleList) and retrieving data from it for a specific row/column.
 *
 * Classes:         TupleListClass (Functions prefix: "TL_")
 *
 * API functions:   none
 *
 * Comments:        See "notice.txt" for copyright and license information.
 *
 */

#include <stdlib.h>
#include <malloc.h>
#include "tuplelist.h"
#include "tuple.h"

TupleListClass *
TL_Constructor(UInt4 fieldcnt)
{
TupleListClass *rv;

	mylog("in TL_Constructor\n");

	rv = (TupleListClass *) malloc(sizeof(TupleListClass));
	if (rv) {

        rv->num_fields = fieldcnt;
        rv->num_tuples = 0;
        rv->list_start = NULL;
        rv->list_end = NULL;
        rv->lastref = NULL;
        rv->last_indexed = -1;
	}

	mylog("exit TL_Constructor\n");

	return rv;
}

void
TL_Destructor(TupleListClass *self)
{
int lf;
TupleNode *node, *tp;

	mylog("TupleList: in DESTRUCTOR\n");

    node = self->list_start;
    while(node != NULL) {
        for (lf=0; lf < self->num_fields; lf++)
            if (node->tuple[lf].value != NULL) {
                free(node->tuple[lf].value);
            }
        tp = node->next;
        free(node);
        node = tp;
    }

	free(self);

	mylog("TupleList: exit DESTRUCTOR\n");
}
			

void *
TL_get_fieldval(TupleListClass *self, Int4 tupleno, Int2 fieldno)
{
Int4 lf;
Int4 delta, from_end;
char end_is_closer, start_is_closer;
TupleNode *rv;

	if (self->last_indexed == -1)
		/* we have an empty tuple list */
		return NULL;

	/* some more sanity checks */
	if ((tupleno >= self->num_tuples) || (tupleno < 0))
		/* illegal tuple number range */
		return NULL;

	if ((fieldno >= self->num_fields) || (fieldno < 0))
		/* illegel field number range */
		return NULL;

 /* check if we are accessing the same tuple that was used in
    the last fetch (e.g: for fetching all the fields one after
    another. Do this to speed things up
 */
	if (tupleno == self->last_indexed)
		return self->lastref->tuple[fieldno].value;

 /* now for the tricky part... */

 /*
 Since random access is quite inefficient for linked lists we use
 the lastref pointer that points to the last element referenced
 by a get_fieldval() call in conjunction with the its index number
 that is stored in last_indexed. (So we use some locality of
 reference principle to speed things up)
 */

	delta = tupleno - self->last_indexed;
	/* if delta is positive, we have to go forward */

	/* now check if we are closer to the start or the end of the list
	than to our last_indexed pointer
	*/
	from_end = (self->num_tuples - 1) - tupleno;

	start_is_closer = labs(delta) > tupleno;
	/* true if we are closer to the start of the list than to the
	last_indexed pointer
	*/

	end_is_closer = labs(delta) > from_end;
	/* true if we are closer at the end of the list */

	if (end_is_closer) {
		/* scanning from the end is the shortest way. so we do that... */
		rv = self->list_end;
		for (lf=0; lf < from_end; lf++)
			rv = rv->prev;
	} else if (start_is_closer) {
		/* the shortest way is to start the search from the head of the list */
		rv = self->list_start;
		for (lf=0; lf < tupleno; lf++)
			rv = rv->next;
	} else {
		/* the closest way is starting from our lastref - pointer */
		rv = self->lastref;
		/* at first determine whether we have to search forward or backwards */
		if (delta < 0) {
			/* we have to search backwards */
			for(lf=0; lf < (-1)*delta; lf++)
				rv = rv->prev;
		} else {
			/* ok, we have to search forward... */
			for (lf=0; lf < delta; lf++)
			rv = rv->next;
		}
	}

	/* now we have got our return pointer, so update the lastref
		and the last_indexed values
	*/
	self->lastref = rv;
	self->last_indexed = tupleno;

	return rv->tuple[fieldno].value;
}



char
TL_add_tuple(TupleListClass *self, TupleNode *new_field)
{
 /* we append the tuple at the end of the doubly linked list
    of the tuples we have already read in
 */

	new_field->prev = NULL;
	new_field->next = NULL;

	if (self->list_start == NULL) {
		/* the list is empty, we have to add the first tuple */
		self->list_start = new_field;
		self->list_end = new_field;
		self->lastref = new_field;
		self->last_indexed = 0;
	} else {
		/* there is already an element in the list, so add the new
			one at the end of the list
		*/
		self->list_end->next = new_field;
		new_field->prev = self->list_end;
		self->list_end = new_field;
	}
	self->num_tuples++;

	/* this method of building a list cannot fail, so we return 1 */
	return 1;
}