首先在源码中这两个类型是这样定义的：
typedef struct ListCell ListCell;
typedef struct List
{
NodeTag type; /* T_List, T_IntList, or T_OidList */
int length;
ListCell *head;
ListCell *tail;
} List;
struct ListCell
{
union
{
void *ptr_value;
int int_value;
Oid oid_value;
} data;
ListCell *next;
};
这两个类型的关系是，ListCell是一个单独的个体，作为一个容器来存储内容以及下一个 ListCell的指针。
1、其中如果这是一个由int或者Oid构成的List，那么ListCell直接存储int或者Oid。若不是，则使用void*来存储，这样可以存储的类型就多了。一般用的时候直接使用强制转换为(Type *)即可使用。
2、next存储的是下一个ListCell，由此可以说明List是一个线性链表，只能向后寻找。
接下来是有ListCell组成的List，List，没有将整个链存储起来，仅仅将由ListCell组成的线性链表的头和尾。在做查询的时候，也仅仅是通过头进行向后查询。同时还存储了链的两个属性：（1）ListCell的个数；（2）List的类型（T_List, T_IntList, or T_OidList）。
List的类型是在构建List的时候指定的：
static List *
new_list(NodeTag type)
{
List *new_list;
ListCell *new_head;
new_head = (ListCell *) palloc(sizeof(*new_head));
new_head->next = NULL;
/* new_head->data is left undefined! */
new_list = (List *) palloc(sizeof(*new_list));
new_list->type = type;
new_list->length = 1;
new_list->head = new_head;
new_list->tail = new_head;
return new_list;
}
遍历List的方法为：
#define foreach(cell, l) \
for ((cell) = list_head(l); (cell) != NULL; (cell) = lnext(cell))
#define for_each_cell(cell, initcell) \
for ((cell) = (initcell); (cell) != NULL; (cell) = lnext(cell))
方法有许多，可以参考 pg_list.h。 另附：pg_list.h
/*-------------------------------------------------------------------------
*
* pg_list.h
* interface for PostgreSQL generic linked list package
*
* This package implements singly-linked homogeneous lists.
*
* It is important to have constant-time length, append, and prepend
* operations. To achieve this, we deal with two distinct data
* structures:
*
* 1. A set of "list cells": each cell contains a data field and
* a link to the next cell in the list or NULL.
* 2. A single structure containing metadata about the list: the
* type of the list, pointers to the head and tail cells, and
* the length of the list.
*
* We support three types of lists:
*
* T_List: lists of pointers
* (in practice usually pointers to Nodes, but not always;
* declared as "void *" to minimize casting annoyances)
* T_IntList: lists of integers
* T_OidList: lists of Oids
*
* (At the moment, ints and Oids are the same size, but they may not
* always be so; try to be careful to maintain the distinction.)
*
*
* Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* src/include/nodes/pg_list.h
*
*-------------------------------------------------------------------------
*/
#ifndef PG_LIST_H
#define PG_LIST_H
#include "nodes/nodes.h"
typedef struct ListCell ListCell;
typedef struct List
{
NodeTag type; /* T_List, T_IntList, or T_OidList */
int length;
ListCell *head;
ListCell *tail;
} List;
struct ListCell
{
union
{
void *ptr_value;
int int_value;
Oid oid_value;
} data;
ListCell *next;
};
/*
* The *only* valid representation of an empty list is NIL; in other
* words, a non-NIL list is guaranteed to have length >= 1 and
* head/tail != NULL
*/
#define NIL ((List *) NULL)
/*
* These routines are used frequently. However, we can't implement
* them as macros, since we want to avoid double-evaluation of macro
* arguments. Therefore, we implement them using static inline functions
* if supported by the compiler, or as regular functions otherwise.
* See STATIC_IF_INLINE in c.h.
*/
#ifndef PG_USE_INLINE
extern ListCell *list_head(const List *l);
extern ListCell *list_tail(List *l);
extern int list_length(const List *l);
#endif /* PG_USE_INLINE */
#if defined(PG_USE_INLINE) || defined(PG_LIST_INCLUDE_DEFINITIONS)
STATIC_IF_INLINE ListCell *
list_head(const List *l)
{
return l ? l->head : NULL;
}
STATIC_IF_INLINE ListCell *
list_tail(List *l)
{
return l ? l->tail : NULL;
}
STATIC_IF_INLINE int
list_length(const List *l)
{
return l ? l->length : 0;
}
#endif /*-- PG_USE_INLINE || PG_LIST_INCLUDE_DEFINITIONS */
/*
* NB: There is an unfortunate legacy from a previous incarnation of
* the List API: the macro lfirst() was used to mean "the data in this
* cons cell". To avoid changing every usage of lfirst(), that meaning
* has been kept. As a result, lfirst() takes a ListCell and returns
* the data it contains; to get the data in the first cell of a
* List, use linitial(). Worse, lsecond() is more closely related to
* linitial() than lfirst(): given a List, lsecond() returns the data
* in the second cons cell.
*/
#define lnext(lc) ((lc)->next)
#define lfirst(lc) ((lc)->data.ptr_value)
#define lfirst_int(lc) ((lc)->data.int_value)
#define lfirst_oid(lc) ((lc)->data.oid_value)
#define linitial(l) lfirst(list_head(l))
#define linitial_int(l) lfirst_int(list_head(l))
#define linitial_oid(l) lfirst_oid(list_head(l))
#define lsecond(l) lfirst(lnext(list_head(l)))
#define lsecond_int(l) lfirst_int(lnext(list_head(l)))
#define lsecond_oid(l) lfirst_oid(lnext(list_head(l)))
#define lthird(l) lfirst(lnext(lnext(list_head(l))))
#define lthird_int(l) lfirst_int(lnext(lnext(list_head(l))))
#define lthird_oid(l) lfirst_oid(lnext(lnext(list_head(l))))
#define lfourth(l) lfirst(lnext(lnext(lnext(list_head(l)))))
#define lfourth_int(l) lfirst_int(lnext(lnext(lnext(list_head(l)))))
#define lfourth_oid(l) lfirst_oid(lnext(lnext(lnext(list_head(l)))))
#define llast(l) lfirst(list_tail(l))
#define llast_int(l) lfirst_int(list_tail(l))
#define llast_oid(l) lfirst_oid(list_tail(l))
/*
* Convenience macros for building fixed-length lists
*/
#define list_make1(x1) lcons(x1, NIL)
#define list_make2(x1,x2) lcons(x1, list_make1(x2))
#define list_make3(x1,x2,x3) lcons(x1, list_make2(x2, x3))
#define list_make4(x1,x2,x3,x4) lcons(x1, list_make3(x2, x3, x4))
#define list_make1_int(x1) lcons_int(x1, NIL)
#define list_make2_int(x1,x2) lcons_int(x1, list_make1_int(x2))
#define list_make3_int(x1,x2,x3) lcons_int(x1, list_make2_int(x2, x3))
#define list_make4_int(x1,x2,x3,x4) lcons_int(x1, list_make3_int(x2, x3, x4))
#define list_make1_oid(x1) lcons_oid(x1, NIL)
#define list_make2_oid(x1,x2) lcons_oid(x1, list_make1_oid(x2))
#define list_make3_oid(x1,x2,x3) lcons_oid(x1, list_make2_oid(x2, x3))
#define list_make4_oid(x1,x2,x3,x4) lcons_oid(x1, list_make3_oid(x2, x3, x4))
/*
* foreach -
* a convenience macro which loops through the list
*/
#define foreach(cell, l) \
for ((cell) = list_head(l); (cell) != NULL; (cell) = lnext(cell))
/*
* for_each_cell -
* a convenience macro which loops through a list starting from a
* specified cell
*/
#define for_each_cell(cell, initcell) \
for ((cell) = (initcell); (cell) != NULL; (cell) = lnext(cell))
/*
* forboth -
* a convenience macro for advancing through two linked lists
* simultaneously. This macro loops through both lists at the same
* time, stopping when either list runs out of elements. Depending
* on the requirements of the call site, it may also be wise to
* assert that the lengths of the two lists are equal.
*/
#define forboth(cell1, list1, cell2, list2) \
for ((cell1) = list_head(list1), (cell2) = list_head(list2); \
(cell1) != NULL && (cell2) != NULL; \
(cell1) = lnext(cell1), (cell2) = lnext(cell2))
/*
* forthree -
* the same for three lists
*/
#define forthree(cell1, list1, cell2, list2, cell3, list3) \
for ((cell1) = list_head(list1), (cell2) = list_head(list2), (cell3) = list_head(list3); \
(cell1) != NULL && (cell2) != NULL && (cell3) != NULL; \
(cell1) = lnext(cell1), (cell2) = lnext(cell2), (cell3) = lnext(cell3))
extern List *lappend(List *list, void *datum);
extern List *lappend_int(List *list, int datum);
extern List *lappend_oid(List *list, Oid datum);
extern ListCell *lappend_cell(List *list, ListCell *prev, void *datum);
extern ListCell *lappend_cell_int(List *list, ListCell *prev, int datum);
extern ListCell *lappend_cell_oid(List *list, ListCell *prev, Oid datum);
extern List *lcons(void *datum, List *list);
extern List *lcons_int(int datum, List *list);
extern List *lcons_oid(Oid datum, List *list);
extern List *list_concat(List *list1, List *list2);
extern List *list_truncate(List *list, int new_size);
extern void *list_nth(const List *list, int n);
extern int list_nth_int(const List *list, int n);
extern Oid list_nth_oid(const List *list, int n);
extern bool list_member(const List *list, const void *datum);
extern bool list_member_ptr(const List *list, const void *datum);
extern bool list_member_int(const List *list, int datum);
extern bool list_member_oid(const List *list, Oid datum);
extern List *list_delete(List *list, void *datum);
extern List *list_delete_ptr(List *list, void *datum);
extern List *list_delete_int(List *list, int datum);
extern List *list_delete_oid(List *list, Oid datum);
extern List *list_delete_first(List *list);
extern List *list_delete_cell(List *list, ListCell *cell, ListCell *prev);
extern List *list_union(const List *list1, const List *list2);
extern List *list_union_ptr(const List *list1, const List *list2);
extern List *list_union_int(const List *list1, const List *list2);
extern List *list_union_oid(const List *list1, const List *list2);
extern List *list_intersection(const List *list1, const List *list2);
/* currently, there's no need for list_intersection_int etc */
extern List *list_difference(const List *list1, const List *list2);
extern List *list_difference_ptr(const List *list1, const List *list2);
extern List *list_difference_int(const List *list1, const List *list2);
extern List *list_difference_oid(const List *list1, const List *list2);
extern List *list_append_unique(List *list, void *datum);
extern List *list_append_unique_ptr(List *list, void *datum);
extern List *list_append_unique_int(List *list, int datum);
extern List *list_append_unique_oid(List *list, Oid datum);
extern List *list_concat_unique(List *list1, List *list2);
extern List *list_concat_unique_ptr(List *list1, List *list2);
extern List *list_concat_unique_int(List *list1, List *list2);
extern List *list_concat_unique_oid(List *list1, List *list2);
extern void list_free(List *list);
extern void list_free_deep(List *list);
extern List *list_copy(const List *list);
extern List *list_copy_tail(const List *list, int nskip);
/*
* To ease migration to the new list API, a set of compatibility
* macros are provided that reduce the impact of the list API changes
* as far as possible. Until client code has been rewritten to use the
* new list API, the ENABLE_LIST_COMPAT symbol can be defined before
* including pg_list.h
*/
#ifdef ENABLE_LIST_COMPAT
#define lfirsti(lc) lfirst_int(lc)
#define lfirsto(lc) lfirst_oid(lc)
#define makeList1(x1) list_make1(x1)
#define makeList2(x1, x2) list_make2(x1, x2)
#define makeList3(x1, x2, x3) list_make3(x1, x2, x3)
#define makeList4(x1, x2, x3, x4) list_make4(x1, x2, x3, x4)
#define makeListi1(x1) list_make1_int(x1)
#define makeListi2(x1, x2) list_make2_int(x1, x2)
#define makeListo1(x1) list_make1_oid(x1)
#define makeListo2(x1, x2) list_make2_oid(x1, x2)
#define lconsi(datum, list) lcons_int(datum, list)
#define lconso(datum, list) lcons_oid(datum, list)
#define lappendi(list, datum) lappend_int(list, datum)
#define lappendo(list, datum) lappend_oid(list, datum)
#define nconc(l1, l2) list_concat(l1, l2)
#define nth(n, list) list_nth(list, n)
#define member(datum, list) list_member(list, datum)
#define ptrMember(datum, list) list_member_ptr(list, datum)
#define intMember(datum, list) list_member_int(list, datum)
#define oidMember(datum, list) list_member_oid(list, datum)
/*
* Note that the old lremove() determined equality via pointer
* comparison, whereas the new list_delete() uses equal(); in order to
* keep the same behavior, we therefore need to map lremove() calls to
* list_delete_ptr() rather than list_delete()
*/
#define lremove(elem, list) list_delete_ptr(list, elem)
#define LispRemove(elem, list) list_delete(list, elem)
#define lremovei(elem, list) list_delete_int(list, elem)
#define lremoveo(elem, list) list_delete_oid(list, elem)
#define ltruncate(n, list) list_truncate(list, n)
#define set_union(l1, l2) list_union(l1, l2)
#define set_uniono(l1, l2) list_union_oid(l1, l2)
#define set_ptrUnion(l1, l2) list_union_ptr(l1, l2)
#define set_difference(l1, l2) list_difference(l1, l2)
#define set_differenceo(l1, l2) list_difference_oid(l1, l2)
#define set_ptrDifference(l1, l2) list_difference_ptr(l1, l2)
#define equali(l1, l2) equal(l1, l2)
#define equalo(l1, l2) equal(l1, l2)
#define freeList(list) list_free(list)
#define listCopy(list) list_copy(list)
extern int length(List *list);
#endif /* ENABLE_LIST_COMPAT */
#endif /* PG_LIST_H */
