PostgreSQL 的 target_list分析(六)

进一步分析 ColumnRef:

查了一下， 原来 makeColumnRef 就在 gram.y 里面：

static Node *                                                                
makeColumnRef(char *colname, List *indirection, 
              int location, core_yyscan_t yyscanner)
{                                                                
    /*                                                            
     * Generate a ColumnRef node, with an A_Indirection node added if there 
     * is any subscripting in the specified indirection list.  However,
     * any field selection at the start of the indirection list must be 
     * transposed into the "fields" part of the ColumnRef node. 
     */                                                            
    ColumnRef  *c = makeNode(ColumnRef);                                                            
    int        nfields = 0;                                                    
    ListCell *l;                                                            
                                                                
    c->location = location; 
    foreach(l, indirection)
    {                                                            
        if (IsA(lfirst(l), A_Indices)) 
        {                                                        
            A_Indirection *i = makeNode(A_Indirection);        
            if (nfields == 0)                                                    
            {                                                    
                /* easy case - all indirection goes to A_Indirection */ 
                c->fields = list_make1(makeString(colname));
                i->indirection = check_indirection(indirection, yyscanner);                                                
            }                                                    
            else                                                    
            {                                                    
                /* got to split the list in two */
                i->indirection = check_indirection(
                    list_copy_tail(indirection, nfields),yyscanner); 
                indirection = list_truncate(indirection, nfields); 
                c->fields = lcons(makeString(colname), indirection);                                                
            }                                                    
            i->arg = (Node *) c; 
            return (Node *) i;                                                    
        }                                                        
        else if (IsA(lfirst(l), A_Star))  
        {                                                        
            /* We only allow '*' at the end of a ColumnRef */ 
            if (lnext(l) != NULL)
                parser_yyerror("improper use of \"*\"");  
        }                                                        
        nfields++;                                                        
    }                                                            
    /* No subscripting, so all indirection gets added to field list */ 
    c->fields = lcons(makeString(colname), indirection);                                                            
    return (Node *) c;                                                            
}                                                                 

这个

ColumnRef  *c = makeNode(ColumnRef);

和

c->fields = list_make1(makeString(colname)); 将 字段 赋予了 ColumnRef。

makeString 来自于 value.c:

00047 /*
00048  *  makeString
00049  *
00050  * Caller is responsible for passing a palloc'd string.
00051  */
00052 Value *
00053 makeString(char *str)
00054 {
00055     Value      *v = makeNode(Value);
00056 
00057     v->type = T_String;
00058     v->val.str = str;
00059     return v;
00060 }

Value 的定义来自于 value.h:

00042 typedef struct Value
00043 {
00044     NodeTag     type;           /* tag appropriately (eg. T_String) */
00045     union ValUnion
00046     {
00047         long        ival;       /* machine integer */
00048         char       *str;        /* string */
00049     }           val;
00050 } Value;

再看 pg_list.h 中的 list_make1:

00142 #define list_make1(x1)              lcons(x1, NIL)

再看 list.c中的  lcons:

00259 lcons(void *datum, List *list)
00260 {
00261     Assert(IsPointerList(list));
00262 
00263     if (list == NIL)
00264         list = new_list(T_List);
00265     else
00266         new_head_cell(list);
00267 
00268     lfirst(list->head) = datum;
00269     check_list_invariants(list);
00270     return list;
00271 }
00272

再看 list==NIL 时的 new_list(T_List)：

00063 new_list(NodeTag type)
00064 {
00065     List       *new_list;
00066     ListCell   *new_head;
00067 
00068     new_head = (ListCell *) palloc(sizeof(*new_head));
00069     new_head->next = NULL;
00070     /* new_head->data is left undefined! */
00071 
00072     new_list = (List *) palloc(sizeof(*new_list));
00073     new_list->type = type;
00074     new_list->length = 1;
00075     new_list->head = new_head;
00076     new_list->tail = new_head;
00077 
00078     return new_list;
00079 }

如下图：