PostgreSQL 的 语法分析的理解(五)

接上文，从 simple_select 中的 target_list ，再看target_list部分的内容：

/*****************************************************************************                                    
 *                                    
 *    target list for SELECT                                
 *                                    
 *****************************************************************************/                                    
                                    
target_list:                                    
            target_el                { $$ = list_make1($1); }        
            | target_list ',' target_el                { $$ = lappend($1, $3); }        
        ;                            
                                    
target_el:    a_expr AS ColLabel                                
                {                    
                    $$ = makeNode(ResTarget);                
                    $$->name = $3;                
                    $$->indirection = NIL;                
                    $$->val = (Node *)$1;                
                    $$->location = @1;                
                }                    
            /*                        
             * We support omitting AS only for column labels that aren't 
             * any known keyword.  There is an ambiguity against postfix 
             * operators: is "a ! b" an infix expression, or a postfix  
             * expression and a column label?  We prefer to resolve this 
             * as an infix expression, which we accomplish by assigning 
             * IDENT a precedence higher than POSTFIXOP.                        
             */                        
            | a_expr IDENT                        
                {                    
                    $$ = makeNode(ResTarget);                
                    $$->name = $2;                
                    $$->indirection = NIL;                
                    $$->val = (Node *)$1;                
                    $$->location = @1;                
                }                    
            | a_expr                        
                {                    
                    $$ = makeNode(ResTarget);                
                    $$->name = NULL;                
                    $$->indirection = NIL;                
                    $$->val = (Node *)$1;                
                    $$->location = @1;                
                }                    
            | '*'                        
                {                    
                    ColumnRef *n = makeNode(ColumnRef);                
                    n->fields = list_make1(makeNode(A_Star));                
                    n->location = @1;                
                                    
                    $$ = makeNode(ResTarget);                
                    $$->name = NULL;                
                    $$->indirection = NIL;                
                    $$->val = (Node *)n;                
                    $$->location = @1;                
                }                    
        ;                            

从上面可以看出：

target_list 是一个递归表达式：

target_list:  target_el

                 | target_list  ','  target_el

而具体 target_el又分为以下几种：

         a_expr AS ColLabel

         a_expr  INDENT

         a_expr

         '*'

这个 target_list 就是指我们平时 select col1,col2,col3 ... from table 中的各个列的组合。