根據 <PostgreSQL 數據庫內核分析>200和201頁的說法，

ResTarget 應該指向 ColumnRef 。這是如何實現的呢？

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; } ;

?和

a_expr: c_expr
{ $$ = $1; } | a_expr TYPECAST Typename { $$ = makeTypeCast($1, $3, @2); } | a_expr COLLATE any_name { CollateClause *n = makeNode(CollateClause); n->arg = $1; n->collname = $3; n->location = @2; $$ = (Node *) n; }
...

和

c_expr: columnref { $$ = $1; }| AexprConst { $$ = $1; }| PARAM opt_indirection { ParamRef *p = makeNode(ParamRef); p->number = $1; p->location = @1; if ($2) { A_Indirection *n = makeNode(A_Indirection); n->arg = (Node *) p; n->indirection = check_indirection($2, yyscanner); $$ = (Node *) n; } else $$ = (Node *) p; } | '(' a_expr ')' opt_indirection { if ($4) { A_Indirection *n = makeNode(A_Indirection); n->arg = $2; n->indirection = check_indirection($4, yyscanner); $$ = (Node *)n; } else $$ = $2; } | case_expr { $$ = $1; } | func_expr { $$ = $1; } | select_with_parens %prec UMINUS { SubLink *n = makeNode(SubLink); n->subLinkType = EXPR_SUBLINK; n->testexpr = NULL; n->operName = NIL; n->subselect = $1; n->location = @1; $$ = (Node *)n; } | EXISTS select_with_parens { SubLink *n = makeNode(SubLink); n->subLinkType = EXISTS_SUBLINK; n->testexpr = NULL; n->operName = NIL; n->subselect = $2; n->location = @1; $$ = (Node *)n; } | ARRAY select_with_parens { SubLink *n = makeNode(SubLink); n->subLinkType = ARRAY_SUBLINK; n->testexpr = NULL; n->operName = NIL; n->subselect = $2; n->location = @1; $$ = (Node *)n; } | ARRAY array_expr { A_ArrayExpr *n = (A_ArrayExpr *) $2; Assert(IsA(n, A_ArrayExpr)); /* point outermost A_ArrayExpr to the ARRAY keyword */ n->location = @1; $$ = (Node *)n; } | row { RowExpr *r = makeNode(RowExpr); r->args = $1; r->row_typeid = InvalidOid; /* not analyzed yet */ r->location = @1; $$ = (Node *)r; } ;

這個時候，距離ColumnRef 就已經不遠了。

然后挑選最簡單的情況：

columnref: ColId { $$ = makeColumnRef($1, NIL, @1, yyscanner); } | ColId indirection { $$ = makeColumnRef($1, $2, @1, yyscanner); } ;

查了一下， 原來 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; }

這個

ColumnRef ?*c = makeNode(ColumnRef);

和

c->fields = list_make1(makeString(colname)); 將 字段 賦予了 ColumnRef。

可以看出，語法分析是一層套著一層。而整個語法分析的目的，就是逐層地構建一個語法分析樹形結構。

總結

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