根据 <PostgreSQL 数据库内核分析>200和201页的说法，
ResTarget 应该指向 ColumnRef 。这是如何实现的呢？
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;
}
;
这个时候，距离ColumnRef 就已经不远了。
然后挑选最简单的情况：
columnref: ColId { $$ = makeColumnRef($1, NIL, @1, yyscanner); } | ColId indirection { $$ = makeColumnRef($1, $2, @1, yyscanner); } ;
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。
可以看出，语法分析是一层套着一层。而整个语法分析的目的，就是逐层地构建一个语法分析树形结构。