PostgreSQL 的 语法分析的理解(二)

对于PostgreSQL的语法分析，修改其 gram.y后，

opt_distinct:
DISTINCT        { $$ = list_make1(NIL); }
| DISTINCT ON '(' expr_list ')'    { $$ = $4; }
| ALL            { $$ = NIL; }
| /*EMPTY*/      { $$ = NIL; }

simple_select:
SELECT    opt_distinct    target_list
into_clause     from_clause     where_clause
group_clause     having_clause    window_clause
{
SelectStmt *n = makeNode(SelectStmt);
n->distinctClause = $2;
n->targetList = $3;
n->intoClause = $4;
n->fromClause = $5;
n->whereClause = $6;
n->groupClause = $7;
n->havingClause = $8;
n->windowClause = $9;
$$ = (Node *)n;
}
| values_clause   { $$ = $1; }
| TABLE relation_expr
{
/* same as SELECT * FROM relation_expr */
ColumnRef *cr = makeNode(ColumnRef);
ResTarget *rt = makeNode(ResTarget);
SelectStmt *n = makeNode(SelectStmt);

cr->fields = list_make1(makeNode(A_Star));
cr->location = -1;

rt->name = NULL;
rt->indirection = NIL;
rt->val = (Node *)cr;
rt->location = -1;

n->targetList = list_make1(rt);
n->fromClause = list_make1($2);
$$ = (Node *)n;
}
| select_clause UNION opt_all select_clause
{
$$ = makeSetOp(SETOP_UNION, $3, $1, $4);
}
| select_clause INTERSECT opt_all select_clause
{
$$ = makeSetOp(SETOP_INTERSECT, $3, $1, $4);
}
| select_clause EXCEPT opt_all select_clause
{
$$ = makeSetOp(SETOP_EXCEPT, $3, $1, $4);
}
;

-->

opt_distinct:
DISTINCT        { $$ = list_make1(NIL);
fprintf(stderr,"In opt_distinct\n");
}
| DISTINCT ON '(' expr_list ')'    { $$ = $4; }
| ALL            { $$ = NIL; }
| /*EMPTY*/      { $$ = NIL; }

-->

simple_select:
SELECT    opt_distinct    target_list
into_clause     from_clause     where_clause
group_clause     having_clause    window_clause
{
SelectStmt *n = makeNode(SelectStmt);
n->distinctClause = $2;
n->targetList = $3;
n->intoClause = $4;
n->fromClause = $5;
n->whereClause = $6;
n->groupClause = $7;
n->havingClause = $8;
n->windowClause = $9;
$$ = (Node *)n;
fprintf(stderr,"In simple select\n“);
}
| values_clause   { $$ = $1; }
| TABLE relation_expr
{
/* same as SELECT * FROM relation_expr */
ColumnRef *cr = makeNode(ColumnRef);
ResTarget *rt = makeNode(ResTarget);
SelectStmt *n = makeNode(SelectStmt);

cr->fields = list_make1(makeNode(A_Star));
cr->location = -1;

rt->name = NULL;
rt->indirection = NIL;
rt->val = (Node *)cr;
rt->location = -1;

n->targetList = list_make1(rt);
n->fromClause = list_make1($2);
$$ = (Node *)n;
}
| select_clause UNION opt_all select_clause
{
$$ = makeSetOp(SETOP_UNION, $3, $1, $4);
}
| select_clause INTERSECT opt_all select_clause
{
$$ = makeSetOp(SETOP_INTERSECT, $3, $1, $4);
}
| select_clause EXCEPT opt_all select_clause
{
$$ = makeSetOp(SETOP_EXCEPT, $3, $1, $4);
}
;

在psql执行查询：select distinct id from a6;

在后台会显示：

In opt_distinct

In simple select