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