PostgreSQL 的 target_list分析(七)

最新推荐文章于 2024-06-19 10:17:36 发布

转载最新推荐文章于 2024-06-19 10:17:36 发布 · 134 阅读

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本文详细介绍了如何修改gram.y文件中的select部分，以确保在执行SQL查询时，后台能够准确地给出各个字段的名称。通过解析语法树节点，包括distinctClause、targetList、intoClause等关键组件，实现对查询结果中字段名称的正确显示。

修改gram.y 的select 部分，看能否找出 target_list中的各个字段名称：

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;
                    ListCell * curr=n->targetList->head; 
                    while ( curr != NULL)
                    {
                       //To display the current ResTarget Node
                       if (curr->data.ptr_value==NULL)
                          fprintf(stderr,"NULL of ptr_value\n");
                       else
                          fprintf(stderr,"Not NULL of ptr_value\n");
                       ResTarget * restar=(ResTarget *)(curr->data.ptr_value);
                       Node * nod=(Node *)(restar->val); 
                       ColumnRef * colref=(ColumnRef *)nod;
                       if (colref ==NULL)
                         fprintf(stderr,"NULL of ColumnRef\n");
                       else
                         fprintf(stderr,"Not NULL of ColumnRef\n");

                      Value * vval=(Value *)(colref->fields->head->data.ptr_value);
                      if (vval == NULL)
                         fprintf(stderr,"NULl of vval\n");
                      else
                         fprintf(stderr,"Not Null of vval\n"); 

                      fprintf(stderr,"colum is: %s\n",vval->val.str);
                      curr=curr->next;
                   }
                }
            | 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);
                }
        ;