PostgreSQL 源码解读 node的模拟实现

 

node的实现是PostgreSQL的查询解析的基础，实现的关键是两个宏，makeNode和newNode。其他节点继承自Node节点，如果增加新的结构体，需要添加NodeTag中添加对应的枚举值，并在equal和nodetoString中添加对于的处理代码。当结构体少是很容易处理，如果结构体过多，维护会比较麻烦。PostgreSQL中大约有300个继承自node的结构体，写代码的人真是需要相当的勇气和毅力呀。

 

#include
<iostream>

#include
<string.h>

#include
<assert.h>

#include
<stdlib.h>

#include
<stdio.h>

using
namespace std;

 

enum NodeTag

{

    T_Stmt,

    T_Value

};

 

typedef
struct
Node

{

    NodeTag type;

}Node;

 

Node *newNodeMacroHolder ;

#define newNode(size, tag) \

    ( \

         assert((size) >= sizeof(Node)),        /* need the tag, at least */ \

         newNodeMacroHolder = (Node *) malloc(size), \

         newNodeMacroHolder->type = (tag), \

         newNodeMacroHolder \

    )

 

#define makeNode(_type_) ((_type_ *)newNode(sizeof(_type_),T_##_type_))

#define nodeTag(nodeptr) (((const Node *)(nodeptr))->type)

 

typedef
struct
Stmt

{

    NodeTag type;

    char *text;

}Stmt;

typedef
struct
Value

{

    NodeTag type;

    long
val;

}Value;

 

bool
equal(void *a,void *b)

{

    if(a == b)

        return
true;

    if (a == NULL || b == NULL)

        return
false;

    if(nodeTag(a) != nodeTag(b))

        return
false;

 

    switch(nodeTag(a)){

        case
T_Stmt:

            return
strcmp(((const
Stmt*)a)->text,((const
Stmt *)b)->text)==0? true:false;

        case
T_Value:

            return ((const
Value *)a)->val==((const
Value *)b)->val;

        default:

            cout<<"error:unknown type"<<endl;

    }

 

    return
false;

}

char * nodetoString(void *obj)

{

    char *r =(char *)malloc(1024);

 

    if (obj == NULL){

        strcpy(r,"<>");

    }

 

    switch(nodeTag(obj)){

        case
T_Stmt:

            sprintf(r,"<Stmt:%s>",((const
Stmt *)obj)->text);

            break;

        case
T_Value:

            sprintf(r,"<Value:%ld>",((const
Value *)obj)->val);

            break;

        default:

            strcpy(r,"<unknown node type>");

    }

 

    return r;

}

 

int
main(int argc,char *argv[])

{

    Stmt *s= makeNode(Stmt);

    if(s){

        char str[]="select * from a";

        s->text=str;

    }

        

    Stmt *t= makeNode(Stmt);

    if(t){

        char str[]="select * from b";

        t->text=str;

    }

    Value *v=makeNode(Value);

    if(v){

        v->val=100;

    }

 

    cout<<"t->text:"<<t->text<<endl;

    

    cout<<"equal:"<<equal(s,t)<<endl;

 

    cout<<nodetoString(t)<<endl;

 

    free(s);

    free(t);

    free(v);

    return 0;

}

 

如果准备使用C语言来实现node结构体的话，尤其是准备采用gcc编译的话，要注意将newNode的宏设置为

/* 针对gcc版本的newNode */

#define newNode(size, tag) \

({    Node *_result; \

    AssertMacro((size) >= sizeof(Node));/* 检测申请的内存大小，>>=sizeof(Node) */ \

    _result = (Node *) palloc0fast(size); /* 申请内存 */ \

    _result->type = (tag); /*设置TypeTag */ \

    _result; /*返回值*/\

})

 

参见我的另一篇笔记《PostgreSQL源码解读 基础结构 node》