本篇内容介绍了“分析数据库实现原理”的有关知识,在实际案例的操作过程中,不少人都会遇到这样的困境,接下来就让小编带领大家学习一下如何处理这些情况吧!希望大家仔细阅读,能够学有所成!Hash连接,如内存足够,
本篇内容介绍了“分析数据库实现原理”的有关知识,在实际案例的操作过程中,不少人都会遇到这样的困境,接下来就让小编带领大家学习一下如何处理这些情况吧!希望大家仔细阅读,能够学有所成!
Hash连接,如内存足够,首先遍历内表创建Hash表,然后遍历外表,对连接键计算HashCode,如一致,则遍历Hash表中具有同一HashCode的链表,值一致,则返回该值。
如内存不够,可遍历两张表,使用同样的Hash函数把表拆分为N个Hash“分区”,遍历内表每一个Hash分区和外表相应的Hash分区,如找到与连接键值一致的数据,则返回该值。
详见代码注释.
#include <stdio.h>
#include <stdlib.h>
#include "hash_join.h"
#define MAX_ELEMENTS 1024
//生成hash code
static int generate_hashcode(int n)
{
return n % HASH_BUCKET;
}
//生成hash桶(写入到文件中,以文件的方式模拟)
static int generate_bucket(FILE *file,char *tag)
{
printf("----------- generate_bucket ---------- \n");
//数组
char buf[MAX_BYTES];
FILE *fd = NULL;
for(;!feof(file);)
{
int x = read_int(file,buf);
if(x == 0)
break;
int hashcode = generate_hashcode(x);
char filename[30];
sprintf(filename,"/cygdrive/d/tmp/hash/%s_%d.csv",tag,hashcode);
//printf("Hash code is %d,Bucket filename is %s.\n",hashcode,filename);
fd = fopen(filename,"a");
if(fd == NULL)
{
printf("Can not open file %s.\n",filename);
return 0;
}
//写入文件中
write_int(fd,x);
fclose(fd);
}
return 1;
}
//把hash表加载到内存中,适用于内存足够的情况
//使用二维数组模拟Hash表,D1 : hash桶,D2 : 桶中的数据
static int load_hashtable(int ht[][MAX_ELEMENTS])
{
printf("----------- load_hashtable ---------- \n");
for(int i=0;i < HASH_BUCKET;i++)
{
//循环桶号
char filename[MAX_BYTES];
//读文件
sprintf(filename,"/cygdrive/d/tmp/hash/inner_%d.csv",i);
FILE *fd = fopen(filename,"r");
if(fd == NULL){
//printf("Can not open file : %s\n",filename);
continue;
}
int j=0;
char buf[MAX_BYTES];
for(;!feof(fd) && j < MAX_ELEMENTS;)
{
//把文件内容放到数组中
int x = read_int(fd,buf);
ht[i][j++] = x;
}
fclose(fd);
}
return 1;
}
//使用内存创建hash表进行hash连接
static void hash_join_onmemory(FILE *outerfile,FILE *innerfile)
{
printf("----------- hash_join_onmemory ---------- \n");
int ht[HASH_BUCKET][MAX_ELEMENTS];
char buffer[MAX_BYTES];
int flag = 0;
//创建hash bucket文件
flag = generate_bucket(innerfile,"inner");
if(!flag)
{
printf("Can not generate bucket file!\n");
return;
}
//加载到hash表中(二维数组模拟)
flag = load_hashtable(ht);
if(!flag)
{
printf("Can not load hash table!\n");
return;
}
//遍历第二个文件,执行JOIN
for(;!feof(outerfile);)
{
//读第二个文件,执行join
int outer = read_int(outerfile,buffer);
//计算hashcode
int hashcode = generate_hashcode(outer);
for(int i=0;i < MAX_ELEMENTS;i++)
{
//遍历hash桶中的数据,找到对应的数据
if(ht[hashcode][i] == outer)
{
printf("Found one,hash bucket is %d,value is : %d.\n",hashcode,outer);
}
}
}
}
//使用磁盘缓存进行hash连接
static void hash_join_ondisk(FILE *outerfile,FILE *innerfile)
{
printf("----------- hash_join_ondisk ---------- \n");
char buffer[MAX_BYTES];
int flag = 0;
//创建hash"桶"文件
flag = generate_bucket(innerfile,"inner");
if(!flag)
{
printf("Can not generate inner bucket file!\n");
return;
}
flag = generate_bucket(outerfile,"outer");
if(!flag)
{
printf("Can not generate outer bucket file!\n");
return;
}
//遍历hash值相同的文件,执行连接
for(int i=0;i < HASH_BUCKET;i++)
{
//从0号桶开始
char innerfname[MAX_BYTES];
char outerfname[MAX_BYTES];
//读文件
sprintf(innerfname,"/cygdrive/d/tmp/hash/%s_%d.csv","inner",i);
sprintf(outerfname,"/cygdrive/d/tmp/hash/%s_%d.csv","outer",i);
FILE *fd_inner = fopen(innerfname,"r");
if(fd_inner == NULL){
//printf("Can not open file : %s\n",filename);
continue;
}
FILE *fd_outer = fopen(outerfname,"r");
if(fd_outer == NULL)
{
continue;
}
for(;!feof(fd_outer);)
{
int v_out = read_int(fd_outer,buffer);
if(v_out == 0)
continue;
for(;!feof(fd_inner);)
{
int v_in = read_int(fd_inner,buffer);
if(v_in == 0)
continue;
if(v_out == v_in)
{
printf("Found one,hash bucket is %d,value is : %d.\n",i,v_out);
}
}
rewind(fd_inner);
}
}
}
//执行Hash连接
void hash_join(char *file1,char * file2,char *flag)
{
printf("----------- hash join ---------- \n");
FILE *outerfile = fopen(file1,"r");
if(outerfile == NULL)
{
printf("Can not open file %s.\n",file1);
return;
}
//打开第二个文件
FILE *innerfile = fopen(file2,"r");
if(innerfile == NULL)
{
printf("Can not open file %s.\n",file2);
return;
}
//执行JOIN
if(strcmp(flag,"memory") == 0)
hash_join_onmemory(outerfile,innerfile);
else
hash_join_ondisk(outerfile,innerfile);
//关闭
fclose(outerfile);
fclose(innerfile);
}
运行输出
$ cat file1.csv
1
2
3
4
5
1
234
2939
9002
20
$ cat file2.csv
11
20
3
40
55
50
234
33
90
1
$ /cygdrive/d/tmp/test.exe file1.csv file2.csv
------------- use memory ------------------
----------- hash join ----------
----------- hash_join_onmemory ----------
----------- generate_bucket ----------
----------- load_hashtable ----------
Found one,hash bucket is 1,value is : 1.
Found one,hash bucket is 3,value is : 3.
Found one,hash bucket is 1,value is : 1.
Found one,hash bucket is 106,value is : 234.
Found one,hash bucket is 20,value is : 20.
------------- use disk ------------------
----------- hash join ----------
----------- hash_join_ondisk ----------
----------- generate_bucket ----------
----------- generate_bucket ----------
Found one,hash bucket is 1,value is : 1.
Found one,hash bucket is 1,value is : 1.
Found one,hash bucket is 3,value is : 3.
Found one,hash bucket is 20,value is : 20.
Found one,hash bucket is 106,value is : 234.
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