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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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