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Hash链表的应用比较常见，其目的就是为了将不同的值映射到不同的位置，查找的时候直接找到相应的位置，而不需要传统的顺序遍历或是二分查找，从而达到减少查询时间的目的。常规的hash是预定义一定的桶(bucket)，规定一个hash函数，然后进行散列。然而Mysql中的hash没有固定的bucket，hash函数也是动态变化的，本文就进行非深入介绍。
基本结构体
Hash的结构体定义以及相关的函数接口定义在include/hash.h和mysys/hash.c两个文件中。下面是HASH结构体的定义。
typedef struct st_hash {
size_t key_offset,key_length; /* Length of key if const length */
size_t blength;
ulong records;
uint flags;
DYNAMIC_ARRAY array; /* Place for hash_keys */
my_hash_get_key get_key;
void (*free)(void *);
CHARSET_INFO *charset;
} HASH;
成员名 说明 key_offset hash时key的offset，在不指定hash函数的情况下有意义 key_length key的长度，用于计算key值 blength 非常重要的辅助结构，初始为1，动态变化，用于hash函数计算，这里理解为bucket length（其实不是真实的bucket数） records 实际的记录数 flags 是否允许存在相同的元素，取值为HASH_UNIQUE(1)或者0 array 存储元素的数组 get_key 用户定义的hash函数，可以为NULL free 析构函数，可以为NULL charset 字符集
HASH结构体里面包含了一个动态数组结构体DYNAMIC_ARRAY，这里就一并介绍了。其定义在include/my_sys.h中。
typedef struct st_dynamic_array
{
uchar *buffer;
uint elements,max_element;
uint alloc_increment;
uint size_of_element;
} DYNAMIC_ARRAY;
成员名 说明 buffer 一块连续的地址空间，用于存储数据，可以看成一个数组空间 elements 元素个数 max_element 元素个数上限 alloc_increment 当元素达到上限时，即buffer满时，按照alloc_increment进行扩展 size_of_element 每个元素的长度
初始化函数
Hash初始化函数对外提供两个，my_hash_init和my_hash_init2，其区别即是否定义了growth_size（用于设置DYNAMIC_ARRAY的alloc_increment）。代码在mysys/hash.c中。
#define my_hash_init(A,B,C,D,E,F,G,H) /
_my_hash_init(A,0,B,C,D,E,F,G,H)
#define my_hash_init2(A,B,C,D,E,F,G,H,I) /
_my_hash_init(A,B,C,D,E,F,G,H,I)
/**
@brief Initialize the hash
@details
Initialize the hash, by defining and giving valid values for
its elements. The failure to allocate memory for the
hash->array element will not result in a fatal failure. The
dynamic array that is part of the hash will allocate memory
as required during insertion.
@param[in,out] hash The hash that is initialized
@param[in] charset The charater set information
@param[in] size The hash size
@param[in] key_offest The key offset for the hash
@param[in] key_length The length of the key used in
the hash
@param[in] get_key get the key for the hash
@param[in] free_element pointer to the function that
does cleanup
@return inidicates success or failure of initialization
@retval 0 success
@retval 1 failure
*/
my_bool
_my_hash_init(HASH *hash, uint growth_size, CHARSET_INFO *charset,
ulong size, size_t key_offset, size_t key_length,
my_hash_get_key get_key,
void (*free_element)(void*), uint flags)
{
DBUG_ENTER("my_hash_init");
DBUG_PRINT("enter",("hash: 0x%lx size: %u", (long) hash, (uint) size));
hash->records=0;
hash->key_offset=key_offset;
hash->key_length=key_length;
hash->blength=1;
hash->get_key=get_key;
hash->free=free_element;
hash->flags=flags;
hash->charset=charset;
DBUG_RETURN(my_init_dynamic_array_ci(&hash->array,
sizeof(HASH_LINK), size, growth_size));
}
array（DYNAMIC_ARRAY结构体）。
动态HASH函数
static inline char*
my_hash_key(const HASH *hash, const uchar *record, size_t *length,
my_bool first)
{
if (hash->get_key)
return (char*) (*hash->get_key)(record,length,first);
*length=hash->key_length;
return (char*) record+hash->key_offset;
}
static uint my_hash_mask(my_hash_value_type hashnr, size_t buffmax,
size_t maxlength)
{
if ((hashnr & (buffmax-1))
return (hashnr & ((buffmax >> 1) -1));
}
my_hash_key参数 说明 hash HASH链表结构 record 带插入的元素的值 length 带插入元素的值长度 first 辅助参数
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my_hash_mask参数 说明 hashnr my_hash_key的计算结果 buffmax hash结构体中的blength maxlength 实际桶的个数
你可能要问我怎么有两个？其实这和我们平时使用的差不多，第一个函数my_hash_key是根据我们的值进行Hash Key计算，一般我们在计算后，会对hash key进行一次模运算，以便计算结果在我们的bucket中。即my_hash_key的结果作为my_hash_mask的第一个输入参数。其实到这里都是非常好理解的，唯一让我蛋疼的是my_hash_mask的实现，其计算结果是和第二和第三个参数有关，即Hash结构体中的blength和records有关。动态变化的，我去..
看到这里我迷惑了，我上网经过各种百度，谷歌，终于让我找到了一封Mysql Expert的回信：
Hi!
"Yan" == Yan Yu
Yan> Dear MySQL experts:
Yan> Thank you so much for your reply to my previous Qs, they are very
Yan> helpful!
Yan> Could someone please help me understand function my_hash_insert()
Yan> in mysys/hash.cc?
Yan> what are lines 352 -429 trying to achieve? Are they just some
Yan> optimization to shuffle existing
Yan> hash entries in the table (since the existing hash entries may be in
Yan> the wrong slot due to chaining
Yan> in the case of hash collision)?
The hash algorithm is based on dynamic hashing without empty slots.
This means that when you insert a new key, in some cases a small set
of old keys needs to be moved to other buckets. This is what the code
does.
Regards,
Monty
红色注释的地方是重点，dynamic hash，原来如此，动态hash，第一次听说，在网上下了个《Dynamic Hash Tables》的论文，下面图解下基本原理。
动态Hash的本质是Hash函数的设计，图中给出的动态hash函数只是论文中提到的一个例子。下面就具体解读下Mysql中的hash插入——my_hash_insert
my_hash_insert非深入解析
首先给出my_hash_insert的源代码,代码在mysys/hash.c中。
my_bool my_hash_insert(HASH *info, const uchar *record)
{
int flag;
size_t idx,halfbuff,first_index;
my_hash_value_type hash_nr;
uchar *UNINIT_VAR(ptr_to_rec),*UNINIT_VAR(ptr_to_rec2);
HASH_LINK *data,*empty,*UNINIT_VAR(gpos),*UNINIT_VAR(gpos2),*pos;
if (HASH_UNIQUE & info->flags)
{
uchar *key= (uchar*) my_hash_key(info, record, &idx, 1);
if (my_hash_search(info, key, idx))
return(TRUE); /* Duplicate entry */
}
flag=0;
if (!(empty=(HASH_LINK*) alloc_dynamic(&info->array)))
return(TRUE); /* No more memory */
data=dynamic_element(&info->array,0,HASH_LINK*);
halfbuff= info->blength >> 1;
idx=first_index=info->records-halfbuff;
if (idx != info->records) /* If some records */
{
do
{
pos=data+idx;
hash_nr=rec_hashnr(info,pos->data);
if (flag == 0) /* First loop; Check if ok */
if (my_hash_mask(hash_nr, info->blength, info->records) != first_index)
break;
if (!(hash_nr & halfbuff))
{ /* Key will not move */
if (!(flag & LOWFIND))
{
if (flag & HIGHFIND)
{
flag=LOWFIND | HIGHFIND;
/* key shall be moved to the current empty position */
gpos=empty;
ptr_to_rec=pos->data;
empty=pos; /* This place is now free */
}
else
{
flag=LOWFIND | LOWUSED; /* key isn't changed */
gpos=pos;
ptr_to_rec=pos->data;
}
}
else
{
if (!(flag & LOWUSED))
{
/* Change link of previous LOW-key */
gpos->data=ptr_to_rec;
gpos->next= (uint) (pos-data);
flag= (flag & HIGHFIND) | (LOWFIND | LOWUSED);
}
gpos=pos;
ptr_to_rec=pos->data;
}
}
else
{ /* key will be moved */
if (!(flag & HIGHFIND))
{
flag= (flag & LOWFIND) | HIGHFIND;
/* key shall be moved to the last (empty) position */
gpos2 = empty; empty=pos;
ptr_to_rec2=pos->data;
}
else
{
if (!(flag & HIGHUSED))
{
/* Change link of previous hash-key and save */
gpos2->data=ptr_to_rec2;
gpos2->next=(uint) (pos-data);
flag= (flag & LOWFIND) | (HIGHFIND | HIGHUSED);
}
gpos2=pos;
ptr_to_rec2=pos->data;
}
}
}
while ((idx=pos->next) != NO_RECORD);
if ((flag & (LOWFIND | LOWUSED)) == LOWFIND)
{
gpos->data=ptr_to_rec;
gpos->next=NO_RECORD;
}
if ((flag & (HIGHFIND | HIGHUSED)) == HIGHFIND)
{
gpos2->data=ptr_to_rec2;
gpos2->next=NO_RECORD;
}
}
/* Check if we are at the empty position */
idx= my_hash_mask(rec_hashnr(info, record), info->blength, info->records + 1);
pos=data+idx;
if (pos == empty)
{
pos->data=(uchar*) record;
pos->next=NO_RECORD;
}
else
{
/* Check if more records in same hash-nr family */
empty[0]=pos[0];
gpos= data + my_hash_rec_mask(info, pos, info->blength, info->records + 1);
if (pos == gpos)
{
pos->data=(uchar*) record;
pos->next=(uint) (empty - data);
}
else
{
pos->data=(uchar*) record;
pos->next=NO_RECORD;
movelink(data,(uint) (pos-data),(uint) (gpos-data),(uint) (empty-data));
}
}
if (++info->records == info->blength)
info->blength+= info->blength;
return(0);
}
同时给出动态hash函数如下：
static uint my_hash_mask(my_hash_value_type hashnr, size_t buffmax,
size_t maxlength)
{
if ((hashnr & (buffmax-1))
return (hashnr & ((buffmax >> 1) -1));
}
可以看出，hash函数是hash key与buffmax的模运算，buffmax即HASH结构中的blength，由my_hash_insert中最后几行代码可知：info->blength+= info->blength; 其初始值为1，即blength = 2^n，而且blengh始终是大于records。这个动态hash函数的基本意思是key%（2^n）。依然用图解这个动态hash函数。
hash函数基本清楚了，但是mysql的具体实现还是比较值得探讨的。那封回信中也提到了without empty slots，是的，它这种实现方式是根据实际的数据量进行桶数的分配。我这里大概说下代码的流程（有兴趣的还需要大家自己仔细琢磨）。
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