Mysql源码学习——没那么简单的Hash

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.hmysys/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
<font size="4"> </font><font size="3">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<em><strong>include/my_sys.h</strong></em> 中。</font>
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));
}
<font size="3">   可以看到,_my_hash_init函数主要是初始化HASH结构体和hash->array(DYNAMIC_ARRAY结构体)。</font>
  • 动态HASH函数

<font size="3">   我们首先来看下hash函数的定义:</font>
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)) < maxlength) return (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
<font size="3">   </font>
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 <yan2...@facebook.com> writes:
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)?
<strong><font color="#ff0000">The hash algorithm is based on dynamic hashing without empty slots.</font></strong>
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。

<a href="http://images.cnblogs.com/cnblogs_com/nocode/201110/201110142107072298.png"><img title="image" style="border-width: 0px; display: block; float: none; margin-left: auto; margin-right: auto;" alt="image" src="http://images.cnblogs.com/cnblogs_com/nocode/201110/201110142107114127.png" border="0" height="910" width="570"></a>
<font size="3"></font>

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)) < maxlength) return (hashnr & (buffmax-1));
  return (hashnr & ((buffmax >> 1) -1));
}
<font size="3">  </font>

   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。

image

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)。

  1. 根据flag去判断是否是否唯一Hash,如果是唯一Hash,去查找Hash表中是否存在重复值(dupliacate entry),存在则报错。
  2. 进行桶分裂,对应代码中的if (idx != info->records)分支。这个分支有些费解,稍微提示下:gpos和ptr_to_rec指示了低位需要移动的数据,gpos2和 ptr_to_rec2只是了高位需要移动的数据。LOWFIND表示低位存在值,LOWUSED表示低位是否进行了调整。HIGH的宏意义基本相同。 if (!(hash_nr & halfbuff)) 用于判断hash值存在高位还是低位。
  3. 计算新值对应的bucket号,插入。如果此位置上存在元素(一般情况都存在,除非是empty,概率比较小),调整原始元素的位置。
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