Redis Hash data structure

一、Redis Hash

1, Redis dictionary implemented as the underlying hash table, a hash table which nodes can have multiple hash tables, each hash table of the node is saved in a dictionary key pair.

2, Redis hash table used by the dictionary dict.h/dicthtdefinition structure:

typedef struct dictht { 

    // hash table array 
    dictEntry ** Table; 

    // hash table size 
    unsigned Long size; 

    // hash table size mask for calculating the index value
     // always equal size -. 1 
    unsigned Long sizemask ; 

    // number of the hash table of an existing node 
    unsigned Long Used; 

} dictht;

1.table property is a genus group, each element of the array pointing to adict.h/dictEntry结构的指针，每个dictEntry结构保存着一个键值对；

2.size attribute records the size of the hash table, that is, the size of the array table, and used property records the hash table there are nodes (key-value pairs) number;

3.sizemask value of the attribute is always equal to size-1, and the property values determined with a hash key which index table should be placed above the array;

3. hash table node uses dictEntrythe structure of said each dictEntrystructure are preserved in a key-value pair:

typedef struct dictEntry { 

    // key 
    void * Key; 

    // value 
    Union {
         void * Val; 
        uint64_t U64; 
        an int64_t S64; 
    } V; 

    // point to the next node hash table, a linked list is formed 
    struct dictEntry * Next; 

} dictEntry;

1.key attribute holds the key to the key, and v is the attribute value of the stored key-value pair, where the value of v may be a pointer, or a uint64_t integer, or an integer int64_t;

2.next property is a pointer to another node in the hash table, the hash value of the pointer may be a plurality of the same key problems connected together, in order to resolve the conflict bond;

Illustrate how, connected together by the next pointer to the same key two index values k1 and k0:

4, Redis in the dictionary dict.h/dictshows the structure of:

typedef struct dict { 

    // Type-specific function 
    dictType * type; 

    // private data 
    void * privdata; 

    // hash table 
    dictht HT [ 2 ]; 

    // rehash index
     // When rehash not performed, a value of -1 
    int rehashidx ; / * rehashing Not in Progress IF rehashidx == -1 * / 

} dict;

1. typeproperties and privdataproperties for different types of key-value pairs to create a set of polymorphic dictionaries;

2. The htproperty is an array that contains two entries, each entry in the array is a dicththash table, under normal circumstances, only use the dictionary ht[0]hash table, ht[1]hash table will only to ht[0]be used when the hash table rehash ;

3. In addition to ht[1]outside, and another rehash related properties that rehashidx: rehash it records the current pace, if not currently performing rehash, then its value -1.

4 illustrates a normal state of the dictionary (not for the rehash) of:

Redis Hash data structure

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