MySQL Hash Table
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The document describes the format, creation, insertion, and deletion processes for hash tables in MySQL. It explains that a hash table contains elements like the type, number of cells, synchronization objects, and pointer to the cell array. It then outlines the functions involved in initializing a hash table, inserting data by searching for the last hash entry and adding the new entry, and deleting an entry by changing pointers to the next hash entry.
MySQL Hash Table
- 2. Contents 7/4/2016 2 • Hash Table Format • Creating a Hash Table • Insertion in Hash Table • Deletion in Hash Table
- 3. Hash Table Format 7/4/2016 3 Type of hash table, type Adaptive hash index flag, adaptive Number of cells in the hash table, n_cells Pointer to cell array, array Number of mutexes or rw_locks, n_sync_obj Synchronization objects (mutex or rw_lock), sync_obj Heaps, heaps Heap, heap Magic number, magic_n void* node mutexes rw_locks hash_table_t
- 4. Hash Table Format 7/4/2016 4 • type: type of hash table HASH_TABLE_SYNC_NONE HASH_TABLE_SYNC_MUTEX HASH_TABLE_SYNC_RW_LOCK • adaptive: TRUE if this is the hash table of the adaptive hash index • n_cells: number of cells in the hash table • array: pointer to cell array node: hash chain node, NULL if none
- 5. Hash Table Format 7/4/2016 5 • n_sync_obj: the number of mutexes or rw_locks, must be a power of 2 or 0 • sync_obj: synchronization objects ① mutexes: NULL or an array of mutexes used to protect segments of hash table ② rw_locks: NULL or an array of read/write locks • heaps: if this is non-NULL, hash chain nodes for external chaining can be allocated from these memory heaps • heap • magic_n: magic number (76561114)
- 6. Creating a Hash Table 7/4/2016 6 buf_pool_ init() buf_pool_init _instance() ha_create() ha_create _func() hash_create() Initialized hash table hash table buf_pool ->page_hash
- 10. hash_create() 7/4/2016 10 Allocate memory for hash table and hash entries Initialize the elements of hash table • ha/hash0hash.cc:293
- 11. ha_create_func() 7/4/2016 11 Create a hash table protected by rw_locks or mutex for buf_pool • ha/ha0ha.cc:80
- 12. Insertion in Hash Table 7/4/2016 12 buf_page_ get_gen() buf_read_ page() buf_read_ page_low() buf_page_init _for_read() buf_page_init() buf_block_init_low() buf_page_init_low() HASH_INSERT fil_io() HASH_SEARCH buf_LRU_get_free_block()
- 13. Page Hash - Hash Table 7/4/2016 13
- 16. HASH_INSERT 7/4/2016 16 DATA (hash entry) NULL DATA (hash entry) NULL struct3333 (hash entry) hash entry hash entry There is no hash entry If there are other hash entries, find last hash entry and insert DATA into the next of the last hash entry • include/hash0hash.h:122
- 17. Deletion in Hash Table 7/4/2016 17 buf_LRU_get _free_block() buf_LRU_scan_and _free_block() buf_LRU_free _page() buf_LRU_block_ remove_hashed() HASH_DELETE buf_LRU_get _free_only() buf_page_can_relocate()
- 18. HASH_DELETE 7/4/2016 18 1. Check whether the next hash entry of DATA is valid 2. Change the pointer to point the next hash entry DATA (hash entry) hash entry hash entry DATA (hash entry) hash entry hash entry (void*)-1 : invalid address • include/hash0hash.h:158