PostgreSQL 源碼解讀（143）- Buffer Manager#8（BufTableHashCode函數(shù)）

本節(jié)簡單介紹了PostgreSQL緩存管理(Buffer Manager)中的實(shí)現(xiàn)函數(shù)ReadBuffer_common->BufferAlloc->BufTableHashCode,該函數(shù)根據(jù)BufferTag計(jì)算Hash Code。

一、數(shù)據(jù)結(jié)構(gòu)

BufferDesc
共享緩沖區(qū)的共享描述符(狀態(tài))數(shù)據(jù)

/*
 * Flags for buffer descriptors
 * buffer描述器標(biāo)記
 *
 * Note: TAG_VALID essentially means that there is a buffer hashtable
 * entry associated with the buffer's tag.
 * 注意:TAG_VALID本質(zhì)上意味著有一個(gè)與緩沖區(qū)的標(biāo)記相關(guān)聯(lián)的緩沖區(qū)散列表?xiàng)l目。
 */
//buffer header鎖定
#define BM_LOCKED               (1U << 22)  /* buffer header is locked */
//數(shù)據(jù)需要寫入(標(biāo)記為DIRTY)
#define BM_DIRTY                (1U << 23)  /* data needs writing */
//數(shù)據(jù)是有效的
#define BM_VALID                (1U << 24)  /* data is valid */
//已分配buffer tag
#define BM_TAG_VALID            (1U << 25)  /* tag is assigned */
//正在R/W
#define BM_IO_IN_PROGRESS       (1U << 26)  /* read or write in progress */
//上一個(gè)I/O出現(xiàn)錯(cuò)誤
#define BM_IO_ERROR             (1U << 27)  /* previous I/O failed */
//開始寫則變DIRTY
#define BM_JUST_DIRTIED         (1U << 28)  /* dirtied since write started */
//存在等待sole pin的其他進(jìn)程
#define BM_PIN_COUNT_WAITER     (1U << 29)  /* have waiter for sole pin */
//checkpoint發(fā)生,必須刷到磁盤上
#define BM_CHECKPOINT_NEEDED    (1U << 30)  /* must write for checkpoint */
//持久化buffer(不是unlogged或者初始化fork)
#define BM_PERMANENT            (1U << 31)  /* permanent buffer (not unlogged,
                                             * or init fork) */
/*
 *  BufferDesc -- shared descriptor/state data for a single shared buffer.
 *  BufferDesc -- 共享緩沖區(qū)的共享描述符(狀態(tài))數(shù)據(jù)
 *
 * Note: Buffer header lock (BM_LOCKED flag) must be held to examine or change
 * the tag, state or wait_backend_pid fields.  In general, buffer header lock
 * is a spinlock which is combined with flags, refcount and usagecount into
 * single atomic variable.  This layout allow us to do some operations in a
 * single atomic operation, without actually acquiring and releasing spinlock;
 * for instance, increase or decrease refcount.  buf_id field never changes
 * after initialization, so does not need locking.  freeNext is protected by
 * the buffer_strategy_lock not buffer header lock.  The LWLock can take care
 * of itself.  The buffer header lock is *not* used to control access to the
 * data in the buffer!
 * 注意:必須持有Buffer header鎖(BM_LOCKED標(biāo)記)才能檢查或修改tag/state/wait_backend_pid字段.
 * 通常來說,buffer header lock是spinlock,它與標(biāo)記位/參考計(jì)數(shù)/使用計(jì)數(shù)組合到單個(gè)原子變量中.
 * 這個(gè)布局設(shè)計(jì)允許我們執(zhí)行原子操作,而不需要實(shí)際獲得或者釋放spinlock(比如,增加或者減少參考計(jì)數(shù)).
 * buf_id字段在初始化后不會出現(xiàn)變化,因此不需要鎖定.
 * freeNext通過buffer_strategy_lock鎖而不是buffer header lock保護(hù).
 * LWLock可以很好的處理自己的狀態(tài).
 * 務(wù)請注意的是:buffer header lock不用于控制buffer中的數(shù)據(jù)訪問!
 *
 * It's assumed that nobody changes the state field while buffer header lock
 * is held.  Thus buffer header lock holder can do complex updates of the
 * state variable in single write, simultaneously with lock release (cleaning
 * BM_LOCKED flag).  On the other hand, updating of state without holding
 * buffer header lock is restricted to CAS, which insure that BM_LOCKED flag
 * is not set.  Atomic increment/decrement, OR/AND etc. are not allowed.
 * 假定在持有buffer header lock的情況下,沒有人改變狀態(tài)字段.
 * 持有buffer header lock的進(jìn)程可以執(zhí)行在單個(gè)寫操作中執(zhí)行復(fù)雜的狀態(tài)變量更新,
 *   同步的釋放鎖(清除BM_LOCKED標(biāo)記).
 * 換句話說,如果沒有持有buffer header lock的狀態(tài)更新,會受限于CAS,
 *   這種情況下確保BM_LOCKED沒有被設(shè)置.
 * 比如原子的增加/減少(AND/OR)等操作是不允許的.
 *
 * An exception is that if we have the buffer pinned, its tag can't change
 * underneath us, so we can examine the tag without locking the buffer header.
 * Also, in places we do one-time reads of the flags without bothering to
 * lock the buffer header; this is generally for situations where we don't
 * expect the flag bit being tested to be changing.
 * 一種例外情況是如果我們已有buffer pinned,該buffer的tag不能改變(在本進(jìn)程之下),
 *   因此不需要鎖定buffer header就可以檢查tag了.
 * 同時(shí),在執(zhí)行一次性的flags讀取時(shí)不需要鎖定buffer header.
 * 這種情況通常用于我們不希望正在測試的flag bit將被改變.
 *
 * We can't physically remove items from a disk page if another backend has
 * the buffer pinned.  Hence, a backend may need to wait for all other pins
 * to go away.  This is signaled by storing its own PID into
 * wait_backend_pid and setting flag bit BM_PIN_COUNT_WAITER.  At present,
 * there can be only one such waiter per buffer.
 * 如果其他進(jìn)程有buffer pinned,那么進(jìn)程不能物理的從磁盤頁面中刪除items.
 * 因此,后臺進(jìn)程需要等待其他pins清除.這可以通過存儲它自己的PID到wait_backend_pid中,
 *   并設(shè)置標(biāo)記位BM_PIN_COUNT_WAITER.
 * 目前,每個(gè)緩沖區(qū)只能由一個(gè)等待進(jìn)程.
 *
 * We use this same struct for local buffer headers, but the locks are not
 * used and not all of the flag bits are useful either. To avoid unnecessary
 * overhead, manipulations of the state field should be done without actual
 * atomic operations (i.e. only pg_atomic_read_u32() and
 * pg_atomic_unlocked_write_u32()).
 * 本地緩沖頭部使用同樣的結(jié)構(gòu),但并不需要使用locks,而且并不是所有的標(biāo)記位都使用.
 * 為了避免不必要的負(fù)載,狀態(tài)域的維護(hù)不需要實(shí)際的原子操作
 * (比如只有pg_atomic_read_u32() and pg_atomic_unlocked_write_u32())
 *
 * Be careful to avoid increasing the size of the struct when adding or
 * reordering members.  Keeping it below 64 bytes (the most common CPU
 * cache line size) is fairly important for performance.
 * 在增加或者記錄成員變量時(shí),小心避免增加結(jié)構(gòu)體的大小.
 * 保持結(jié)構(gòu)體大小在64字節(jié)內(nèi)(通常的CPU緩存線大小)對于性能是非常重要的.
 */
typedef struct BufferDesc
{
    //buffer tag
    BufferTag   tag;            /* ID of page contained in buffer */
    //buffer索引編號(0開始),指向相應(yīng)的buffer pool slot
    int         buf_id;         /* buffer's index number (from 0) */
    /* state of the tag, containing flags, refcount and usagecount */
    //tag狀態(tài),包括flags/refcount和usagecount
    pg_atomic_uint32 state;
    //pin-count等待進(jìn)程ID
    int         wait_backend_pid;   /* backend PID of pin-count waiter */
    //空閑鏈表鏈中下一個(gè)空閑的buffer
    int         freeNext;       /* link in freelist chain */
    //緩沖區(qū)內(nèi)容鎖
    LWLock      content_lock;   /* to lock access to buffer contents */
} BufferDesc;

BufferTag
Buffer tag標(biāo)記了buffer存儲的是磁盤中哪個(gè)block

/*
 * Buffer tag identifies which disk block the buffer contains.
 * Buffer tag標(biāo)記了buffer存儲的是磁盤中哪個(gè)block
 *
 * Note: the BufferTag data must be sufficient to determine where to write the
 * block, without reference to pg_class or pg_tablespace entries.  It's
 * possible that the backend flushing the buffer doesn't even believe the
 * relation is visible yet (its xact may have started before the xact that
 * created the rel).  The storage manager must be able to cope anyway.
 * 注意:BufferTag必須足以確定如何寫block而不需要參照pg_class或者pg_tablespace數(shù)據(jù)字典信息.
 * 有可能后臺進(jìn)程在刷新緩沖區(qū)的時(shí)候深圳不相信關(guān)系是可見的(事務(wù)可能在創(chuàng)建rel的事務(wù)之前).
 * 存儲管理器必須可以處理這些事情.
 *
 * Note: if there's any pad bytes in the struct, INIT_BUFFERTAG will have
 * to be fixed to zero them, since this struct is used as a hash key.
 * 注意:如果在結(jié)構(gòu)體中有填充的字節(jié),INIT_BUFFERTAG必須將它們固定為零，因?yàn)檫@個(gè)結(jié)構(gòu)體用作散列鍵.
 */
typedef struct buftag
{
    //物理relation標(biāo)識符
    RelFileNode rnode;          /* physical relation identifier */
    ForkNumber  forkNum;
    //相對于relation起始的塊號
    BlockNumber blockNum;       /* blknum relative to begin of reln */
} BufferTag;

HTAB
哈希表的頂層控制結(jié)構(gòu).

/*
 * Top control structure for a hashtable --- in a shared table, each backend
 * has its own copy (OK since no fields change at runtime)
 * 哈希表的頂層控制結(jié)構(gòu).
 * 在這個(gè)共享哈希表中,每一個(gè)后臺進(jìn)程都有自己的拷貝
 * (之所以沒有問題是因?yàn)閒ork出來后,在運(yùn)行期沒有字段會變化)
 */
struct HTAB
{
    //指向共享的控制信息
    HASHHDR    *hctl;           /* => shared control information */
    //段開始目錄
    HASHSEGMENT *dir;           /* directory of segment starts */
    //哈希函數(shù)
    HashValueFunc hash;         /* hash function */
    //哈希鍵比較函數(shù)
    HashCompareFunc match;      /* key comparison function */
    //哈希鍵拷貝函數(shù)
    HashCopyFunc keycopy;       /* key copying function */
    //內(nèi)存分配器
    HashAllocFunc alloc;        /* memory allocator */
    //內(nèi)存上下文
    MemoryContext hcxt;         /* memory context if default allocator used */
    //表名(用于錯(cuò)誤信息)
    char       *tabname;        /* table name (for error messages) */
    //如在共享內(nèi)存中,則為T
    bool        isshared;       /* true if table is in shared memory */
    //如為T,則固定大小不能擴(kuò)展
    bool        isfixed;        /* if true, don't enlarge */
    /* freezing a shared table isn't allowed, so we can keep state here */
    //不允許凍結(jié)共享表,因此這里會保存相關(guān)狀態(tài)
    bool        frozen;         /* true = no more inserts allowed */
    /* We keep local copies of these fixed values to reduce contention */
    //保存這些固定值的本地拷貝,以減少?zèng)_突
    //哈希鍵長度(以字節(jié)為單位)
    Size        keysize;        /* hash key length in bytes */
    //段大小,必須為2的冪
    long        ssize;          /* segment size --- must be power of 2 */
    //段偏移,ssize的對數(shù)
    int         sshift;         /* segment shift = log2(ssize) */
};
/*
 * Header structure for a hash table --- contains all changeable info
 * 哈希表的頭部結(jié)構(gòu) -- 存儲所有可變信息
 *
 * In a shared-memory hash table, the HASHHDR is in shared memory, while
 * each backend has a local HTAB struct.  For a non-shared table, there isn't
 * any functional difference between HASHHDR and HTAB, but we separate them
 * anyway to share code between shared and non-shared tables.
 * 在共享內(nèi)存哈希表中,HASHHDR位于共享內(nèi)存中,每一個(gè)后臺進(jìn)程都有一個(gè)本地HTAB結(jié)構(gòu).
 * 對于非共享哈希表,HASHHDR和HTAB沒有任何功能性的不同,
 * 但無論如何,我們還是把它們區(qū)分為共享和非共享表.
 */
struct HASHHDR
{
    /*
     * The freelist can become a point of contention in high-concurrency hash
     * tables, so we use an array of freelists, each with its own mutex and
     * nentries count, instead of just a single one.  Although the freelists
     * normally operate independently, we will scavenge entries from freelists
     * other than a hashcode's default freelist when necessary.
     * 在高并發(fā)的哈希表中,空閑鏈表會成為競爭熱點(diǎn),因此我們使用空閑鏈表數(shù)組,
     *   數(shù)組中的每一個(gè)元素都有自己的mutex和條目統(tǒng)計(jì),而不是使用一個(gè).
     *
     * If the hash table is not partitioned, only freeList[0] is used and its
     * spinlock is not used at all; callers' locking is assumed sufficient.
     * 如果哈希表沒有分區(qū),那么只有freelist[0]元素是有用的,自旋鎖沒有任何用處;
     * 調(diào)用者鎖定被認(rèn)為已足夠OK.
     */
    FreeListData freeList[NUM_FREELISTS];
    /* These fields can change, but not in a partitioned table */
    //這些域字段可以改變,但不適用于分區(qū)表
    /* Also, dsize can't change in a shared table, even if unpartitioned */
    //同時(shí),就算是非分區(qū)表,共享表的dsize也不能改變
    //目錄大小
    long        dsize;          /* directory size */
    //已分配的段大小(<= dbsize)
    long        nsegs;          /* number of allocated segments (<= dsize) */
    //正在使用的最大桶ID
    uint32      max_bucket;     /* ID of maximum bucket in use */
    //進(jìn)入整個(gè)哈希表的模掩碼
    uint32      high_mask;      /* mask to modulo into entire table */
    //進(jìn)入低于半個(gè)哈希表的模掩碼
    uint32      low_mask;       /* mask to modulo into lower half of table */
    /* These fields are fixed at hashtable creation */
    //下面這些字段在哈希表創(chuàng)建時(shí)已固定
    //哈希鍵大小(以字節(jié)為單位)
    Size        keysize;        /* hash key length in bytes */
    //所有用戶元素大小(以字節(jié)為單位)
    Size        entrysize;      /* total user element size in bytes */
    //分區(qū)個(gè)數(shù)(2的冪),或者為0
    long        num_partitions; /* # partitions (must be power of 2), or 0 */
    //目標(biāo)的填充因子
    long        ffactor;        /* target fill factor */
    //如目錄是固定大小,則該值為dsize的上限值
    long        max_dsize;      /* 'dsize' limit if directory is fixed size */
    //段大小,必須是2的冪
    long        ssize;          /* segment size --- must be power of 2 */
    //端偏移,ssize的對數(shù)
    int         sshift;         /* segment shift = log2(ssize) */
    //一次性分配的條目個(gè)數(shù)
    int         nelem_alloc;    /* number of entries to allocate at once */
#ifdef HASH_STATISTICS
    /*
     * Count statistics here.  NB: stats code doesn't bother with mutex, so
     * counts could be corrupted a bit in a partitioned table.
     * 統(tǒng)計(jì)信息.
     * 注意:統(tǒng)計(jì)相關(guān)的代碼不會影響mutex,因此對于分區(qū)表,統(tǒng)計(jì)可能有一點(diǎn)點(diǎn)問題
     */
    long        accesses;
    long        collisions;
#endif
};
/*
 * HASHELEMENT is the private part of a hashtable entry.  The caller's data
 * follows the HASHELEMENT structure (on a MAXALIGN'd boundary).  The hash key
 * is expected to be at the start of the caller's hash entry data structure.
 * HASHELEMENT是哈希表?xiàng)l目的私有部分.
 * 調(diào)用者的數(shù)據(jù)按照HASHELEMENT結(jié)構(gòu)組織(位于MAXALIGN的邊界).
 * 哈希鍵應(yīng)位于調(diào)用者h(yuǎn)ash條目數(shù)據(jù)結(jié)構(gòu)的開始位置.
 */
typedef struct HASHELEMENT
{
    //鏈接到相同桶中的下一個(gè)條目
    struct HASHELEMENT *link;   /* link to next entry in same bucket */
    //該條目的哈希函數(shù)結(jié)果
    uint32      hashvalue;      /* hash function result for this entry */
} HASHELEMENT;
/* Hash table header struct is an opaque type known only within dynahash.c */
//哈希表頭部結(jié)構(gòu),非透明類型,用于dynahash.c
typedef struct HASHHDR HASHHDR;
/* Hash table control struct is an opaque type known only within dynahash.c */
//哈希表控制結(jié)構(gòu),非透明類型,用于dynahash.c
typedef struct HTAB HTAB;
/* Parameter data structure for hash_create */
//hash_create使用的參數(shù)數(shù)據(jù)結(jié)構(gòu)
/* Only those fields indicated by hash_flags need be set */
//根據(jù)hash_flags標(biāo)記設(shè)置相應(yīng)的字段
typedef struct HASHCTL
{
    //分區(qū)個(gè)數(shù)(必須是2的冪)
    long        num_partitions; /* # partitions (must be power of 2) */
    //段大小
    long        ssize;          /* segment size */
    //初始化目錄大小
    long        dsize;          /* (initial) directory size */
    //dsize上限
    long        max_dsize;      /* limit to dsize if dir size is limited */
    //填充因子
    long        ffactor;        /* fill factor */
    //哈希鍵大小(字節(jié)為單位)
    Size        keysize;        /* hash key length in bytes */
    //參見上述數(shù)據(jù)結(jié)構(gòu)注釋
    Size        entrysize;      /* total user element size in bytes */
    //
    HashValueFunc hash;         /* hash function */
    HashCompareFunc match;      /* key comparison function */
    HashCopyFunc keycopy;       /* key copying function */
    HashAllocFunc alloc;        /* memory allocator */
    MemoryContext hcxt;         /* memory context to use for allocations */
    //共享內(nèi)存中的哈希頭部結(jié)構(gòu)地址
    HASHHDR    *hctl;           /* location of header in shared mem */
} HASHCTL;
/* A hash bucket is a linked list of HASHELEMENTs */
//哈希桶是HASHELEMENTs鏈表
typedef HASHELEMENT *HASHBUCKET;
/* A hash segment is an array of bucket headers */
//hash segment是桶數(shù)組
typedef HASHBUCKET *HASHSEGMENT;
/*
 * Hash functions must have this signature.
 * Hash函數(shù)必須有它自己的標(biāo)識
 */
typedef uint32 (*HashValueFunc) (const void *key, Size keysize);
 /*
 * Key comparison functions must have this signature.  Comparison functions
 * return zero for match, nonzero for no match.  (The comparison function
 * definition is designed to allow memcmp() and strncmp() to be used directly
 * as key comparison functions.)
 * 哈希鍵對比函數(shù)必須有自己的標(biāo)識.
 * 如匹配則對比函數(shù)返回0,不匹配返回非0.
 * (對比函數(shù)定義被設(shè)計(jì)為允許在對比鍵值時(shí)可直接使用memcmp()和strncmp())
 */
typedef int (*HashCompareFunc) (const void *key1, const void *key2,
 Size keysize);
 /*
 * Key copying functions must have this signature.  The return value is not
 * used.  (The definition is set up to allow memcpy() and strlcpy() to be
 * used directly.)
 * 鍵拷貝函數(shù)必須有自己的標(biāo)識.
 * 返回值無用.
 */
typedef void *(*HashCopyFunc) (void *dest, const void *src, Size keysize);
/*
 * Space allocation function for a hashtable --- designed to match malloc().
 * Note: there is no free function API; can't destroy a hashtable unless you
 * use the default allocator.
 * 哈希表的恐懼分配函數(shù) -- 被設(shè)計(jì)為與malloc()函數(shù)匹配.
 * 注意:這里沒有釋放函數(shù)API;不能銷毀哈希表,除非使用默認(rèn)的分配器.
 */
typedef void *(*HashAllocFunc) (Size request);

FreeListData
在一個(gè)分區(qū)哈希表中,每一個(gè)空閑鏈表與特定的hashcodes集合相關(guān),通過下面的FREELIST_IDX()宏進(jìn)行定義.
nentries跟蹤有這些hashcodes的仍存活的hashtable條目個(gè)數(shù).

/*
 * Per-freelist data.
 * 空閑鏈表數(shù)據(jù).
 *
 * In a partitioned hash table, each freelist is associated with a specific
 * set of hashcodes, as determined by the FREELIST_IDX() macro below.
 * nentries tracks the number of live hashtable entries having those hashcodes
 * (NOT the number of entries in the freelist, as you might expect).
 * 在一個(gè)分區(qū)哈希表中,每一個(gè)空閑鏈表與特定的hashcodes集合相關(guān),通過下面的FREELIST_IDX()宏進(jìn)行定義.
 * nentries跟蹤有這些hashcodes的仍存活的hashtable條目個(gè)數(shù).
 * (注意不要搞錯(cuò),不是空閑的條目個(gè)數(shù))
 *
 * The coverage of a freelist might be more or less than one partition, so it
 * needs its own lock rather than relying on caller locking.  Relying on that
 * wouldn't work even if the coverage was the same, because of the occasional
 * need to "borrow" entries from another freelist; see get_hash_entry().
 * 空閑鏈表的覆蓋范圍可能比一個(gè)分區(qū)多或少,因此需要自己的鎖而不能僅僅依賴調(diào)用者的鎖.
 * 依賴調(diào)用者鎖在覆蓋面一樣的情況下也不會起效,因?yàn)榕紶栃枰獜牧硪粋€(gè)自由列表“借用”條目,詳細(xì)參見get_hash_entry()
 *
 * Using an array of FreeListData instead of separate arrays of mutexes,
 * nentries and freeLists helps to reduce sharing of cache lines between
 * different mutexes.
 * 使用FreeListData數(shù)組而不是一個(gè)獨(dú)立的mutexes,nentries和freelists數(shù)組有助于減少不同mutexes之間的緩存線共享.
 */
typedef struct
{
    //該空閑鏈表的自旋鎖
    slock_t     mutex;          /* spinlock for this freelist */
    //相關(guān)桶中的條目個(gè)數(shù)
    long        nentries;       /* number of entries in associated buckets */
    //空閑元素鏈
    HASHELEMENT *freeList;      /* chain of free elements */
} FreeListData;

BufferLookupEnt

/* entry for buffer lookup hashtable */
//檢索hash表的條目
typedef struct
{
    //磁盤page的tag
    BufferTag   key;            /* Tag of a disk page */
    //相關(guān)聯(lián)的buffer ID
    int         id;             /* Associated buffer ID */
} BufferLookupEnt;

二、源碼解讀

BufTableHashCode
BufTableHashCode函數(shù)根據(jù)BufferTag計(jì)算Hash Code,主要的邏輯在函數(shù)hash_any中實(shí)現(xiàn)

/*
 * BufTableHashCode
 *      Compute the hash code associated with a BufferTag
 * 根據(jù)BufferTag計(jì)算Hash Code
 *
 * This must be passed to the lookup/insert/delete routines along with the
 * tag.  We do it like this because the callers need to know the hash code
 * in order to determine which buffer partition to lock, and we don't want
 * to do the hash computation twice (hash_any is a bit slow).
 * 該函數(shù)的返回值需要作為參數(shù)傳遞給與tag相關(guān)的檢索/插入/刪除處理過程.
 * 之所以這樣處理是因?yàn)檎{(diào)用者需要知道Hash Code以便確定那個(gè)buffer partition需要鎖定,
 *   而且我們不希望多次計(jì)算hash(hash_any有一點(diǎn)點(diǎn)慢).
 */
uint32
BufTableHashCode(BufferTag *tagPtr)
{
    return get_hash_value(SharedBufHash, (void *) tagPtr);
}
/*
 * get_hash_value -- exported routine to calculate a key's hash value
 * get_hash_value -- exported過程,用于計(jì)算key的hash值
 *
 * We export this because for partitioned tables, callers need to compute
 * the partition number (from the low-order bits of the hash value) before
 * searching.
 * 之所以export這個(gè)過程是因?yàn)榉謪^(qū)表,調(diào)用者需要在搜索前計(jì)算分區(qū)編號(根據(jù)hash值的lower-order bits)
 */
uint32
get_hash_value(HTAB *hashp, const void *keyPtr)
{
    return hashp->hash(keyPtr, hashp->keysize);
}
 /*
 * tag_hash: hash function for fixed-size tag values
 * tag_hash:固定tag大小的hash函數(shù)
 */
uint32
tag_hash(const void *key, Size keysize)
{
    return DatumGetUInt32(hash_any((const unsigned char *) key,
                                   (int) keysize));
}
 /*
 * DatumGetUInt32
 *      Returns 32-bit unsigned integer value of a datum.
 * DatumGetUInt32返回datum的32位無符號整型值
 */
#define DatumGetUInt32(X) ((uint32) (X))

hash_any
hash_any函數(shù)hash一個(gè)可變長鍵值到一個(gè)32位值

/*
 * hash_any() -- hash a variable-length key into a 32-bit value
 *      k       : the key (the unaligned variable-length array of bytes)
 *      len     : the length of the key, counting by bytes
 * hash_any() -- hash一個(gè)可變長鍵值到一個(gè)32位值.
 *      k       : the key(未對齊的可變長字節(jié)數(shù)組)
 *
 * Returns a uint32 value.  Every bit of the key affects every bit of
 * the return value.  Every 1-bit and 2-bit delta achieves avalanche.
 * About 6*len+35 instructions. The best hash table sizes are powers
 * of 2.  There is no need to do mod a prime (mod is sooo slow!).
 * If you need less than 32 bits, use a bitmask.
 * 返回?zé)o符號32位整型值.
 * key的每一位都會影響返回值的每一位.每一個(gè)1位和2位增量都會產(chǎn)生雪崩.
 * 大概有6*len+35個(gè)指令.最好的hash表大小是2的冪.不需要進(jìn)行很慢的mod操作.
 * 如果需要少于32bits的值,那使用bitmask. 
 *
 * This procedure must never throw elog(ERROR); the ResourceOwner code
 * relies on this not to fail.
 * 這個(gè)過程永遠(yuǎn)都不要拋出elog(ERROR);依賴此函數(shù)的ResourceOwner代碼永遠(yuǎn)都不會出現(xiàn)異常.
 *
 * Note: we could easily change this function to return a 64-bit hash value
 * by using the final values of both b and c.  b is perhaps a little less
 * well mixed than c, however.
 * 注意:不能輕易的改變該函數(shù),通過使用b和c的最后值來返回64-bit的hash值.b的混合度可能沒有c好
 * 
 */
Datum
hash_any(register const unsigned char *k, register int keylen)
{
    register uint32 a,
                b,
                c,
                len;
    /* Set up the internal state */
    //設(shè)置內(nèi)部狀態(tài),初始化a/b/c
    len = keylen;
    a = b = c = 0x9e3779b9 + len + 3923095;
    /* If the source pointer is word-aligned, we use word-wide fetches */
    //如果源指針是字對齊的,那么我們使用字寬提取
    if (((uintptr_t) k & UINT32_ALIGN_MASK) == 0)
    {
        //源數(shù)據(jù)是對齊的
        /* Code path for aligned source data */
        register const uint32 *ka = (const uint32 *) k;
        /* handle most of the key */
        while (len >= 12)
        {
            a += ka[0];
            b += ka[1];
            c += ka[2];
            mix(a, b, c);
            ka += 3;
            len -= 12;
        }
        /* handle the last 11 bytes */
        //處理后面11個(gè)字節(jié)
        k = (const unsigned char *) ka;
#ifdef WORDS_BIGENDIAN//大碼端
        switch (len)
        {
            case 11:
                c += ((uint32) k[10] << 8);
                /* fall through */
            case 10:
                c += ((uint32) k[9] << 16);
                /* fall through */
            case 9:
                c += ((uint32) k[8] << 24);
                /* fall through */
            case 8:
                /* the lowest byte of c is reserved for the length */
                b += ka[1];
                a += ka[0];
                break;
            case 7:
                b += ((uint32) k[6] << 8);
                /* fall through */
            case 6:
                b += ((uint32) k[5] << 16);
                /* fall through */
            case 5:
                b += ((uint32) k[4] << 24);
                /* fall through */
            case 4:
                a += ka[0];
                break;
            case 3:
                a += ((uint32) k[2] << 8);
                /* fall through */
            case 2:
                a += ((uint32) k[1] << 16);
                /* fall through */
            case 1:
                a += ((uint32) k[0] << 24);
                /* case 0: nothing left to add */
        }
#else                           /* 小碼端; !WORDS_BIGENDIAN */
        switch (len)
        {
            case 11:
                c += ((uint32) k[10] << 24);
                /* fall through */
            case 10:
                c += ((uint32) k[9] << 16);
                /* fall through */
            case 9:
                c += ((uint32) k[8] << 8);
                /* fall through */
            case 8:
                /* the lowest byte of c is reserved for the length */
                b += ka[1];
                a += ka[0];
                break;
            case 7:
                b += ((uint32) k[6] << 16);
                /* fall through */
            case 6:
                b += ((uint32) k[5] << 8);
                /* fall through */
            case 5:
                b += k[4];
                /* fall through */
            case 4:
                a += ka[0];
                break;
            case 3:
                a += ((uint32) k[2] << 16);
                /* fall through */
            case 2:
                a += ((uint32) k[1] << 8);
                /* fall through */
            case 1:
                a += k[0];
                /* case 0: nothing left to add */
        }
#endif                          /* WORDS_BIGENDIAN */
    }
    else//---------- 非字對齊
    {
        /* Code path for non-aligned source data */
        /* handle most of the key */
        while (len >= 12)
        {
#ifdef WORDS_BIGENDIAN
            a += (k[3] + ((uint32) k[2] << 8) + ((uint32) k[1] << 16) + ((uint32) k[0] << 24));
            b += (k[7] + ((uint32) k[6] << 8) + ((uint32) k[5] << 16) + ((uint32) k[4] << 24));
            c += (k[11] + ((uint32) k[10] << 8) + ((uint32) k[9] << 16) + ((uint32) k[8] << 24));
#else                           /* !WORDS_BIGENDIAN */
            a += (k[0] + ((uint32) k[1] << 8) + ((uint32) k[2] << 16) + ((uint32) k[3] << 24));
            b += (k[4] + ((uint32) k[5] << 8) + ((uint32) k[6] << 16) + ((uint32) k[7] << 24));
            c += (k[8] + ((uint32) k[9] << 8) + ((uint32) k[10] << 16) + ((uint32) k[11] << 24));
#endif                          /* WORDS_BIGENDIAN */
            mix(a, b, c);
            k += 12;
            len -= 12;
        }
        /* handle the last 11 bytes */
#ifdef WORDS_BIGENDIAN
        switch (len)
        {
            case 11:
                c += ((uint32) k[10] << 8);
                /* fall through */
            case 10:
                c += ((uint32) k[9] << 16);
                /* fall through */
            case 9:
                c += ((uint32) k[8] << 24);
                /* fall through */
            case 8:
                /* the lowest byte of c is reserved for the length */
                b += k[7];
                /* fall through */
            case 7:
                b += ((uint32) k[6] << 8);
                /* fall through */
            case 6:
                b += ((uint32) k[5] << 16);
                /* fall through */
            case 5:
                b += ((uint32) k[4] << 24);
                /* fall through */
            case 4:
                a += k[3];
                /* fall through */
            case 3:
                a += ((uint32) k[2] << 8);
                /* fall through */
            case 2:
                a += ((uint32) k[1] << 16);
                /* fall through */
            case 1:
                a += ((uint32) k[0] << 24);
                /* case 0: nothing left to add */
        }
#else                           /* !WORDS_BIGENDIAN */
        switch (len)
        {
            case 11:
                c += ((uint32) k[10] << 24);
                /* fall through */
            case 10:
                c += ((uint32) k[9] << 16);
                /* fall through */
            case 9:
                c += ((uint32) k[8] << 8);
                /* fall through */
            case 8:
                /* the lowest byte of c is reserved for the length */
                b += ((uint32) k[7] << 24);
                /* fall through */
            case 7:
                b += ((uint32) k[6] << 16);
                /* fall through */
            case 6:
                b += ((uint32) k[5] << 8);
                /* fall through */
            case 5:
                b += k[4];
                /* fall through */
            case 4:
                a += ((uint32) k[3] << 24);
                /* fall through */
            case 3:
                a += ((uint32) k[2] << 16);
                /* fall through */
            case 2:
                a += ((uint32) k[1] << 8);
                /* fall through */
            case 1:
                a += k[0];
                /* case 0: nothing left to add */
        }
#endif                          /* WORDS_BIGENDIAN */
    }
    final(a, b, c);
    /* report the result */
    return UInt32GetDatum(c);
}
/*----------
 * mix -- mix 3 32-bit values reversibly.
 *
 * This is reversible, so any information in (a,b,c) before mix() is
 * still in (a,b,c) after mix().
 *
 * If four pairs of (a,b,c) inputs are run through mix(), or through
 * mix() in reverse, there are at least 32 bits of the output that
 * are sometimes the same for one pair and different for another pair.
 * This was tested for:
 * * pairs that differed by one bit, by two bits, in any combination
 *   of top bits of (a,b,c), or in any combination of bottom bits of
 *   (a,b,c).
 * * "differ" is defined as +, -, ^, or ~^.  For + and -, I transformed
 *   the output delta to a Gray code (a^(a>>1)) so a string of 1's (as
 *   is commonly produced by subtraction) look like a single 1-bit
 *   difference.
 * * the base values were pseudorandom, all zero but one bit set, or
 *   all zero plus a counter that starts at zero.
 *
 * This does not achieve avalanche.  There are input bits of (a,b,c)
 * that fail to affect some output bits of (a,b,c), especially of a.  The
 * most thoroughly mixed value is c, but it doesn't really even achieve
 * avalanche in c.
 *
 * This allows some parallelism.  Read-after-writes are good at doubling
 * the number of bits affected, so the goal of mixing pulls in the opposite
 * direction from the goal of parallelism.  I did what I could.  Rotates
 * seem to cost as much as shifts on every machine I could lay my hands on,
 * and rotates are much kinder to the top and bottom bits, so I used rotates.
 *----------
 */
#define mix(a,b,c) \
{ \
  a -= c;  a ^= rot(c, 4);  c += b; \
  b -= a;  b ^= rot(a, 6);  a += c; \
  c -= b;  c ^= rot(b, 8);  b += a; \
  a -= c;  a ^= rot(c,16);  c += b; \
  b -= a;  b ^= rot(a,19);  a += c; \
  c -= b;  c ^= rot(b, 4);  b += a; \
}
/*
 * UInt32GetDatum
 *      Returns datum representation for a 32-bit unsigned integer.
 */
#define UInt32GetDatum(X) ((Datum) (X))

三、跟蹤分析

N/A

四、參考資料

PG Source Code
Buffer Manager