PostgreSQL 源码解读（240）- HTAB简介

来源：这里教程网时间：2026-03-14 19:45:50 作者：

本节简单介绍了PostgreSQL中的数据结构HTAB.
### 一、数据结构
```
/*
 * Top control structure for a hashtable --- in a shared table, each backend
 * has its own copy (OK since no fields change at runtime)
 * 哈希表的顶层控制结构.
 * 在这个共享哈希表中,每一个后台进程都有自己的拷贝
 * (之所以没有问题是因为fork出来后,在运行期没有字段会变化)
 */
struct HTAB
{
    //指向共享的控制信息
    HASHHDR    *hctl;           /* => shared control information */
    //段目录
    HASHSEGMENT *dir;           /* directory of segment starts */
    //哈希函数
    HashValueFunc hash;         /* hash function */
    //哈希键比较函数
    HashCompareFunc match;      /* key comparison function */
    //哈希键拷贝函数
    HashCopyFunc keycopy;       /* key copying function */
    //内存分配器
    HashAllocFunc alloc;        /* memory allocator */
    //内存上下文
    MemoryContext hcxt;         /* memory context if default allocator used */
    //表名(用于错误信息)
    char       *tabname;        /* table name (for error messages) */
    //如在共享内存中,则为T
    bool        isshared;       /* true if table is in shared memory */
    //如为T,则固定大小不能扩展
    bool        isfixed;        /* if true, don't enlarge */
    /* freezing a shared table isn't allowed, so we can keep state here */
    //不允许冻结共享表,因此这里会保存相关状态
    bool        frozen;         /* true = no more inserts allowed */
    /* We keep local copies of these fixed values to reduce contention */
    //保存这些固定值的本地拷贝,以减少冲突
    //哈希键长度(以字节为单位)
    Size        keysize;        /* hash key length in bytes */
    //段大小,必须为2的幂
    long        ssize;          /* segment size --- must be power of 2 */
    //段偏移,ssize的对数
    int         sshift;         /* segment shift = log2(ssize) */
};
/*
 * Header structure for a hash table --- contains all changeable info
 * 哈希表的头部结构 -- 存储所有可变信息
 *
 * 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.
 * 在共享内存哈希表中,HASHHDR位于共享内存中,每一个后台进程都有一个本地HTAB结构.
 * 对于非共享哈希表,HASHHDR和HTAB没有任何功能性的不同,
 * 但无论如何,我们还是把它们区分为共享和非共享表.
 */
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.
     * 在高并发的哈希表中,空闲链表会成为竞争热点,因此我们使用空闲链表数组,
     *   数组中的每一个元素都有自己的mutex和条目统计,而不是使用一个.
     *
     * 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.
     * 如果哈希表没有分区,那么只有freelist[0]元素是有用的,自旋锁没有任何用处;
     * 调用者锁定被认为已足够OK.
     */
    /* Number of freelists to be used for a partitioned hash table. */
    //#define NUM_FREELISTS           32
    FreeListData freeList[NUM_FREELISTS];
    /* These fields can change, but not in a partitioned table */
    //这些域字段可以改变,但不适用于分区表
    /* Also, dsize can't change in a shared table, even if unpartitioned */
    //同时,就算是非分区表,共享表的dsize也不能改变
    //目录大小
    long        dsize;          /* directory size */
    //已分配的段大小(<= dsize)
    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 */
    //进入整个哈希表的模掩码
    uint32      high_mask;      /* mask to modulo into entire table */
    //进入低位哈希表的模掩码
    uint32      low_mask;       /* mask to modulo into lower half of table */
    /* These fields are fixed at hashtable creation */
    //下面这些字段在哈希表创建时已固定
    //哈希键大小(以字节为单位)
    Size        keysize;        /* hash key length in bytes */
    //所有用户元素大小(以字节为单位)
    Size        entrysize;      /* total user element size in bytes */
    //分区个数(2的幂),或者为0
    long        num_partitions; /* # partitions (must be power of 2), or 0 */
    //目标的填充因子
    long        ffactor;        /* target fill factor */
    //如目录是固定大小,则该值为dsize的上限值
    long        max_dsize;      /* 'dsize' limit if directory is fixed size */
    //段大小,必须是2的幂
    long        ssize;          /* segment size --- must be power of 2 */
    //段偏移,ssize的对数
    int         sshift;         /* segment shift = log2(ssize) */
    //一次性分配的条目个数
    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.
     * 统计信息.
     * 注意:统计相关的代码不会影响mutex,因此对于分区表,统计可能有一点点问题
     */
    long        accesses;
    long        collisions;
#endif
};
/*
 * Per-freelist data.
 * 空闲链表数据.
 *
 * 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).
 * 在一个分区哈希表中,每一个空闲链表与特定的hashcodes集合相关,通过下面的FREELIST_IDX()宏进行定义.
 * nentries跟踪有这些hashcodes的仍存活的hashtable条目个数.
 * (注意不要搞错,不是空闲的条目个数)
 *
 * 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().
 * 空闲链表的覆盖范围可能比一个分区多或少,因此需要自己的锁而不能仅仅依赖调用者的锁.
 * 依赖调用者锁在覆盖面一样的情况下也不会起效,因为偶尔需要从另一个自由列表“借用”条目,详细参见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数组而不是一个独立的mutexes,nentries和freelists数组有助于减少不同mutexes之间的缓存线共享.
 */
typedef struct
{
    //该空闲链表的自旋锁
    slock_t     mutex;          /* spinlock for this freelist */
    //相关桶中的条目个数
    long        nentries;       /* number of entries in associated buckets */
    //空闲元素链
    HASHELEMENT *freeList;      /* chain of free elements */
} FreeListData;
/*
 * 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是哈希表条目的私有部分.
 * 调用者的数据按照HASHELEMENT结构组织(位于MAXALIGN的边界).
 * 哈希键应位于调用者hash条目数据结构的开始位置.
 */
typedef struct HASHELEMENT
{
    //链接到相同桶中的下一个条目
    struct HASHELEMENT *link;   /* link to next entry in same bucket */
    //该条目的哈希函数结果
    uint32      hashvalue;      /* hash function result for this entry */
} HASHELEMENT;
/* Hash table header struct is an opaque type known only within dynahash.c */
//哈希表头部结构,非透明类型,用于dynahash.c
typedef struct HASHHDR HASHHDR;
/* Hash table control struct is an opaque type known only within dynahash.c */
//哈希表控制结构,非透明类型,用于dynahash.c
typedef struct HTAB HTAB;
/* Parameter data structure for hash_create */
//hash_create使用的参数数据结构
/* Only those fields indicated by hash_flags need be set */
//根据hash_flags标记设置相应的字段
typedef struct HASHCTL
{
    //分区个数(必须是2的幂)
    long        num_partitions; /* # partitions (must be power of 2) */
    //段大小
    long        ssize;          /* segment size */
    //初始化目录大小
    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 */
    //哈希键大小(字节为单位)
    Size        keysize;        /* hash key length in bytes */
    //参见上述数据结构注释
    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 */
    //共享内存中的哈希头部结构地址
    HASHHDR    *hctl;           /* location of header in shared mem */
} HASHCTL;

/* A hash bucket is a linked list of HASHELEMENTs */
//哈希桶是HASHELEMENTs链表
typedef HASHELEMENT *HASHBUCKET;
/* A hash segment is an array of bucket headers */
//hash segment是桶数组
typedef HASHBUCKET *HASHSEGMENT;
/*
 * Hash functions must have this signature.
 * Hash函数必须有它自己的标识
 */
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.)
 * 哈希键对比函数必须有自己的标识.
 * 如匹配则对比函数返回0,不匹配返回非0.
 * (对比函数定义被设计为允许在对比键值时可直接使用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.)
 * 键拷贝函数必须有自己的标识.
 * 返回值无用.
 */
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.
 * 哈希表的恐惧分配函数 -- 被设计为与malloc()函数匹配.
 * 注意:这里没有释放函数API;不能销毁哈希表,除非使用默认的分配器.
 */
typedef void *(*HashAllocFunc) (Size request);
```
其结构如下图所示：
![](https://www.herecours.com/d/file/efpub/2026/14-14/2026031419293474391.jpg?x-oss-process=style/bb)

### 二、跟踪分析
测试脚本
```
\pset footer off
\pset tuples_only on
\o /tmp/drop.sql
SELECT 'drop table if exists tbl' || id || ' ;' as "--"
       FROM generate_series(1, 20000) AS id;
\i /tmp/drop.sql
\pset footer off
\pset tuples_only on
\o /tmp/create.sql
SELECT 'CREATE TABLE tbl' || id || ' (id int);' as "--"
       FROM generate_series(1, 10000) AS id;
begin;
\o /tmp/ret.txt
\i /tmp/create.sql
```
跟踪分析
```
...
HASHSEGMENT *dir --> HASHELEMENT ***dir;
dir --> HASHELEMENT ***
(gdb) p *hctl
$1 = {freeList = {{mutex = 0 '\000', nentries = 312, freeList = 0x7fd906ab84c0}, {mutex = 0 '\000', 
      nentries = 298, freeList = 0x7fd907097c40}, {mutex = 0 '\000', nentries = 292, 
      freeList = 0x7fd906ac2520}, {mutex = 0 '\000', nentries = 321, freeList = 0x7fd906ac8120}, {
      mutex = 0 '\000', nentries = 341, freeList = 0x7fd907229980}, {mutex = 0 '\000', nentries = 334, 
      freeList = 0x7fd906ad3f08}, {mutex = 0 '\000', nentries = 316, freeList = 0x7fd906ad6fb8}, {
      mutex = 0 '\000', nentries = 299, freeList = 0x7fd906ade550}, {mutex = 0 '\000', nentries = 328, 
      freeList = 0x7fd906ae1600}, {mutex = 0 '\000', nentries = 328, freeList = 0x7fd906ae62e8}, {
      mutex = 0 '\000', nentries = 308, freeList = 0x7fd906aeb660}, {mutex = 0 '\000', nentries = 327, 
      freeList = 0x7fd90706f338}, {mutex = 0 '\000', nentries = 346, freeList = 0x7fd906af6bc0}, {
      mutex = 0 '\000', nentries = 323, freeList = 0x7fd907237bc0}, {mutex = 0 '\000', nentries = 304, 
      freeList = 0x7fd9071ddb40}, {mutex = 0 '\000', nentries = 311, freeList = 0x7fd906b06238}, {
      mutex = 0 '\000', nentries = 292, freeList = 0x7fd90707b620}, {mutex = 0 '\000', nentries = 303, 
      freeList = 0x7fd90723dd20}, {mutex = 0 '\000', nentries = 302, freeList = 0x7fd906b137e0}, {
      mutex = 0 '\000', nentries = 307, freeList = 0x7fd9070873c8}, {mutex = 0 '\000', nentries = 314, 
      freeList = 0x7fd90723bb68}, {mutex = 0 '\000', nentries = 279, freeList = 0x7fd906b22678}, {
      mutex = 0 '\000', nentries = 297, freeList = 0x7fd907073e08}, {mutex = 0 '\000', nentries = 309, 
      freeList = 0x7fd90721f888}, {mutex = 0 '\000', nentries = 317, freeList = 0x7fd906b33880}, {
      mutex = 0 '\000', nentries = 283, freeList = 0x7fd907086168}, {mutex = 0 '\000', nentries = 331, 
      freeList = 0x7fd906b3d838}, {mutex = 0 '\000', nentries = 330, freeList = 0x7fd906b41f38}, {
      mutex = 0 '\000', nentries = 313, freeList = 0x7fd906b46440}, {mutex = 0 '\000', nentries = 304, 
      freeList = 0x7fd906b4b5c0}, {mutex = 0 '\000', nentries = 310, freeList = 0x7fd90720ed80}, {
      mutex = 0 '\000', nentries = 323, freeList = 0x7fd906b575a0}}, dsize = 256, nsegs = 16, 
  max_bucket = 4095, high_mask = 8191, low_mask = 4095, keysize = 16, entrysize = 152, num_partitions = 16, 
  ffactor = 1, max_dsize = 256, ssize = 256, sshift = 8, nelem_alloc = 48}
(gdb) p *hashp
$2 = {hctl = 0x7fd906aae980, dir = 0x7fd906aaecd8, hash = 0xa79ac6 <tag_hash>, match = 0x47cb70 <memcmp@plt>, 
  keycopy = 0x47d0a0 <memcpy@plt>, alloc = 0x8c3419 <ShmemAllocNoError>, hcxt = 0x0, 
  tabname = 0x160f1d0 "LOCK hash", isshared = true, isfixed = false, frozen = false, keysize = 16, 
  ssize = 256, sshift = 8}
(gdb) p *hashp->dir
$3 = (HASHSEGMENT) 0x7fd906aaf500
(gdb) p hashp->dir
$4 = (HASHSEGMENT *) 0x7fd906aaecd8
(gdb) p **hashp->dir
$5 = (HASHBUCKET) 0x7fd907212dd0
(gdb) p ***hashp->dir
$6 = {link = 0x7fd9071a7b90, hashvalue = 1748602880}
(gdb) n
949     if (action == HASH_ENTER || action == HASH_ENTER_NULL)
(gdb) 
956         if (!IS_PARTITIONED(hctl) && !hashp->frozen &&
(gdb) 
965     bucket = calc_bucket(hctl, hashvalue); --> hash桶
(gdb) 
967     segment_num = bucket >> hashp->sshift; --> 桶号右移8位得到段号
(gdb) 
968     segment_ndx = MOD(bucket, hashp->ssize); --> 桶号取模得到段内偏移
(gdb) 
970     segp = hashp->dir[segment_num]; --> 获取段（HASHELEMENT **）
(gdb) 
972     if (segp == NULL)
(gdb) p bucket
$7 = 2072
(gdb) p segment_num
$8 = 8
(gdb) p segment_ndx
$9 = 24
(gdb) p segp --> 
$10 = (HASHSEGMENT) 0x7fd906ab3500
(gdb) 
(gdb) n
975     prevBucketPtr = &segp[segment_ndx]; --> HASHELEMENT **
(gdb) 
976     currBucket = *prevBucketPtr; --> HASHELEMENT *
(gdb) 
981     match = hashp->match;       /* save one fetch in inner loop */
(gdb) p prevBucketPtr
$12 = (HASHBUCKET *) 0x7fd906ab35c0
(gdb) p currBucket
$13 = (HASHBUCKET) 0x7fd90714da68
(gdb)

```
DONE!
### 三、参考资料
N/A