在PostgreSQL的数据目录中，每个关系的数据都存储在所谓的fork中
- 主fork（无后缀）
- 空闲空间映射（后缀：
fsm） - 可见性映射（后缀：
vm） - 初始化fork（后缀：
init）
包含实际数据（可能分布在多个文件中）的主fork始终存在。
源代码
/*
* Stuff for fork names.
*
* The physical storage of a relation consists of one or more forks.
* The main fork is always created, but in addition to that there can be
* additional forks for storing various metadata. ForkNumber is used when
* we need to refer to a specific fork in a relation.
*/
typedef enum ForkNumber
{
InvalidForkNumber = -1,
MAIN_FORKNUM = 0,
FSM_FORKNUM,
VISIBILITYMAP_FORKNUM,
INIT_FORKNUM
...
} ForkNumber;
/*
* Lookup table of fork name by fork number.
*
* If you add a new entry, remember to update the errhint in
* forkname_to_number() below, and update the SGML documentation for
* pg_relation_size().
*/
const char *const forkNames[] = {
"main", /* MAIN_FORKNUM */
"fsm", /* FSM_FORKNUM */
"vm", /* VISIBILITYMAP_FORKNUM */
"init" /* INIT_FORKNUM */
};
示例
具有可见性和空闲空间映射fork的关系
-rw------- 1 postgres postgres 36249600 21 Dec 15:59 16384 -rw------- 1 postgres postgres 32768 21 Dec 15:59 16384_fsm -rw------- 1 postgres postgres 8192 21 Dec 15:59 16384_vm