一、一切皆为Oid

在Linux中一切皆为文件，在postgresql中一切皆为Oid。

1.1 什么是Oid

Object identifier(Oid), 对象标识符。 在postgresql内部，所有的数据库对象都是通过相应的Oid进行管理。

typedef unsigned int Oid;
1

Oid在代码中是一个4字节的无符号的整数。

1.2 数据库Oid

postgres@2f9d6ce41c2b:~$ /usr/local/pgsql/bin/psql 
psql (14.2)
Type "help" for help.

postgres=# select datname,oid from pg_database where datname='test';
 datname |  oid  
---------+-------
 test    | 16384
(1 row)

postgres=# 
1
2
3
4
5
6
7
8
9
10
11

postgres@2f9d6ce41c2b:~$ ls /usr/local/pgsql/data/base/ -l
total 24
drwx------ 2 postgres postgres 4096 Jun  6 14:38 1
drwx------ 2 postgres postgres 4096 Jun  6 14:38 12971
drwx------ 2 postgres postgres 4096 Jul  4 12:50 12972
drwx------ 2 postgres postgres 4096 Jun  7 12:19 16384
drwx------ 2 postgres postgres 4096 Jun 11 02:01 24577
drwx------ 2 postgres postgres 4096 Jul  4 12:50 40960
1
2
3
4
5
6
7
8

1.3 表Oid

postgres@2f9d6ce41c2b:~$ /usr/local/pgsql/bin/psql test
psql (14.2)
Type "help" for help.

test=# create table stu (name varchar(32), age int, sex char(2));
CREATE TABLE
test=# \dS+ stu
                                                  Table "public.stu"
 Column |         Type          | Collation | Nullable | Default | Storage  | Compression | Stats target | Description 
--------+-----------------------+-----------+----------+---------+----------+-------------+--------------+-------------
 name   | character varying(32) |           |          |         | extended |             |              | 
 age    | integer               |           |          |         | plain    |             |              | 
 sex    | character(2)          |           |          |         | extended |             |              | 
Access method: heap

test=# select oid,relname from pg_class where relname  = 'stu';
  oid  | relname 
-------+---------
 49152 | stu
(1 row)

test=# 

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23

postgres@2f9d6ce41c2b:~$ ls /usr/local/pgsql/data/base/16384/49152 -hal
-rw------- 1 postgres postgres 0 Jul  4 12:57 /usr/local/pgsql/data/base/16384/49152
1
2

二、和MySQL对比

postgresql和mysql都是将数据库作为目录，对应的表则在对应的目录下，方便查找以及管理。

三、 数据目录结构

root@2f9d6ce41c2b:/# tree /usr/local/pgsql/data/ -L 1
/usr/local/pgsql/data/
|-- PG_VERSION 
|-- base
|-- global
|-- pg_commit_ts
|-- pg_dynshmem
|-- pg_hba.conf
|-- pg_ident.conf
|-- pg_logical
|-- pg_multixact
|-- pg_notify
|-- pg_replslot
|-- pg_serial
|-- pg_snapshots
|-- pg_stat
|-- pg_stat_tmp
|-- pg_subtrans
|-- pg_tblspc
|-- pg_twophase
|-- pg_wal
|-- pg_xact
|-- postgresql.auto.conf 
|-- postgresql.conf 
|-- postmaster.opts
`-- postmaster.pid

17 directories, 7 files
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28