0 ~ 2是保留的txid，它们比任何普通txid都要旧。
0：invalidtransactionid，表示无效的事务id
1：bootstraptransactionid，表示系统表初始化时的事务id，比任何普通的事务id都旧。
2：frozentransactionid，冻结的事务id，比任何普通的事务id都旧。
大于2的事务id都是普通的事务id，即从3开始就是普通的事务id。
PostgreSQL 9.3或者之前的版本事务冻结,xmin 会被改成2.
PostgreSQL 9.4或者之后的版本中事务冻结,infomask = xmin_frozen.
/************************************************************************************/
/*
* information stored in t_infomask:
*/
#define HEAP_HASNULL 0x0001 /* has null attribute(s) */
#define HEAP_HASVARWIDTH 0x0002 /* has variable-width attribute(s) */
#define HEAP_HASEXTERNAL 0x0004 /* has external stored attribute(s) */
#define HEAP_HASOID_OLD 0x0008 /* has an object-id field */
#define HEAP_XMAX_KEYSHR_LOCK 0x0010 /* xmax is a key-shared locker */
#define HEAP_COMBOCID 0x0020 /* t_cid is a combo cid */
#define HEAP_XMAX_EXCL_LOCK 0x0040 /* xmax is exclusive locker */
#define HEAP_XMAX_LOCK_ONLY 0x0080 /* xmax, if valid, is only a locker */
/* xmax is a shared locker */
#define HEAP_XMAX_SHR_LOCK (HEAP_XMAX_EXCL_LOCK | HEAP_XMAX_KEYSHR_LOCK)
#define HEAP_LOCK_MASK (HEAP_XMAX_SHR_LOCK | HEAP_XMAX_EXCL_LOCK | \
HEAP_XMAX_KEYSHR_LOCK)
#define HEAP_XMIN_COMMITTED 0x0100 /* t_xmin committed */
#define HEAP_XMIN_INVALID 0x0200 /* t_xmin invalid/aborted */
#define HEAP_XMIN_FROZEN (HEAP_XMIN_COMMITTED|HEAP_XMIN_INVALID)
#define HEAP_XMAX_COMMITTED 0x0400 /* t_xmax committed */
#define HEAP_XMAX_INVALID 0x0800 /* t_xmax invalid/aborted */
#define HEAP_XMAX_IS_MULTI 0x1000 /* t_xmax is a MultiXactId */
#define HEAP_UPDATED 0x2000 /* this is UPDATEd version of row */
#define HEAP_MOVED_OFF 0x4000 /* moved to another place by pre-9.0
* VACUUM FULL; kept for binary
* upgrade support */
#define HEAP_MOVED_IN 0x8000 /* moved from another place by pre-9.0
* VACUUM FULL; kept for binary
* upgrade support */
#define HEAP_MOVED (HEAP_MOVED_OFF | HEAP_MOVED_IN)
#define HEAP_XACT_MASK 0xFFF0 /* visibility-related bits */
/************************************************************************************/
HEAP_XMIN_FROZEN(HEAP_XMIN_COMMITTED|HEAP_XMIN_INVALID)
HEAP_XMIN_COMMITTED 0x0100 /* t_xmin committed */
HEAP_XMIN_INVALID 0x0200 /* t_xmin invalid/aborted */
pageinspect 模块提供函数让你从低层次观察数据库页面的内容
postgres=# create extension pageinspect;
CREATE EXTENSION
postgres=# \dx
List of installed extensions
Name | Version | Schema | Description
-------------+---------+------------+-------------------------------------------------------
pageinspect | 1.6 | public | inspect the contents of database pages at a low level
plpgsql | 1.0 | pg_catalog | PL/pgSQL procedural language
(2 rows)
制造表膨胀场景,执行vacuum freeze test;
制造表膨胀
Session 1
开启一个事务，让其持有backend_xid
postgres=# create table test(id int,info text);
CREATE TABLE
postgres=# insert into test values(1,'test');
INSERT 0 1
postgres=# insert into test values(2,'test');
INSERT 0 1
postgres=# begin;
BEGIN
postgres=# select pg_backend_pid();
pg_backend_pid
----------------
3968
(1 row)
postgres=# select txid_current();
txid_current
--------------
611
(1 row)
表膨胀实验 2
session 2
另外一个会话不断进行更新
postgres=# update test set info = 'c1c2';\watch 0.01
等一会就可以看到表膨胀了
db1=# \dt+ test
List of relations
Schema | Name | Type | Owner | Size | Description
--------+------+-------+----------+--------+-------------
public | test | table | postgres | 712 kB |
(1 row)
执行vacuum freeze test;
vacuum freeze test;
postgres=# select t_infomask,to_hex(t_infomask) from heap_page_items(get_raw_page('test', 0));
t_infomask | to_hex
------------+--------
1794 | 702
1794 | 702
9474 | 2502
9474 | 2502
9474 | 2502
9474 | 2502
9474 | 2502
9474 | 2502
9474 | 2502
9474 | 2502
拆分
0x2502
----------
0x2000
0x0500 - > 0x0100 0x0400
0x0002
HEAP_XMIN_FROZEN(HEAP_XMIN_COMMITTED|HEAP_XMIN_INVALID)
HEAP_XMIN_COMMITTED 0x0100 /* t_xmin committed */
HEAP_XMIN_INVALID 0x0200 /* t_xmin invalid/aborted */
#define HEAP_XMIN_FROZEN (HEAP_XMIN_COMMITTED|HEAP_XMIN_INVALID)
0x0100 | 0x0200 = 768
hex(768) = 0x0300
int('0x0300',16) = 768
