Detailed explanation of PostgreSQL version identification-Mysql Tutorial-php.cn

Detailed explanation of PostgreSQL version identification

jacklove

Release： 2018-06-11 17:22:42

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Insert\delete\updateLocate via ctid, and view the changes in the record xmin\xmax.

Xid: Database transaction ID;

Xmin: Row header xidInformation, xmin represents the transaction that inserted the recordID

Xmax : Indicates deletion or lockThe transaction of this recordID

xid_snapshot: Ended in the current cluster Transaction

Clog: Transaction submission status log

Definition of record format: htup_details.h:POSTGRES heap tuple header definitions.

1) View all xid related functions. What is needed here is txid_currentFunction

##2）

can be seen Current transaction ID

postgres=# select * from txid_current();
 txid_current
--------------
         1676
(1 row)
 
3）进行一次insert后，看事务ID已经+1
postgres=# insert into tt values(1);
INSERT 0 1
postgres=# select ctid,xmin,xmax,cmin,cmax,id from tt;
 ctid  | xmin | xmax | cmin | cmax | id
-------+------+------+------+------+----
 (0,1) | 1677 |    0 |    0 |    0 |  1
(1 row)

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After opening a transaction, proceed update

postgres=# begin;
BEGIN
 
postgres=# update tt set id=3;
UPDATE 1
postgres=# select ctid,xmin,xmax,cmin,cmax,id from tt;
 ctid  | xmin | xmax | cmin | cmax | id
-------+------+------+------+------+----
 (0,2) | 1678 |    0 |    0 |    0 |  3
(1 row)

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5) View

postgres=#  select ctid,xmin,xmax,cmin,cmax,id from tt;
 ctid  | xmin | xmax | cmin | cmax | id
-------+------+------+------+------+----
 (0,1) | 1677 | 1678 |    0 |    0 |  1
(1 row)

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## in another session to see the current unfinished transactions or unopened transactions Transaction

postgres=# select *from txid_current_snapshot();
 txid_current_snapshot
-----------------------
 1684:1684:
(1 row)
 
postgres=# select * from txid_current();
 txid_current
--------------
         1684
(1 row)

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Record whether the transaction is submitted, in In this file,

bit

-rw-------. 1 pg pg 8192 Jun 10 04:19 0000
[pg@localhost pg_clog]$ pwd
/home/pg/data/pg_clog

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