Explaining MVCC
xmin is only a part of the story of managing MVCC. PostgreSQL labels every tuple in the database with four different fields, named xmin (already described), xmax, cmin, and cmax. Similar to what you learned about xmin, in order to make those fields appear in a query result, you need to explicitly reference them – for instance:
forumdb=> SELECT xmin, xmax, cmin, cmax, * FROM tags ORDER BY tag;
xmin | xmax | cmin | cmax | pk | tag | parent
------+------+------+------+----+------+--------
4854 | 0 | 0 | 0 | 24 | c++ |
4853 | 0 | 0 | 0 | 23 | java |
4852 | 0 | 0 | 0 | 22 | perl |
4855 | 0 | 0 | 0 | 25 | unix |
(4 rows)
The meaning of xmin has been already described in a previous section: it indicates the transaction identifier of the transaction that created the tuple. The xmax field, on the other hand, indicates the xid of the transaction that invalidated the tuple, for example, because it has deleted the data...