Inconsistent results after XID wraparound

[yugo-n]

After xid wraparound, inserting a tuple into a base table causes inconsistent results.

• The view definition

  test=# \d+ mv
                                   Materialized view "public.mv"
  Column |  Type   | Collation | Nullable | Default | Storage | Compression | Stats target | Description 
  --------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
  x      | integer |           |          |         | plain   |             |              | 
  y      | integer |           |          |         | plain   |             |              | 
  View definition:
  SELECT t.i AS x,
  t2.i AS y
  FROM t,
  t t2;
  Access method: heap
  Incremental view maintenance: yes

• After inserting the first tuple, use pg_resetwal to simulate xid wraparound

  
  test=# insert into t values (1);
  INSERT 0 1vance xid by
  test=# \q

$ vacuumdb -F -a; pg_ctl stop; pg_resetwal -x 0x100000010 -D data_ivm; pg_ctl start

- After that, inserting the second row results in the inconsistency

test=# select xmin, * from t; xmin | i ------+--- 728 | 1 (1 row)

test=# select * from mv; x | y ---+--- 1 | 1 (1 row)

test=# insert into t values (2); INSERT 0 1 test=# select xmin, * from t; xmin | i ------+--- 728 | 1 27 | 2 (2 rows)

test=# select * from mv; x | y ---+--- 1 | 1 1 | 2 2 | 2 (3 rows)

test=# select * from t, t as t2; i | i ---+--- 1 | 1 1 | 2 2 | 1 2 | 2 (4 rows)

[yugo-n]

The cause of the problem is the way to calculate "pre-update" state of base tables. When multiple tables are updated or the view contains a self-join, we need to calculate table states that was before it is modified. Currently we look at xmin and cmin system column for this purpose, but after a tuple is frozen, its xmin no longer has any meaning, so this way doesn't work.

[yugo-n]

Also, we can find similar inconsistency when using sub-transaction because xmin values do not always monotonically increasing by command executions. Here is an example:

test=# \d+ mv
                                     Materialized view "public.mv"
 Column |  Type   | Collation | Nullable | Default | Storage | Compression | Stats target | Description 
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
 x      | integer |           |          |         | plain   |             |              | 
 y      | integer |           |          |         | plain   |             |              | 
View definition:
 SELECT t.i AS x,
    t2.i AS y
   FROM t,
    t t2;
Access method: heap
Incremental view maintenance: yes

test=# begin ;
BEGIN
test=*# savepoint p;
SAVEPOINT
test=*# insert into t values (1);
INSERT 0 1
test=*# select xmin, * from t;
 xmin | i 
------+---
   31 | 1
(1 row)

test=*# select * from mv;
 x | y 
---+---
 1 | 1
(1 row)

test=*# release savepoint p;
RELEASE
test=*# insert into t values (1);
INSERT 0 1
test=*# select xmin, * from t;
 xmin | i 
------+---
   31 | 1
   30 | 1
(2 rows)

test=*# select * from mv;
 x | y 
---+---
 1 | 1
 1 | 1
 1 | 1
(3 rows)

test=*# select * from t, t t2;
 i | i 
---+---
 1 | 1
 1 | 1
 1 | 1
 1 | 1
(4 rows)