On Tue, Aug 23, 2016 at 10:10 AM, Antonin Houska <a...@cybertec.at> wrote: > I'm failing to understand why  mentions "table-rewriting forms of ALTER > TABLE" besides TRUNCATE command. > > For TRUNCATE it seems clear: if transaction A takes the snapshot before it > accesses the table first time (typically because isolation level is at least > REPEATABLE READ) and transaction B manages to commit TRUNCATE soon enough, > then A sees pg_class entry of the table already affected by B, which has the > new (empty) relfilenode. (The original pg_class entry is no longer visible by > catalog snapshot, nor does it contain valid OID of the original relfilenode.) > > But IMO heap rewriting changes neither table contents, nor visibility. Can > anyone explain what I miss?
CLUSTER preserves xmin/xmax when rewriting, but ALTER TABLE does not. rhaas=# create table foo (a int); CREATE TABLE rhaas=# insert into foo values (1); INSERT 0 1 rhaas=# insert into foo values (2); INSERT 0 1 rhaas=# select xmin, a from foo; xmin | a ------+--- 934 | 1 935 | 2 (2 rows) rhaas=# alter table foo alter a type text; ALTER TABLE rhaas=# select xmin, a from foo; xmin | a ------+--- 936 | 1 936 | 2 (2 rows) This is sad. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers