Re: Why copy_relation_data only use wal when WAL archiving is enabled

> Jacky Leng wrote:
>> If I run the database under non-archiving mode, and execute the following
>> command:
>> alter table t set tablespace tblspc1;
>> Isn't it possible that the "new t" cann't be recovered?
>
> No. At the end of copy_relation_data we call smgrimmedsync, which fsyncs
> the new relation file.

Usually it's true, but how about this situation:
* First, do the following series:
* Create two tablespace SPC1, SPC2;
* Create table T1 in SPC1 and insert some values into it, suppose T1's
oid/relfilenode is OID1;
* Drop table T1;----------OID1 was released in pg_class and can be
reused.
* Do anything that will make the next oid that'll be allocated from
pg_class be OID1, e.g. insert
many many tuples into a relation with oid;
* Create table T2 in SPC2, and insert some values into it, and its
oid/relfilenode is OID1;
* Alter table T2 set tablespace SPC1;---------T2 goes to SPC1 and uses
the same file name with old T1;
* Second, suppose that no checkpoint has occured during the upper
series--authough not quite possible;
* Kill the database abnormaly;
* Restart the database;

Let's analyze what will happen during the recovery process:
* When T1 is re-created, it finds that its file has already been
there--actually this file is T2's;
* "T1" ' s file(actually T2's) is re-dropped;
* ....
* T2 is re-created, and finds that its file has disappeared, so it re-create
one;
* As copy_relation_data didn't record any xlog about T2's AlterTableSpace
op,
after recovery, we'll find that T2 is empty!!!

> --
> Heikki Linnakangas
> EnterpriseDB http://www.enterprisedb.com
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