Re: WAL, xl_heap_insert and tuple oid mystry

On 11.04.2011 23:35, jagan wrote:
> Hi,
> Suppose I create a table as follows:
>
> CREATE TABLE test2 (name TEXT, age INTEGER) WITH oids;
>
> Now, for every tuple in this table is associated with a unique oid, which I can retrieve by:
>
> SELECT oid, name, age FROM test2;
>
> which works great. So far so good.
>
> Now, if look at the corresponding WAL entry for any insert into this relation, it creates the following WAL entry which I can decode.
>
> XLogRecord ---> Followed by --> xl_heap_insert struct -->followed by --> xl_heap_header --> Followed by --> tuple data
>
> If I use the macro HeapTupleHeaderGetOid([xl_heap_header struct])

Yeah, that won't work because xl_heap_header is not a HeapTupleHeader.

> or equivalently if I test ([xl_heap_header struct]->t_infomask& HEAP_HASOID), it tells me that the tuple oid is not stored with this record.

That's strange, the flag should be set.

> Where is the OID of tuple stored in a WAL record of a tuple? If not with xl_heap_header, where is it stored? Is it stored at all?

It's stored in the "tuple data" portion. See the code that writes the
WAL record, here:

http://git.postgresql.org/gitweb?p=postgresql.git;a=blob;f=src/backend/access/heap/heapam.c;h=1fbd8b39b4a73f6aee337fccf2dcce548d0850bb;hb=HEAD#l1987

Maybe the WAL record you're looking at is a full-page image? A record
with a full-page image includes a verbatim copy of the page, and the
individual tuple is omitted in that case.

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