Re: what happens when...?

Gregory Stark wrote:
> How do we handle this situation?
>
> We go to insert a record in the heap, find no free space, so we extend the
> table and insert it into a new page. Then we insert an index entry pointing
> to the new tuple. Then some other backend (or bgwriter) comes along and
> decides the index page is a good candidate for eviction and forces an xlog
> buffer flush for that buffer. Then the system crashes.

Let me reiterate:

1. extend table
2. insert heap tuple
3. insert index tuple
4. flush index page
5. crash

> Now when the system comes back up the index will have a pointer to a page
> beyond the end of the heap. Even if we have a WAL log entry for the extension
> the index pointer would be pointing to a zeroed block so vacuum would never
> get the chance to note the tuple is dead and remove the index pointer.

There's a hole in your logic. The xlog flush in step 4 is also going to
flush the xlog record of 1-3. By the time 3 is replayed, the heap page
has already been reconstructed.

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