Обсуждение: what happens when...?

How do we handle this situation?
We go to insert a record in the heap, find no free space, so we extend thetable and insert it into a new page. Then we
insertan index entry pointingto the new tuple. Then some other backend (or bgwriter) comes along anddecides the index
pageis a good candidate for eviction and forces an xlogbuffer flush for that buffer. Then the system crashes.
 

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.

I know there's some special code in lazyvacuum to handle zeroed pages but I
don't think it does anything special to note those zeroed pages and check
index entries against them, does it?

--  Gregory Stark EnterpriseDB          http://www.enterprisedb.com

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