Re: page is uninitialized --- fixing

On Tue, 2009-06-09 at 17:28 -0400, Tom Lane wrote:

> Is there any particular reason to suppose that the empty pages appeared
> during a crash recovery?
> 
> Have you read through md.c?  I seem to recall there are some slightly
> squirrelly choices made there about segment-extension behavior.  Maybe
> it could've done the wrong thing here during normal operation.

The only way I can see this happening is

1. backup: pg_start_backup()
2. user1: creates temp table
3. user2: Vacuum causes partial truncation of table
4. backup: copies table's files
5. backup: pg_stop_backup()

When we replay this, we do so with a base backup that starts recovering
at XLogRecPtr of start_backup(), but using a file copied at a later
XLogRecPtr.

I don't see any way thru md.c that would allow reading a block beyond
the end of the currently existing file (except in recovery).

I'm fairly certain that this didn't happen because the table in question
was pg_attribute and the main problem was that it had not been vacuumed
for a very long time. Which leaves me perplexed.

An erroneous value from file seek would lead to this though, so perhaps
that is the easiest explanation: damage to file metadata in memory.

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Training, Services and Support