Writeable CTEs and empty relations

Hi,

While playing around with another issue with the patch, I came across
the following:

=> create table foo(a int);
CREATE TABLE
=> with t as (insert into foo values(0)) select * from foo;
a
---
(0 rows)

I traced this down to heapam.c, which has this:

/*
* return null immediately if relation is empty
*/
if (scan->rs_nblocks == 0)
{
Assert(!BufferIsValid(scan->rs_cbuf));
tuple->t_data = NULL;
return;
}

and

/*
* Determine the number of blocks we have to scan.
*
* It is sufficient to do this once at scan start, since any tuples added
* while the scan is in progress will be invisible to my snapshot anyway.
* (That is not true when using a non-MVCC snapshot. However, we couldn't
* guarantee to return tuples added after scan start anyway, since they
* might go into pages we already scanned. To guarantee consistent
* results for a non-MVCC snapshot, the caller must hold some higher-level
* lock that ensures the interesting tuple(s) won't change.)
*/
scan->rs_nblocks = RelationGetNumberOfBlocks(scan->rs_rd);

This doesn't exactly work anymore since we modify the snapshot after
calling ExecInitScan(). I'm not really familiar with this part of the
code, so I'm asking: is there a simple enough way around this? Would
updating scan->rs_nblocks before scanning the first tuple be OK?

Regards,
Marko Tiikkaja