Re: Early WIP/PoC for inlining CTEs

On 07/27/2018 10:10 AM, David Fetter wrote:
> On Fri, Jul 27, 2018 at 02:55:26PM +1200, Thomas Munro wrote:
>> On Thu, Jul 26, 2018 at 7:14 AM, David Fetter <david@fetter.org> wrote:
>>> Please find attached the next version, which passes 'make check'.
>>
>> ... but not 'make check-world' (contrib/postgres_fdw's EXPLAIN is different).
> 
> Please find attached a patch that does.
> 
> It doesn't always pass make installcheck-world, but I need to sleep
> rather than investigate that at the moment.

I took a quick look at this patch and I have a couple of comments.

1) Is it really safe, for backwards compatibility reasons, to inline 
when there are volatile functions? I imagine that it is possible that 
there are people who rely on that volatile functions inside a CTE are 
always run.

Imagine this case:

WITH cte AS (
   SELECT x, volatile_f(x) FROM tab ORDER BY x
)
SELECT * FROM cte LIMIT 10;

It could change behavior if volatile_f() has side effects and we inline 
the CTE. Is backwards compatibility for cases like this worth 
preserving? People can get the old behavior by adding OFFSET 0 or 
MATERIALIZED, but existing code would break.

2) The code in inline_cte_walker() is quite finicky. It looks correct to 
me but it is hard to follow and some simple bug could easily be hiding 
in there. I wonder if this code could be rewritten in some way to make 
it easier to follow.

3) Can you recall what the failing test was because I have so far not 
managed to reproduce it?

Andreas