Re: Early WIP/PoC for inlining CTEs

Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> writes:
> Here, uncommenting that NOT actually changes the result, from 22 rows to
> 4 rows, because we end up generating multiple worktable scans and the
> recursion logic is not set up to handle that.

Ugh.

> So what I think we need to do here is to forbid inlining if (a) the
> refcount is greater than 1 and (b) the CTE in question contains,
> recursively anywhere inside its rtable or the rtables of any of its
> nested CTEs, a "self_reference" RTE.

That's kind of "ugh" too: it sounds expensive, and doing it in a way
that doesn't produce false positives would be even more complicated.

Idle uncaffeinated speculation: is it practical to fix the restriction
about multiple worktable scans?

Also, I thought of a somewhat-related scenario that the code isn't
accounting for: you can break the restrictions about single evaluation
with nested WITHs, like

with x as not materialized (with y as materialized (select random() r) select * from y)
select * from x, x x1;

In this particular example, we're saved from computing random() twice
by the checks for volatile functions. But without that, y is inlined
and computed twice, e.g.

explain verbose with x as not materialized (with y as (select now() r) select * from y)
select * from x, x x1;
QUERY PLAN
------------------------------------------------
Nested Loop (cost=0.00..0.06 rows=1 width=16)
Output: (now()), (now())
-> Result (cost=0.00..0.01 rows=1 width=8)
Output: now()
-> Result (cost=0.00..0.01 rows=1 width=8)
Output: now()
(6 rows)

As a user I think I'd find that surprising, and bad if y were expensive.

Is it practical to inline the outer "x" level and still compute "y"
only once? If not, I think we need to disallow inlining anything
that contains a "with".

regards, tom lane