Re: Possible problem in Custom Scan API

Tom Lane wrote:
> Uh, no, construction of a custom plan node is entirely driven by the
> PlanCustomPath method as far as I can see. You're free to ignore what
> create_scan_plan did and insert your own tlist.

Are you sure? Even if it's true, this targetlist should still contain each
and every Var that's been requested. If I'm correct, the only way to ensure
that is to call build_path_tlist(), which is static (oops!). Perhaps I
could make my own, but it uses replace_nestloop_params() (again, static),
and the problem goes further and further.

This effectively means that it would be nice if I could just use the
existing machinery. The fix would be quite trivial.

By the way, what if our CustomScan is a top node? Let's take a look at
create_plan():

/* Recursively process the path tree, demanding the correct tlist result */
plan = create_plan_recurse(root, best_path, CP_EXACT_TLIST);

...
if (!IsA(plan, ModifyTable))
apply_tlist_labeling(plan->targetlist, root->processed_tlist);
...

If I spoil the targetlist, everything blows up in apply_tlist_labeling():

Assert(list_length(dest_tlist) == list_length(src_tlist));

since the lengths may differ. It's much safer to use the tlist provided by
the core code, but alas, sometimes it's not an option.

> In particular, if what
> your custom path actually is is a rescan of something like an
> IndexOnlyScan, why don't you just copy the IOS's result tlist?

I'm actively working on a prototype of partition pruning. It could be much
simpler if I just patched the core, but we have a working extension for 9.5
and 9.6, and we can't throw it away just yet. I wouldn't bother you if I
didn't encounter a broken query :)

--
Dmitry Ivanov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company