SupportRequestSimplify and SQL SRF

Hello,

I've been trying to play with support functions for a use-case of ours, and
found some behaviors I don't know are expected or not.

The use case: we have some complicated queries, where whole-branches of the
execution tree could be cut if some things were evaluated at planning time.
Take the following simplified example:

CREATE OR REPLACE FUNCTION myfunction(t1_id int) AS $$
SELECT *
FROM sometable t1
JOIN sometable t2 on t2.t1_id = t1.id
WHERE id = t1_id AND t1.somecolumn IS NOT NULL
$$ language SQL;

If I were to incorporate this function in a larger query, the planner will
choose a plan based on a generic value of t1_id and may estimate a large
rowcount after inlining.

What I want to do is to evaluate whether id = t1_id AND somecolumn is NOT
NULL at planification time, and replace the function by another one if this
can be pruned altogether.

So, what I've been doing is to implement a support function for
SupportRequestSimplify, and If the predicate doesn't match any row, replace
the FuncExpr by a new one, calling a different function.

This seems to work great, but I have several questions:

1) Is it valid to make SPI calls in a support function to do this kind of
simplification ?

2) My new FuncExpr doesn't get inlined. This is because in
inline_set_returning_function, we check that after the call to
eval_const_expressions we still call the same function. I think it would be
better to first simplify the function if we can, and only then record the
function oid and call the rest of the machinery. I tested that naively by
calling eval_const_expressions early in inline_set_returning_function and
it seems to do the trick. A proper patch would likely only call the support
function at this stage.

What do you think ?

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