Re: Optimze usage of immutable functions as relation

26.07.2019 21:26, Tom Lane wrote:
> I took a quick look at this and I have a couple of gripes ---
>
> * The naming and documentation of transform_const_function_to_result
> seem pretty off-point to me.  ISTM the real goal of that function is to
> pull up constant values from RTE_FUNCTION RTEs.  Replacing the RTE
> with an RTE_RESULT is just a side-effect that's needed so that we
> don't generate a useless FunctionScan plan node.  I'd probably call
> it pull_up_constant_function or something like that.
>
> * It would be useful for the commentary to point out that in principle we
> could pull up any immutable (or, probably, even just stable) expression;
> but we don't, (a) for fear of multiple evaluations of the result costing
> us more than we can save, and (b) because a primary goal is to let the
> constant participate in further const-folding, and of course that won't
> happen for a non-Const.
Fixed
> * The test cases are really pretty bogus, because int4(1) or int4(0) are
> not function invocations at all.  The parser thinks those are no-op type
> casts, and so what comes out of parse analysis is already just a plain
> Const.  Thus, not one of these test cases is actually verifying that
> const-folding of an immutable function has happened before we try to
> pull up.  While you could dodge the problem today by, say, writing
> int8(0) which isn't a no-op cast, I'd recommend staying away from
> typename() notation altogether.  There's too much baggage there and too
> little certainty that you wrote a function call not something else.
> The existing test cases you changed, with int4(sin(1)) and so on,
> are better examples of something that has to actively be folded to
> a constant.

Thank you for pointing out on specific of int4() function,
I updated tests to use dummy plpgsql function.
I'm not sure if tests of various join types are redundant but I left them.

As far as I understand, the principal motivation of this patch was to 
optimize
function scan joins that occur in FTS queries. For example:
select * from test_tsquery, to_tsquery('english', 'new') q where 
txtsample @@ q;
So I also added another test to tsearch.sql to illustrate difference 
between optimized and not optimized plans.

-- 
Anastasia Lubennikova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company