|From:||Anastasia Lubennikova <a(dot)lubennikova(at)postgrespro(dot)ru>|
|To:||Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>|
|Subject:||Re: Optimze usage of immutable functions as relation|
|Views:||Raw Message | Whole Thread | Download mbox | Resend email|
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.
> * 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
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.
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