From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Philip Semanchuk <philip(at)americanefficient(dot)com> |
Cc: | PostgreSQL General <pgsql-general(at)lists(dot)postgresql(dot)org> |
Subject: | Re: Why is my function inlined only when STABLE? |
Date: | 2022-03-29 18:24:41 |
Message-ID: | 51174.1648578281@sss.pgh.pa.us |
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Lists: | pgsql-general |
Philip Semanchuk <philip(at)americanefficient(dot)com> writes:
> I have a function that isn't being inlined, and I would appreciate help to understand why that's the case.
The example you show *is* inline-able, as you can easily prove with EXPLAIN.
regression=# CREATE OR REPLACE FUNCTION f(foo text)
RETURNS text
AS $$
SELECT substring(foo FROM 1 FOR 2)
$$ LANGUAGE sql IMMUTABLE PARALLEL SAFE;
CREATE FUNCTION
regression=# explain verbose select f(f1) from text_tbl;
QUERY PLAN
----------------------------------------------------------------
Seq Scan on public.text_tbl (cost=0.00..1.02 rows=2 width=32)
Output: "substring"(f1, 1, 2)
(2 rows)
No f() anywhere there.
I think the test methodology you used is faulty, because it does not
distinguish between "inline-able" and "foldable to a constant".
Given an immutable function applied to constant(s), the planner prefers
to fold to a constant by just executing the function. The inline-ing
transformation is considered only when that case doesn't apply.
regards, tom lane
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