Hi,
I have a custom function where the ROWS hint is getting ignored. I think it’s
because the function is getting inlined, but I’d like a second opinion.
Here’s my working (contrived) example.
CREATE TABLE my_table (
id int primary key GENERATED ALWAYS AS IDENTITY,
base_value int NOT NULL
);
INSERT INTO my_table (base_value) VALUES (42);
CREATE OR REPLACE FUNCTION fn_get_deltas(base_value int)
RETURNS TABLE (delta int, total int) AS $$
SELECT
generate_series(1, CASE base_value WHEN 42 THEN 4 ELSE 20 END),
base_value + generate_series(1, CASE base_value WHEN 42 THEN 4 ELSE 20
END)
$$ LANGUAGE sql IMMUTABLE PARALLEL SAFE ROWS 10;
EXPLAIN
SELECT base_value, delta, total
FROM my_table
CROSS JOIN LATERAL (SELECT delta, total FROM fn_get_deltas(base_value)) AS foo
+------------------------------------------------------------------+
| QUERY PLAN |
|------------------------------------------------------------------|
| Nested Loop (cost=0.00..107427.80 rows=2260000 width=12) |
| -> Seq Scan on my_table (cost=0.00..32.60 rows=2260 width=4) |
| -> Result (cost=0.00..27.52 rows=1000 width=8) |
| -> ProjectSet (cost=0.00..5.02 rows=1000 width=4) |
| -> Result (cost=0.00..0.01 rows=1 width=0) |
+------------------------------------------------------------------+
The plan estimates 1000 rows from the CROSS JOIN despite the “ROWS 10” hint on
my function. I think this is because the planner never sees fn_get_deltas() —
it has been inlined by the query preprocessor because fn_get_deltas() meets the
criteria for inlining
(https://wiki.postgresql.org/wiki/Inlining_of_SQL_functions). Instead of 10
rows, the planner uses its default assumption of 1000 rows.
If I change the function to VOLATILE to prevent inlining, I get this plan.
+-------------------------------------------------------------------------+
| QUERY PLAN |
|-------------------------------------------------------------------------|
| Nested Loop (cost=0.25..484.85 rows=22600 width=12) |
| -> Seq Scan on my_table (cost=0.00..32.60 rows=2260 width=4) |
| -> Function Scan on fn_get_deltas (cost=0.25..0.35 rows=10 width=8) |
+-------------------------------------------------------------------------+
I would prefer to have the function inlined for better performance, but I can
declare it VOLATILE if that’s necessary to give decent estimates to the
planner. Am I correctly reading the situation? If so, is there another solution
that allows inlining *and* making the ROWS hint visible to the planner?
Thanks a bunch
Philip
