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From: PG Bug reporting form <noreply@postgresql.org>
To: pgsql-bugs@lists.postgresql.org
Cc: feasiblechart@gmail.com
Subject: BUG #19615: COVAR_POP / COVAR_SAMP / REGR_SXY return 0.0 instead of NaN
Date: Tue, 11 Aug 2026 02:34:38 +0000
Message-ID: <19615-c7e390593416f6b6@postgresql.org> (raw)

The following bug has been logged on the website:

Bug reference:      19615
Logged by:          Junwen An
Email address:      feasiblechart@gmail.com
PostgreSQL version: 19beta2
Operating system:   Linux Ubuntu
Description:        

I found `COVAR_POP`/`REGR_SXY` returns 0.0 instead of NaN when one arg is
constant and the other has Inf (not first), which might be unexpected. I
could reproduce it on 19beta1, 19beta2, and 20devel, but not on 18.4.

Minimal repro:

CREATE TABLE t (y double precision);
INSERT INTO t VALUES (3), ('Infinity'), (4);

SELECT COVAR_POP(0::float8, y) FROM t;
-- Expected 1 row: NaN
-- Actual:         0.0

SELECT COVAR_POP(y, 0::float8) FROM t;
-- Expected 1 row: NaN
-- Actual:         0.0

SELECT COVAR_SAMP(0::float8, y) FROM t;
-- Expected 1 row: NaN
-- Actual:         0.0

SELECT REGR_SXY(0::float8, y) FROM t;
-- Expected 1 row: NaN
-- Actual:         0.0

Did some further experiments, and it seems this behavior also depends on the
position of 'Inf'

WITH t(ord, y) AS (
    VALUES
        (1, 'Infinity'::float8),
        (2, 3::float8),
        (3, 4::float8)
)
SELECT
    covar_pop(0::float8, y ORDER BY ord)      AS inf_first,
    covar_pop(0::float8, y ORDER BY ord DESC) AS inf_last
FROM t;

 inf_first | inf_last
-----------+----------
 NaN       | 0

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