From 2a3de9add95a6d0635f70a93509a156c1a08a806 Mon Sep 17 00:00:00 2001
From: "Chao Li (Evan)" <lic@highgo.com>
Date: Wed, 14 Oct 2026 18:40:56 +0800
Subject: [PATCH v1] Fix numeric log() with an infinite argument and a base at
 or below one.

Return negative infinity when the base is between zero and one, and
raise division by zero when the base is one, matching the finite-input
behavior. Previously, both cases incorrectly returned positive infinity.

Add regression tests for both cases.

Author: Chao Li <lic@highgo.com>
Reviewed-by:
Discussion: https://postgr.es/m/
Backpatch-through: 14
---
 src/backend/utils/adt/numeric.c       | 21 ++++++++++++++++-----
 src/test/regress/expected/numeric.out |  8 ++++++++
 src/test/regress/sql/numeric.sql      |  2 ++
 3 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/src/backend/utils/adt/numeric.c b/src/backend/utils/adt/numeric.c
index f4dfec56659..add1406c518 100644
--- a/src/backend/utils/adt/numeric.c
+++ b/src/backend/utils/adt/numeric.c
@@ -3869,13 +3869,13 @@ numeric_log(PG_FUNCTION_ARGS)
 		sign2 = numeric_sign_internal(num2);
 		if (sign1 < 0 || sign2 < 0)
 			ereport(ERROR,
-					(errcode(ERRCODE_INVALID_ARGUMENT_FOR_LOG),
-					 errmsg("cannot take logarithm of a negative number")));
+					errcode(ERRCODE_INVALID_ARGUMENT_FOR_LOG),
+					errmsg("cannot take logarithm of a negative number"));
 		/* fail on zero inputs, as log_var would */
 		if (sign1 == 0 || sign2 == 0)
 			ereport(ERROR,
-					(errcode(ERRCODE_INVALID_ARGUMENT_FOR_LOG),
-					 errmsg("cannot take logarithm of zero")));
+					errcode(ERRCODE_INVALID_ARGUMENT_FOR_LOG),
+					errmsg("cannot take logarithm of zero"));
 		if (NUMERIC_IS_PINF(num1))
 		{
 			/* log(Inf, Inf) reduces to Inf/Inf, so it's NaN */
@@ -3885,7 +3885,18 @@ numeric_log(PG_FUNCTION_ARGS)
 			PG_RETURN_NUMERIC(make_result(&const_zero));
 		}
 		Assert(NUMERIC_IS_PINF(num2));
-		/* log(finite-positive, Inf) is Inf */
+
+		/*
+		 * log(base, Inf) = Inf / ln(base), so the sign depends on whether
+		 * base is above or below 1.  If base = 1, fail as log_var() would.
+		  */ init_var_from_num(num1, &arg1);
+		sign1 = cmp_var(&arg1, &const_one);
+		if (sign1 == 0)
+			ereport(ERROR,
+					errcode(ERRCODE_DIVISION_BY_ZERO),
+					errmsg("division by zero"));
+		if (sign1 < 0)
+			PG_RETURN_NUMERIC(make_result(&const_ninf));
 		PG_RETURN_NUMERIC(make_result(&const_pinf));
 	}
 
diff --git a/src/test/regress/expected/numeric.out b/src/test/regress/expected/numeric.out
index c58e232a263..cec2bc44b4c 100644
--- a/src/test/regress/expected/numeric.out
+++ b/src/test/regress/expected/numeric.out
@@ -933,6 +933,14 @@ FROM v AS v1(x1), v AS v2(x2);
       NaN |      NaN |                NaN
 (16 rows)
 
+SELECT log('0.5'::numeric, 'inf');
+    log    
+-----------
+ -Infinity
+(1 row)
+
+SELECT log('1'::numeric, 'inf');
+ERROR:  division by zero
 SELECT log('0'::numeric, '10');
 ERROR:  cannot take logarithm of zero
 SELECT log('10'::numeric, '0');
diff --git a/src/test/regress/sql/numeric.sql b/src/test/regress/sql/numeric.sql
index 640c6d92f4c..5fd5c02f636 100644
--- a/src/test/regress/sql/numeric.sql
+++ b/src/test/regress/sql/numeric.sql
@@ -713,6 +713,8 @@ SELECT x1, x2,
   log(x1, x2)
 FROM v AS v1(x1), v AS v2(x2);
 
+SELECT log('0.5'::numeric, 'inf');
+SELECT log('1'::numeric, 'inf');
 SELECT log('0'::numeric, '10');
 SELECT log('10'::numeric, '0');
 SELECT log('-inf'::numeric, '10');
-- 
2.50.1 (Apple Git-155)