From c101d83a3dd6d23f33bfbbbd696c0ea11abcbaf9 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Sun, 21 May 2017 13:05:17 -0400 Subject: [PATCH] Fix precision and rounding issues in money multiplication and division. The cash_div_intX functions applied rint() to the result of the division. That's not merely useless (because the result is already an integer) but it causes precision loss for values larger than 2^52 or so, because of the forced conversion to float8. On the other hand, the cash_mul_fltX functions neglected to apply rint() to their multiplication results, thus possibly causing off-by-one outputs. Per C standard, arithmetic between any integral value and a float value is performed in float format. Thus, cash_mul_flt4 and cash_div_flt4 produced answers good to only about six digits, even when the float value is exact. We can improve matters noticeably by widening the float inputs to double. (It's tempting to consider using "long double" arithmetic if available, but that's probably too much of a stretch for a back-patched fix.) Also, document that cash_div_intX operators truncate rather than round. Per bug #14663 from Richard Pistole. Back-patch to all supported branches. Discussion: https://postgr.es/m/22403.1495223615@sss.pgh.pa.us --- doc/src/sgml/datatype.sgml | 5 ++++ src/backend/utils/adt/cash.c | 16 ++++++------ src/test/regress/expected/money.out | 38 +++++++++++++++++++++++++++++ src/test/regress/sql/money.sql | 10 ++++++++ 4 files changed, 61 insertions(+), 8 deletions(-) diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml index 464ce83d30e..39bbd87e677 100644 --- a/doc/src/sgml/datatype.sgml +++ b/doc/src/sgml/datatype.sgml @@ -977,6 +977,11 @@ SELECT '52093.89'::money::numeric::float8; + Division of a money value by an integer value is performed + with truncation of the fractional part towards zero. To get a rounded + result, divide by a floating-point value, or cast the money + value to numeric before dividing and back to money + afterwards. (The latter is preferable to avoid risking precision loss.) When a money value is divided by another money value, the result is double precision (i.e., a pure number, not money); the currency units cancel each other out in the division. diff --git a/src/backend/utils/adt/cash.c b/src/backend/utils/adt/cash.c index b336185df7e..d329efaa9a8 100644 --- a/src/backend/utils/adt/cash.c +++ b/src/backend/utils/adt/cash.c @@ -621,7 +621,7 @@ cash_mul_flt8(PG_FUNCTION_ARGS) float8 f = PG_GETARG_FLOAT8(1); Cash result; - result = c * f; + result = rint(c * f); PG_RETURN_CASH(result); } @@ -636,7 +636,7 @@ flt8_mul_cash(PG_FUNCTION_ARGS) Cash c = PG_GETARG_CASH(1); Cash result; - result = f * c; + result = rint(f * c); PG_RETURN_CASH(result); } @@ -671,7 +671,7 @@ cash_mul_flt4(PG_FUNCTION_ARGS) float4 f = PG_GETARG_FLOAT4(1); Cash result; - result = c * f; + result = rint(c * (float8) f); PG_RETURN_CASH(result); } @@ -686,7 +686,7 @@ flt4_mul_cash(PG_FUNCTION_ARGS) Cash c = PG_GETARG_CASH(1); Cash result; - result = f * c; + result = rint((float8) f * c); PG_RETURN_CASH(result); } @@ -707,7 +707,7 @@ cash_div_flt4(PG_FUNCTION_ARGS) (errcode(ERRCODE_DIVISION_BY_ZERO), errmsg("division by zero"))); - result = rint(c / f); + result = rint(c / (float8) f); PG_RETURN_CASH(result); } @@ -756,7 +756,7 @@ cash_div_int8(PG_FUNCTION_ARGS) (errcode(ERRCODE_DIVISION_BY_ZERO), errmsg("division by zero"))); - result = rint(c / i); + result = c / i; PG_RETURN_CASH(result); } @@ -808,7 +808,7 @@ cash_div_int4(PG_FUNCTION_ARGS) (errcode(ERRCODE_DIVISION_BY_ZERO), errmsg("division by zero"))); - result = rint(c / i); + result = c / i; PG_RETURN_CASH(result); } @@ -858,7 +858,7 @@ cash_div_int2(PG_FUNCTION_ARGS) (errcode(ERRCODE_DIVISION_BY_ZERO), errmsg("division by zero"))); - result = rint(c / s); + result = c / s; PG_RETURN_CASH(result); } diff --git a/src/test/regress/expected/money.out b/src/test/regress/expected/money.out index 538235c4cc2..efb48f1acd6 100644 --- a/src/test/regress/expected/money.out +++ b/src/test/regress/expected/money.out @@ -185,6 +185,44 @@ SELECT * FROM money_data; $123.46 (1 row) +-- rounding vs. truncation in division +SELECT '878.08'::money / 11::float8; + ?column? +---------- + $79.83 +(1 row) + +SELECT '878.08'::money / 11::float4; + ?column? +---------- + $79.83 +(1 row) + +SELECT '878.08'::money / 11::int; + ?column? +---------- + $79.82 +(1 row) + +SELECT '878.08'::money / 11::smallint; + ?column? +---------- + $79.82 +(1 row) + +-- check for precision loss in division +SELECT '90000000000000099.00'::money / 10::int; + ?column? +--------------------------- + $9,000,000,000,000,009.90 +(1 row) + +SELECT '90000000000000099.00'::money / 10::smallint; + ?column? +--------------------------- + $9,000,000,000,000,009.90 +(1 row) + -- Cast int4/int8 to money SELECT 1234567890::money; money diff --git a/src/test/regress/sql/money.sql b/src/test/regress/sql/money.sql index 09b9476b706..f0db5fa4322 100644 --- a/src/test/regress/sql/money.sql +++ b/src/test/regress/sql/money.sql @@ -57,6 +57,16 @@ DELETE FROM money_data; INSERT INTO money_data VALUES ('$123.459'); SELECT * FROM money_data; +-- rounding vs. truncation in division +SELECT '878.08'::money / 11::float8; +SELECT '878.08'::money / 11::float4; +SELECT '878.08'::money / 11::int; +SELECT '878.08'::money / 11::smallint; + +-- check for precision loss in division +SELECT '90000000000000099.00'::money / 10::int; +SELECT '90000000000000099.00'::money / 10::smallint; + -- Cast int4/int8 to money SELECT 1234567890::money; SELECT 12345678901234567::money;