pingcap/tidb

Arithmetic results are not consistent with MySQL when overflow occurs

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#4,475 opened on Sep 8, 2017

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component/expressionhelp wantedpriority/P4type/compatibility

Description

Please answer these questions before submitting your issue. Thanks!

  1. What did you do? If possible, provide a recipe for reproducing the error.
SET @a:=999999999999999;
select @a + @a;    -- 1999999999999998
SET @a:=9999999999999999;
select @a + @a;    -- 20000000000000000
SET @a:=99999999999999999;
select @a + @a;   -- 20000000000000000
select @a * @a; -- 1000000000000000000000000000000000000
SELECT -@a - @a;  -- -2000000000000000000
SELECT @a / 0.5; -- 2000000000000000000
select round(999999999999999999, -18);  -- tidb: 999999999999999872, mysql: 1000000000000000000
select round(9999999999999999999, -18);   -- tidb: 10446744073709552640, mysql: 100000000000000000000
  1. What did you expect to see?
@a + @a
1999999999999998
@a + @a
19999999999999998
@a + @a
199999999999999998
ERROR 1690 (22003) at line 7: BIGINT value is out of range in '((@`a`) * (@`a`))'
-@a - @a
-199999999999999998
@a / 0.5
199999999999999998.0000
round(999999999999999999, -18)
1000000000000000000
round(9999999999999999999, -18)
10000000000000000000
  1. What did you see instead?
@a + @a
1999999999999998
@a + @a
20000000000000000
@a + @a
200000000000000000
@a * @a
10000000000000000000000000000000000
-@a - @a
-200000000000000000
@a / 0.5
200000000000000000
round(999999999999999999, -18)
999999999999999872
round(9999999999999999999, -18)
10446744073709552640
  1. What version of TiDB are you using (tidb-server -V)?

Contributor guide

Tech stack
gosql
Domain
database
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Go programmingTiDB expression evaluationMySQL BIGINT overflow semantics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Investigate TiDB's arithmetic operation implementation in the expression package (e.g., expressions/builtin arithmetic.go) to understand how overflow is handled. Compare with MySQL's behavior for BIGINT overflow. Write tests to reproduce the issue and ensure fix matches expected MySQL outputs. Note that the issue has been open since 2017 with no assignee, so check recent comments or maintainer input before starting.