Fix wrong result of cast(float as decimal) when overflow happens

[guo-shaoge]

What problem does this PR solve?

Issue Number: close #3998

Problem Summary:

What is changed and how it works?

When cast float32 to decimal, use Float64 as intermediate type to avoid overflow.

WATCHOUT: there is performance regression and compatibility issue. Explained in the following comments.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Fix wrong result of cast(float as decimal) when overflow happens

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• fuzhe1989
• windtalker

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[guo-shaoge]

After change from Float32 to Float64. Result are different from older version.

drop table if exists t1;
create table t1(c1 float, c2 double);
insert into t1 values(123.123456789123456789, 123.123456789123456789);
alter table t1 set tiflash replica 1;
select cast(c1 as decimal(50,20)) from t1;
select cast(c1 as decimal(50,20)) from t1;

TiDB:

mysql>  select cast(c1 as decimal(50,20)) from t1;
+----------------------------+
| cast(c1 as decimal(50,20)) |
+----------------------------+
|   123.12345886230469000000 |
+----------------------------+
1 row in set (0.00 sec)

mysql>  select cast(c2 as decimal(50,20)) from t1;
+----------------------------+
| cast(c2 as decimal(50,20)) |
+----------------------------+
|   123.12345678912345000000 |
+----------------------------+
1 row in set (0.00 sec)

TiFlash Before:

MySQL [test]> select cast(c1 as decimal(50, 20)) from t1;
+-----------------------------+
| cast(c1 as decimal(50, 20)) |
+-----------------------------+
|    123.12344859372018466817 |
+-----------------------------+
1 row in set (0.25 sec)

MySQL [test]> select cast(c2 as decimal(50, 20)) from t1;
+-----------------------------+
| cast(c2 as decimal(50, 20)) |
+-----------------------------+
|    123.12345678912344293376 |
+-----------------------------+
1 row in set (0.02 sec)

TiFlash After:

MySQL [test]> select * from t1;
+-----------+--------------------+
| c1        | c2                 |
+-----------+--------------------+
| 123.12346 | 123.12345678912345 |
+-----------+--------------------+
1 row in set (0.02 sec)
MySQL [test]> select cast(c1 as decimal(50,20)) from t1;
+----------------------------+
| cast(c1 as decimal(50,20)) |
+----------------------------+
|   123.12345886230470197248 |
+----------------------------+
1 row in set (0.21 sec)

MySQL [test]> select cast(c2 as decimal(50,20)) from t1;
+----------------------------+
| cast(c2 as decimal(50,20)) |
+----------------------------+
|   123.12345678912344293376 |
+----------------------------+
1 row in set (0.02 sec)

[guo-shaoge]

/run-all-tests

[sre-bot]

Coverage for changed files

Filename                            Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
FunctionsTiDBConversion.h               856               432    49.53%          89                26    70.79%        1940               635    67.27%         496               321    35.28%
tests/gtest_tidb_conversion.cpp        4645              1044    77.52%          37                 0   100.00%        1453                 0   100.00%        1468               746    49.18%
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                                  5501              1476    73.17%         126                26    79.37%        3393               635    81.28%        1964              1067    45.67%

Coverage summary

Functions  MissedFunctions  Executed  Lines   MissedLines  Cover
17040      9576             43.80%    191889  97050        49.42%

full coverage report (for internal network access only)

[guo-shaoge]

Performance regression for cast(float32 to decimal):

Before:

CastToDecimalBench/float32_to_decimal_60_30/iterations:1000     572218 ns       572220 ns         1000

After:

CastToDecimalBench/float32_to_decimal_60_30/iterations:1000    1007571 ns      1007572 ns         1000

[guo-shaoge]

/cc @fuzhe1989
/cc @windtalker

[ti-chi-bot]

@guo-shaoge: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

You only need to trigger /merge once, and if the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 330948c

[sre-bot]

Coverage for changed files

Filename                            Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
FunctionsTiDBConversion.h               856               432    49.53%          89                26    70.79%        1940               635    67.27%         496               321    35.28%
tests/gtest_tidb_conversion.cpp        4645              1044    77.52%          37                 0   100.00%        1455                 0   100.00%        1468               746    49.18%
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                                  5501              1476    73.17%         126                26    79.37%        3395               635    81.30%        1964              1067    45.67%

Coverage summary

Functions  MissedFunctions  Executed  Lines   MissedLines  Cover
17040      9576             43.80%    191891  97033        49.43%

full coverage report (for internal network access only)

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4386.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4387.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4388.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4389.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4390.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4391.