expression: unify casting real to string in tidb (tikv#16975) | tidb-test=pr/2324

[solotzg]

What problem does this PR solve?

Issue Number: close #51109

Problem Summary:

• The default implementation about casting float32/float64 into string is supported by lib strconv. strconv use Ryu algorithm to calculate significant digits.
• TiKV use method f32::to_string & f64::to_string to implement casting real to str. It can NOT guarantee precision if the number of significant digits exceeds the upper limit (f32: 6, f64: 15).
• If don't use decimal exponent format, the size of target string might be too large.

What changed and how does it work?

Unify the behavior of converting real types to strings in tidb and tikv(tikv/tikv#16975).

• use ryu lib in tikv to implement casting real type to str type.
• if abs(num) >= 1e15 or abs(num) < 1e-15 then use decimal exponent format

Check List

Tests

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

  • I checked and no code files have been changed.

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

Please refer to Release Notes Language Style Guide to write a quality release note.

Fix the incorrect result when casting REAL type to STRING type

[tiprow]

Hi @solotzg. Thanks for your PR.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.8800%. Comparing base (ce74191) to head (5e6945f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #53129        +/-   ##
================================================
+ Coverage   72.4150%   73.8800%   +1.4649%     
================================================
  Files          1492       1492                
  Lines        429231     429258        +27     
================================================
+ Hits         310828     317136      +6308     
+ Misses        99123      92184      -6939     
- Partials      19280      19938       +658     

Flag	Coverage Δ
integration	26.2688% <77.4193%> (?)
unit	71.2438% <100.0000%> (-0.0653%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	53.9957% <ø> (ø)
parser	∅ <ø> (∅)
br	50.1840% <ø> (+8.7438%)	⬆️

[solotzg]

/retest

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[solotzg]

/test mysql-test

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[solotzg]

/retest

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[solotzg]

/retest

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[solotzg]

/cc @wshwsh12 @XuHuaiyu

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gengliqi, wshwsh12, XuHuaiyu

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-05-11 09:22:54.058931214 +0000 UTC m=+1299527.816066789: ☑️ agreed by XuHuaiyu.
• 2024-05-11 09:24:33.870458489 +0000 UTC m=+1299627.627594061: ☑️ agreed by wshwsh12.

[solotzg]

/test mysql-test

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[solotzg]

/cherrypick release-8.1

[ti-chi-bot]

@solotzg: new pull request created to branch release-8.1: #53207.

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/cherrypick release-8.1

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