datum: check len(s) before calculating and checking runeCount

[ekexium]

What problem does this PR solve?

Issue Number: close #53942

Problem Summary:

RuneCountInString can be unexpectedly time-consuming for certain workloads. Since the number of runes must be smaller than or equal to the length of the string, we can fast check the length first. Only calculate the number of runes when necessary.

Before:

go test -run=^$ -bench=BenchmarkInsertIntoSelect -benchmem -benchtime=100x
     100	 136332696 ns/op	106229159 B/op	  507278 allocs/op

This PR:

go test -run=^$ -bench=BenchmarkInsertIntoSelect -benchmem -benchtime=100x
     100	 123812334 ns/op	106041608 B/op	  506652 allocs/op

What changed and how does it work?

Check List

Tests

• Unit test: already covered by existing cases and benchmarks
• 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.

None

[tiprow]

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[codecov]

Codecov Report

Attention: Patch coverage is 79.31034% with 6 lines in your changes missing coverage. Please review.

Project coverage is 55.8063%. Comparing base (a865c86) to head (f3e03bf).
Report is 20 commits behind head on master.

Additional details and impacted files

@@                Coverage Diff                @@
##             master     #53941         +/-   ##
=================================================
- Coverage   72.6206%   55.8063%   -16.8144%     
=================================================
  Files          1507       1628        +121     
  Lines        431669     603639     +171970     
=================================================
+ Hits         313481     336869      +23388     
- Misses        98878     243593     +144715     
- Partials      19310      23177       +3867     

Flag	Coverage Δ
integration	37.1702% <79.3103%> (?)
unit	71.4500% <75.8620%> (-0.1592%)	⬇️

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

Components	Coverage Δ
dumpling	52.9656% <ø> (ø)
parser	∅ <ø> (∅)
br	50.4460% <ø> (+8.2959%)	⬆️

[cfzjywxk]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cfzjywxk, zyguan

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-06-13 01:43:51.40513252 +0000 UTC m=+601785.458444444: ☑️ agreed by cfzjywxk.
• 2024-06-13 03:29:20.856604816 +0000 UTC m=+608114.909916737: ☑️ agreed by zyguan.