expression: support builtin function "benchmark"

[vikingM]

What problem does this PR solve?

#6774

What is changed and how it works?

add the benchmark function
now this function args are (int,string)

Check List

Tests

• Manual test
  select benchmark(10000,md5(rand()));

Code changes
in expression/builtin_info.go
add the related codes

---

This change is 

[sre-bot]

Hi contributor, thanks for your PR.

This patch needs to be approved by someone of admins. They should reply with "/ok-to-test" to accept this PR for running test automatically.

[CLAassistant]

All committers have signed the CLA.

[zz-jason]

@vikingM Please fix the CI.

[vikingM]

the conflict file just because when i create a json object, i found in "datumsToConstants" think it is a string object

[zz-jason]

Please merge the latest master branch and resolve conflicts.

[zz-jason]

@vikingM Please merge the latest master and resolve conflicts.

[XuHuaiyu]

Hi, @vikingM
Please add some integration cases for benchmark.

[zz-jason]

@vikingM any update? there is a conflict between this and the master branch. Could you update to the latest master and resolve the conflicts in your spare time?

[vikingM]

So sorry. I'll resolve the conflict before Friday. Zhang Jian <[email protected]> 于2019年1月8日周二 下午2:42写道：

…

@vikingM <https://github.com/vikingM> any update? there is a conflict between this and the master branch. Could you update to the latest master and resolve the conflicts in your spare time? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#8313 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AYd4cazXL6CHfoNuDYKWPPSjFP4rMTE5ks5vBD3EgaJpZM4YdYIP> .

[codecov-io]

Codecov Report

Merging #8313 into master will decrease coverage by 0.04%.
The diff coverage is 44.71%.

@@            Coverage Diff            @@
##           master   #8313      +/-   ##
=========================================
- Coverage   67.55%   67.5%   -0.05%     
=========================================
  Files         363     363              
  Lines       75301   75423     +122     
=========================================
+ Hits        50868   50918      +50     
- Misses      19940   19997      +57     
- Partials     4493    4508      +15

Impacted Files	Coverage Δ
expression/builtin_info.go	54.1% <44.71%> (-7.07%)	⬇️
util/systimemon/systime_mon.go	80% <0%> (-20%)	⬇️
expression/schema.go	94.11% <0%> (-0.85%)	⬇️
executor/join.go	78.29% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bb2d56a...69ba828. Read the comment docs.

[qw4990]

Could you please add some cases that generate some errors or nil results~?

[zz-jason]

@vikingM I'm sorry to inform you that builtin function benchmark() has been supported in this PR #9252, I'm going to close this PR now. Anyway, thanks for your contribution.