add setting exact_rows_before_limit

[MaxWk]

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Changelog category (leave one):

• New Feature

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):
Added a setting exact_rows_before_limit (0/1). When enabled, ClickHouse will provide exact value for rows_before_limit_at_least statistic, but with the cost that the data before limit will have to be read completely.
This closes #6613.

Detailed description / Documentation draft:

A setting exact_rows_before_limit = 0|1. When enabled, LimitBlockInputStream will always_read_till_end (we already have this feature in code) and the field rows_before_limit_at_least will have precise value.
#6613

By adding documentation, you'll allow users to try your new feature immediately, not when someone else will have time to document it later. Documentation is necessary for all features that affect user experience in any way. You can add brief documentation draft above, or add documentation right into your patch as Markdown files in docs folder.

If you are doing this for the first time, it's recommended to read the lightweight Contributing to ClickHouse Documentation guide first.

Information about CI checks: https://clickhouse.tech/docs/en/development/continuous-integration/

[abyss7]

Changelog entry (and the in-code description) is obscure for users: you should describe the new setting at the user-level scenarios - since typical user doesn't know what is LimitBlockInputStream, where it's located in the pipeline and how it affects typical SELECT queries.

Also please add the test.

[MaxWk]

Changelog entry (and the in-code description) is obscure for users: you should describe the new setting at the user-level scenarios - since typical user doesn't know what is LimitBlockInputStream, where it's located in the pipeline and how it affects typical SELECT queries.

Also please add the test.

Hi abyss7 ,thank you for your quick reply. I have edit the Changelog entry.

[CLAassistant]

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3 out of 4 committers have signed the CLA.

✅ MaxWk
✅ alexey-milovidov
✅ qoega
❌ 万康

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[alexey-milovidov]

@MaxWk Why it is in draft stage? The change should be quite easy and we need this feature...

[alexey-milovidov]

Everything looks alright :)

[alexey-milovidov]

@Mergifyio update

[mergify]

update

✅ Branch has been successfully updated

[alexey-milovidov]

@Mergifyio update

[mergify]

update

✅ Branch has been successfully updated

[MaxWk]

feature

Sorry for late reply, This pull request was forgotten by me, I thought it had been merged

[alexey-milovidov]

Still not merged - the test about this feature has failed.

[m1khal3v]

Any news here?

[Cimus]

Update

[supermarat]

really need

[MaxWk]

update

[MaxWk]

@alexey-milovidov @abyss7 I'm sorry it took so long to do this, I've been so busy with work lately, please help me review
this

[MaxWk]

@Mergifyio update

[mergify]

update

✅ Branch has been successfully updated

[MaxWk]

@qoega These errors seems not relate to my update

[qoega]

Yes. Flaky check just shows that timeout was hit before the number of ribs or wanted to run. Absolutely normal if test is long.

[alexey-milovidov]

The code LGTM.