Better multi-column aggregation support with StringView

[XiangpengHao]

Which issue does this PR close?

Related to #7000. Previous pr was automatically closed because the string-view2 branch is merged.

Rationale for this change

I get some time to implement the multi-column aggregation with StringView, the implementation is inspired by @jayzhan211 's #10976.

However, the performance is mixed, I see almost no performance diff with ClickBench query 18:

SELECT "UserID", extract(minute FROM to_timestamp_seconds("EventTime")) AS m, "SearchPhrase", COUNT(*) FROM hits GROUP BY "UserID", m, "SearchPhrase" ORDER BY COUNT(*) DESC LIMIT 10;

This is becuase the average size of SearchPhrase is only 4, meaning the string is always inlined to view and thus not being reused.

However, I made this query (q14, changed SearchPhrase to URL)

SELECT "SearchEngineID", "URL", COUNT(*) AS c FROM hits WHERE "URL" <> '' GROUP BY "SearchEngineID", "URL" ORDER BY c DESC LIMIT 10;

Without this patch, it takes 7.4s; with this patch, it takes 5.9s, a 20% improvement.

With this patch, the output string will reuse the string values in the buffer, so it is good.
However, when interning the values, we need to hash twice: (1) the string itself to build BytesViewMap (2) the group index - u32, which adds overhead.

What changes are included in this PR?

Things not implemented yet:

• EmtiTo::First(n): I need to think a bit more about how to efficiently reuse the BytesViewBuilder

Are these changes tested?

Are there any user-facing changes?

No

[github-actions]

Thank you for your contribution. Unfortunately, this pull request is stale because it has been open 60 days with no activity. Please remove the stale label or comment or this will be closed in 7 days.