Optimize group by constant keys

[Avogar]

Changelog category (leave one):

• Performance Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Optimize group by constant keys. Will optimize queries with group by _file/_path after #53529

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

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[robot-clickhouse]

This is an automated comment for commit a075d03 with description of existing statuses. It's updated for the latest CI running

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[Avogar]

[novikd]

Why is it better? I didn't understand why we need this class.

[Avogar]

Otherwise we will need to write ~16 if/else branches to check all 4 bool arguments (maybe some of them won't actually appear, but still a lot).
Like https://pastila.nl/?01f3324d/a170ed3f747f063f81f5821188dd8aa3#B7nUAYOQzbzMvGUQ6wClTw==
And IMHO do it in a few lines with some dispatcher class is better. But you can disagree and I can bring back this batch of if/else

[KochetovNicolai]

I do not like dispatchers as well. I think the issue is with executeImplBatch and we should refactor it somehow.

[KochetovNicolai]

The change is good and clear generally.
However, I do not like how our dispatching is done.
Looks like something is wrong with a function that takes 4 bool template args.
I would like to refactor it somehow, but do not have good ideas right now.

[Avogar]

Actually, the new bool flag all_keys_are_const is not called on each row as other flags, so I can just pass it as not template argument and bring previous code back in dispatching with 3 bool template arguments

[nickitat]

I don't really mind BoolArgsToTemplateArgsDispatcher, but I do mind all the if-s that we had to add. we essntially already have this method of calculation - it is called without_key. so I assume that we need almost no changes in Aggregator itself (except that now key columns wouldn't be added to the output header afaik).
so would be better to make it a plan optimisation and get rid of one more AST optimisation:

:) explain syntax select count(), pi() from numbers_mt(1e6) group by pi(), e()

EXPLAIN SYNTAX
SELECT
    count(),
    pi()
FROM numbers_mt(1000000.)
GROUP BY
    pi(),
    e()

┌─explain───────────────────┐
│ SELECT                    │
│     count(),              │
│     pi()                  │
│ FROM numbers_mt(1000000.) │
└───────────────────────────┘

[Avogar]

we essntially already have this method of calculation - it is called without_key

But afaiu that's not the same and we cannot reuse it. We can still have different keys, but some blocks of data can contain constant columns with keys. For example:

select count() from file('data{1,2,3}.parquet') group by _file

Reading from file will return blocks with constant column _file, but different blocks can have different values

[nickitat]

we essntially already have this method of calculation - it is called without_key

But afaiu that's not the same and we cannot reuse it. We can still have different keys, but some blocks of data can contain constant columns with keys. For example:

select count() from file('data{1,2,3}.parquet') group by _file

Reading from file will return blocks with constant column _file, but different blocks can have different values

ok, I indeed assumed that we're talking about globally constant keys.