Add experimental support for descending sorting keys in MergeTree tables

[amosbird]

Changelog category (leave one):

• New Feature

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

This PR introduces a new setting allow_experimental_reverse_key that enables support for descending sort order in MergeTree sorting keys. This is useful for time series analysis, especially TopN queries. Example usage:

CREATE TABLE example
(
    time DateTime,
    key Int32,
    value String
) ENGINE = MergeTree
ORDER BY (time DESC, key);  -- Descending order for 'time' field
SETTINGS allow_experimental_reverse_key = 1;

SELECT * from example WHERE key = 'xxx' ORDER BY time DESC LIMIT 10;  -- This will apply more efficient ReadInOrder optimization instead of ReadInReverseOrder

Some preliminary perf test:

create table event_log_asc (dt DateTime, key UInt16) engine MergeTree order by dt;

insert into event_log_asc select now() - number, rand() from numbers(400000000);

optimize table event_log_asc final;

create table event_log_desc (dt DateTime, key UInt16) engine MergeTree order by dt desc settings allow_experimental_reverse_key = 1;

insert into event_log_desc select * from event_log_asc;

optimize table event_log_desc final;

SELECT *
FROM event_log_asc
WHERE key = 123
ORDER BY dt DESC
LIMIT 10

10 rows in set. Elapsed: 0.109 sec. Processed 27.53 million rows, 61.34 MB (253.50 million rows/s., 564.70 MB/s.)

SELECT *
FROM event_log_desc
WHERE key = 123
ORDER BY dt DESC
LIMIT 10

10 rows in set. Elapsed: 0.024 sec. Processed 25.44 million rows, 56.27 MB (1.06 billion rows/s., 2.35 GB/s.)

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

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[robot-ch-test-poll2]

This is an automated comment for commit 663d26e with description of existing statuses. It's updated for the latest CI running

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[Algunenano]

Please mark the setting as EXPERIMENTAL in the flags (the 0 at the end). Otherwise it's not experimental, just it has a different name

[orloffv]

seems to need validation in the code

[alexey-milovidov]

It does not give any speedup:
https://pastila.nl/?0000bed8/597da07056e3b91a6420a54b22a852d9#kPeUfAAhFC62LfTR1Qaa2A==

[alexey-milovidov]

Although, there is good speedup when I read a small number of columns.

[amosbird]

It does not give any speedup:

Are you using this query to verify?

SELECT *
FROM hits_desc
ORDER BY EventTime DESC
LIMIT 10
FORMAT Vertical

You can try adding non-primary key filters that cannot be satisfied by the last few granules alone (e.g., filters that result in fewer than 10 matches).