Prohibited LowCardinality when importing tables with dates #87586
Description
Company or project name
oecd.ai
Describe what's wrong
Since version 25.8 I'm suddenly getting:
DB::Exception: Creating columns of type LowCardinality(Date) is prohibited by default due to expected negative impact on performance. It can be enabled with the allow_suspicious_low_cardinality_types setting. (SUSPICIOUS_TYPE_FOR_LOW_CARDINALITY)
Does it reproduce on the most recent release?
Yes
How to reproduce
ClickHouse client version 25.8.4.13 (official build).
This works fine
CREATE TABLE pq_test1 ENGINE = MergeTree ORDER BY (year, src, dst)
SETTINGS allow_nullable_key = 1
AS SELECT * FROM url('https://storage.googleapis.com/cliclhouse-bug-repro-42/pq_test.parquet', Parquet)This fails (notice there's additional date hp=2025-09-24 in provided url path, the rest is the same)
CREATE TABLE pq_test2 ENGINE = MergeTree ORDER BY (year, src, dst)
SETTINGS allow_nullable_key = 1
AS SELECT * FROM url('https://storage.googleapis.com/cliclhouse-bug-repro-42/hp=2025-09-24/pq_test.parquet', Parquet)Received exception from server (version 25.8.4):
Code: 455. DB::Exception: Received from localhost:9000. DB::Exception: Creating columns of type LowCardinality(Date) is prohibited by default due to expected negative impact on performance. It can be enabled with the `allow_suspicious_low_cardinality_types` setting. (SUSPICIOUS_TYPE_FOR_LOW_CARDINALITY)
The interesting thing is that there isn't any column of type Date in that table itself ... but Clickhouse somehow infers the hp=2025-09-24 from the url and treats it as a table row.
DESCRIBE TABLE url('https://storage.googleapis.com/cliclhouse-bug-repro-42/hp=2025-09-24/pq_test.parquet', Parquet)
Query id: ad710a68-ad47-408f-8628-ba06db8194b3
┌─name─────────┬─type─────────────────┬─default_type─┬─default_expression─┬─comment─┬─codec_expression─┬─ttl_expression─┐
1. │ year │ Nullable(Int64) │ │ │ │ │ │
2. │ src │ Nullable(String) │ │ │ │ │ │
3. │ dst │ Nullable(String) │ │ │ │ │ │
4. │ Deals │ Nullable(Int64) │ │ │ │ │ │
5. │ Sum_of_deals │ Nullable(Float64) │ │ │ │ │ │
6. │ hp │ LowCardinality(Date) │ │ │ │ │ │
└──────────────┴──────────────────────┴──────────────┴────────────────────┴─────────┴──────────────────┴────────────────┘
6 rows in set. Elapsed: 0.072 sec. Expected behavior
No response
Error message and/or stacktrace
No response
Additional context
I assume that the real problem may be that ClickHouse is parsing the hp=2025-09-24 as some extra Hive metadata and is trying to implicitly add that as a hidden column to the table being created.
However the reported error message is not mentioning any Hive thing and is generally not helpful in this situation.