Try to detect file format automatically during schema inference if it's unknown

[Avogar]

Changelog category (leave one):

• New Feature

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

Try to detect file format automatically during schema inference if it's unknown in file/s3/hdfs/url/azureBlobStorage engines.
Closes #50576.

Documentation entry for user-facing changes

Examples:

:) select * from format('{"a" : 42, "b" : "Data1"}, {"a" : 43, "b" : "Data2" }')

┌──a─┬─b─────┐
│ 42 │ Data1 │
│ 43 │ Data2 │
└────┴───────┘

:) select * from format('42,"Data1"\n43,"Data2"')

┌─c1─┬─c2────┐
│ 42 │ Data1 │
│ 43 │ Data2 │
└────┴───────┘

:) select * from format('42\tData1\n43\tData2')

┌─c1─┬─c2────┐
│ 42 │ Data1 │
│ 43 │ Data2 │
└────┴───────┘

:) select * from format('(42, ''Data1''), (43, ''Data2'')')

┌─c1─┬─c2────┐
│ 42 │ Data1 │
│ 43 │ Data2 │
└────┴───────┘

• Documentation is written (mandatory for new features)

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[robot-clickhouse]

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[serxa]

Few small remarks

[serxa]

Minor. Is it better?

A question. rows_read can only increase. Is this correct if we pass here twice for the same file? Or this should never happen?

Suggested change

	max_rows_to_read -= schema_reader->getNumRowsRead();
	max_rows_to_read -= rows_read;

[serxa]

It is really what we want? Consider a tab-separated file with some textual sentences that may contain commas. There might be a sentence with a lot of commas. This will correspond to a greater number of columns, which IMO is not the "best" fit. Are schema readers smart enough to figure this out under the hood?

[Avogar]

Are schema readers smart enough to figure this out under the hood?

I would say yes. In your example we will be able to infer the schema from such TSV file only if all first 25k rows will have the same number of commas (so we will have the same number of columns in each row for CSV format, otherwise CSV schema reader won't extract the schema), and this is unlikely to happpen.

[serxa]

Can we specify the exact file name that caused the failure?

[Avogar]

It will be done below:

https://github.com/Avogar/ClickHouse/blob/db2c15c0a6824b9fc7aa4a8216db44d7bcacf5bf/src/Formats/ReadSchemaUtils.cpp#L525-L531

[serxa]

Is it okay that we still need to do all the parsing even if columns are explicitly specified?

[Avogar]

But we still don't know the format, so we will anyway need to do some parsing.
Yes, with known structure we can do something better, for example try to use format readers instead of schema inference as we know the structure, and it will find the best format for sure. But I considered this use case with unknown format and known structure as really rare to be used by users and didn't want to implement separate logic for it