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Querying in ClickHouse CloudThe data in this system table is held locally on each node in ClickHouse Cloud. Obtaining a complete view of all data, therefore, requires the clusterAllReplicas function. See here for further details.

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Description

Contains information about all cached file schemas.

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Columns

  • storage (String) — Storage name: File, URL, S3 or HDFS.
  • source (String) — File source.
  • format (String) — Format name.
  • additional_format_info (String) — Additional information required to identify the schema. For example, format specific settings.
  • registration_time (DateTime) — Timestamp when schema was added in cache.
  • schema (Nullable(String)) — Cached schema.
  • number_of_rows (Nullable(UInt64)) — Number of rows in the file in given format. It’s used for caching trivial count() from data files and for caching number of rows from the metadata during schema inference.
  • schema_inference_mode (Nullable(String)) — Scheme inference mode.

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Example

Let’s say we have a file data.jsonl with this content: 
{"id" :  1, "age" :  25, "name" :  "Josh", "hobbies" :  ["football", "cooking", "music"]}
{"id" :  2, "age" :  19, "name" :  "Alan", "hobbies" :  ["tennis", "art"]}
{"id" :  3, "age" :  32, "name" :  "Lana", "hobbies" :  ["fitness", "reading", "shopping"]}
{"id" :  4, "age" :  47, "name" :  "Brayan", "hobbies" :  ["movies", "skydiving"]}
Place data.jsonl in the user_files_path directory. You can find this by looking in your ClickHouse configuration files. The default is:
<user_files_path>/var/lib/clickhouse/user_files/</user_files_path>
Open clickhouse-client and run the DESCRIBE query: 
DESCRIBE file('data.jsonl') SETTINGS input_format_try_infer_integers=0;

┌─name────┬─type────────────────────┬─default_type─┬─default_expression─┬─comment─┬─codec_expression─┬─ttl_expression─┐
│ id      │ Nullable(Float64)       │              │                    │         │                  │                │
│ age     │ Nullable(Float64)       │              │                    │         │                  │                │
│ name    │ Nullable(String)        │              │                    │         │                  │                │
│ hobbies │ Array(Nullable(String)) │              │                    │         │                  │                │
└─────────┴─────────────────────────┴──────────────┴────────────────────┴─────────┴──────────────────┴────────────────┘
Let’s see the content of the system.schema_inference_cache table: 
SELECT *
FROM system.schema_inference_cache
FORMAT Vertical

Row 1:
──────
storage:                File
source:                 /home/droscigno/user_files/data.jsonl
format:                 JSONEachRow
additional_format_info: schema_inference_hints=, max_rows_to_read_for_schema_inference=25000, schema_inference_make_columns_nullable=true, try_infer_integers=false, try_infer_dates=true, try_infer_datetimes=true, try_infer_numbers_from_strings=true, read_bools_as_numbers=true, try_infer_objects=false
registration_time:      2022-12-29 17:49:52
schema:                 id Nullable(Float64), age Nullable(Float64), name Nullable(String), hobbies Array(Nullable(String))

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See Also