ClickHouse/docs/en/operations/system-tables/schema_inference_cache.md

slug
/en/operations/system-tables/schema_inference_cache

Schema inference cache

Contains information about all cached file schemas.

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 (String) - Cached schema.

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"]}

:::tip 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))

See also

• Automatic schema inference from input data