INFER_SCHEMA

自动检测文件元信息 schema 并获取列定义。

infer_schema 当前支持以下文件格式：

• Parquet - 原生支持 schema 推导
• CSV - 支持自定义分隔符和表头检测
• NDJSON - 以换行符分隔的 JSON 文件

压缩支持：所有格式还支持扩展名为 .zip、.xz、.zst 的压缩文件。

语法

INFER_SCHEMA(
  LOCATION => '{ internalStage | externalStage }'
  [ PATTERN => '<regex_pattern>']
  [ FILE_FORMAT => '<format_name>' ]
  [ MAX_RECORDS_PRE_FILE => <number> ]
  [ MAX_FILE_COUNT => <number> ]
)

参数

示例

Parquet 文件

-- Create stage and export data
CREATE STAGE test_parquet;
COPY INTO @test_parquet FROM (SELECT number FROM numbers(10)) FILE_FORMAT = (TYPE = 'PARQUET');

-- Infer schema from parquet files using pattern
SELECT * FROM INFER_SCHEMA(
    location => '@test_parquet',
    pattern => '.*[.]parquet'
);

结果：

+-------------+-----------------+----------+----------+----------+
| column_name | type            | nullable | filenames| order_id |
+-------------+-----------------+----------+----------+----------+
| number      | BIGINT UNSIGNED |    false | data_... |        0 |
+-------------+-----------------+----------+----------+----------+

CSV 文件

-- Create stage and export CSV data
CREATE STAGE test_csv;
COPY INTO @test_csv FROM (SELECT number FROM numbers(10)) FILE_FORMAT = (TYPE = 'CSV');

-- Create a CSV file format
CREATE FILE FORMAT csv_format TYPE = 'CSV';

-- Infer schema using pattern and file format
SELECT * FROM INFER_SCHEMA(
    location => '@test_csv',
    pattern => '.*[.]csv',
    file_format => 'csv_format'
);

结果：

+-------------+---------+----------+----------+----------+
| column_name | type    | nullable | filenames| order_id |
+-------------+---------+----------+----------+----------+
| column_1    | BIGINT  |     true | data_... |        0 |
+-------------+---------+----------+----------+----------+

对于带表头的 CSV 文件：

-- Create CSV file format with header support
CREATE FILE FORMAT csv_headers_format
TYPE = 'CSV'
field_delimiter = ','
skip_header = 1;

-- Export data with headers
CREATE STAGE test_csv_headers;
COPY INTO @test_csv_headers FROM (
  SELECT number as user_id, 'user_' || number::string as user_name
  FROM numbers(5)
) FILE_FORMAT = (TYPE = 'CSV', output_header = true);

-- Infer schema with headers
SELECT * FROM INFER_SCHEMA(
    location => '@test_csv_headers',
    file_format => 'csv_headers_format'
);

限制记录数以加快推导速度：

-- Sample only first 5 records for schema inference
SELECT * FROM INFER_SCHEMA(
    location => '@test_csv',
    pattern => '.*[.]csv',
    file_format => 'csv_format',
    max_records_pre_file => 5
);

NDJSON 文件

-- Create stage and export NDJSON data
CREATE STAGE test_ndjson;
COPY INTO @test_ndjson FROM (SELECT number FROM numbers(10)) FILE_FORMAT = (TYPE = 'NDJSON');

-- Infer schema using pattern and NDJSON format
SELECT * FROM INFER_SCHEMA(
    location => '@test_ndjson',
    pattern => '.*[.]ndjson',
    file_format => 'NDJSON'
);

结果：

+-------------+---------+----------+----------+----------+
| column_name | type    | nullable | filenames| order_id |
+-------------+---------+----------+----------+----------+
| number      | BIGINT  |     true | data_... |        0 |
+-------------+---------+----------+----------+----------+

限制记录数以加快推导速度：

-- Sample only first 5 records for schema inference
SELECT * FROM INFER_SCHEMA(
    location => '@test_ndjson',
    pattern => '.*[.]ndjson',
    file_format => 'NDJSON',
    max_records_pre_file => 5
);

使用多个文件进行 Schema 合并

当文件具有不同的 schema 时，infer_schema 会智能地将它们合并：

-- Suppose you have multiple CSV files with different schemas:
-- file1.csv: id(INT), name(VARCHAR)
-- file2.csv: id(INT), name(VARCHAR), age(INT)
-- file3.csv: id(FLOAT), name(VARCHAR), age(INT)

SELECT * FROM INFER_SCHEMA(
    location => '@my_stage/',
    pattern => '.*[.]csv',
    file_format => 'csv_format'
);

结果会显示合并后的 schema：

+-------------+---------+----------+-----------+----------+
| column_name | type    | nullable | filenames | order_id |
+-------------+---------+----------+-----------+----------+
| id          | VARCHAR |     true | file1,... |        0 |  -- INT+FLOAT→VARCHAR
| name        | VARCHAR |     true | file1,... |        1 |
| age         | BIGINT  |     true | file1,... |        2 |  -- Missing in file1→nullable
+-------------+---------+----------+-----------+----------+

模式匹配和文件数量限制

使用模式匹配从多个文件中推导 schema：

-- Infer schema from all CSV files in the directory
SELECT * FROM INFER_SCHEMA(
    location => '@my_stage/',
    pattern => '.*[.]csv'
);

限制处理的文件数量以提升性能：

-- Process only the first 5 matching files
SELECT * FROM INFER_SCHEMA(
    location => '@my_stage/',
    pattern => '.*[.]csv',
    max_file_count => 5
);

压缩文件

infer_schema 会自动处理压缩文件：

-- Works with compressed CSV files
SELECT * FROM INFER_SCHEMA(location => '@my_stage/data.csv.zip');

-- Works with compressed NDJSON files
SELECT * FROM INFER_SCHEMA(
    location => '@my_stage/data.ndjson.xz',
    file_format => 'NDJSON',
    max_records_pre_file => 50
);

根据推导出的 Schema 创建表

infer_schema 函数会显示 schema，但不会创建表。要根据推导出的 schema 创建表，请执行以下操作：

-- Create table structure from file schema
CREATE TABLE my_table AS
SELECT * FROM @my_stage/ (pattern=>'.*[.]parquet')
LIMIT 0;

-- Verify the table structure
DESC my_table;