docs/source/user-guide/cli/functions.md

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# CLI Specific Functions

`datafusion-cli` comes with build-in functions that are not included in the
DataFusion SQL engine by default. These functions are:

## `parquet_metadata`

The `parquet_metadata` table function can be used to inspect detailed metadata
about a parquet file such as statistics, sizes, and other information. This can
be helpful to understand how parquet files are structured.

For example, to see information about the `"WatchID"` column in the
`hits.parquet` file, you can use:

```sql
SELECT path_in_schema, row_group_id, row_group_num_rows, stats_min, stats_max, total_compressed_size
FROM parquet_metadata('hits.parquet')
WHERE path_in_schema = '"WatchID"'
LIMIT 3;

+----------------+--------------+--------------------+---------------------+---------------------+-----------------------+
| path_in_schema | row_group_id | row_group_num_rows | stats_min           | stats_max           | total_compressed_size |
+----------------+--------------+--------------------+---------------------+---------------------+-----------------------+
| "WatchID"      | 0            | 450560             | 4611687214012840539 | 9223369186199968220 | 3883759               |
| "WatchID"      | 1            | 612174             | 4611689135232456464 | 9223371478009085789 | 5176803               |
| "WatchID"      | 2            | 344064             | 4611692774829951781 | 9223363791697310021 | 3031680               |
+----------------+--------------+--------------------+---------------------+---------------------+-----------------------+
3 rows in set. Query took 0.053 seconds.
```

The returned table has the following columns for each row for each column chunk
in the file. Please refer to the [Parquet Documentation] for more information in
the meaning of these fields.

[parquet documentation]: https://parquet.apache.org/

| column_name             | data_type | Description                                                                                         |
| ----------------------- | --------- | --------------------------------------------------------------------------------------------------- |
| filename                | Utf8      | Name of the file                                                                                    |
| row_group_id            | Int64     | Row group index the column chunk belongs to                                                         |
| row_group_num_rows      | Int64     | Count of rows stored in the row group                                                               |
| row_group_num_columns   | Int64     | Total number of columns in the row group (same for all row groups)                                  |
| row_group_bytes         | Int64     | Number of bytes used to store the row group (not including metadata)                                |
| column_id               | Int64     | ID of the column                                                                                    |
| file_offset             | Int64     | Offset within the file that this column chunk's data begins                                         |
| num_values              | Int64     | Total number of values in this column chunk                                                         |
| path_in_schema          | Utf8      | "Path" (column name) of the column chunk in the schema                                              |
| type                    | Utf8      | Parquet data type of the column chunk                                                               |
| stats_min               | Utf8      | The minimum value for this column chunk, if stored in the statistics, cast to a string              |
| stats_max               | Utf8      | The maximum value for this column chunk, if stored in the statistics, cast to a string              |
| stats_null_count        | Int64     | Number of null values in this column chunk, if stored in the statistics                             |
| stats_distinct_count    | Int64     | Number of distinct values in this column chunk, if stored in the statistics                         |
| stats_min_value         | Utf8      | Same as `stats_min`                                                                                 |
| stats_max_value         | Utf8      | Same as `stats_max`                                                                                 |
| compression             | Utf8      | Block level compression (e.g. `SNAPPY`) used for this column chunk                                  |
| encodings               | Utf8      | All block level encodings (e.g. `[PLAIN_DICTIONARY, PLAIN, RLE]`) used for this column chunk        |
| index_page_offset       | Int64     | Offset in the file of the [`page index`], if any                                                    |
| dictionary_page_offset  | Int64     | Offset in the file of the dictionary page, if any                                                   |
| data_page_offset        | Int64     | Offset in the file of the first data page, if any                                                   |
| total_compressed_size   | Int64     | Number of bytes the column chunk's data after encoding and compression (what is stored in the file) |
| total_uncompressed_size | Int64     | Number of bytes the column chunk's data after encoding                                              |

[`page index`]: https://github.com/apache/parquet-format/blob/master/PageIndex.md

## `metadata_cache`

The `metadata_cache` function shows information about the default File Metadata Cache that is used by the
[`ListingTable`] implementation in DataFusion. This cache is used to speed up
reading metadata from files when scanning directories with many files.

For example, after creating a table with the [CREATE EXTERNAL TABLE](../sql/ddl.md#create-external-table)
command:

```sql
> create external table hits
  stored as parquet
  location 's3://clickhouse-public-datasets/hits_compatible/athena_partitioned/';
```

You can inspect the metadata cache by querying the `metadata_cache` function:

```sql
> select * from metadata_cache();
+----------------------------------------------------+---------------------+-----------------+---------------------------------------+---------+---------------------+------+------------------+
| path                                               | file_modified       | file_size_bytes | e_tag                                 | version | metadata_size_bytes | hits | extra            |
+----------------------------------------------------+---------------------+-----------------+---------------------------------------+---------+---------------------+------+------------------+
| hits_compatible/athena_partitioned/hits_61.parquet | 2022-07-03T15:40:34 | 117270944       | "5db11cad1ca0d80d748fc92c914b010a-6"  | NULL    | 212949              | 0    | page_index=false |
| hits_compatible/athena_partitioned/hits_32.parquet | 2022-07-03T15:37:17 | 94506004        | "2f7db49a9fe242179590b615b94a39d2-5"  | NULL    | 278157              | 0    | page_index=false |
| hits_compatible/athena_partitioned/hits_40.parquet | 2022-07-03T15:38:07 | 142508647       | "9e5852b45a469d5a05bf270a286eab8a-8"  | NULL    | 212917              | 0    | page_index=false |
| hits_compatible/athena_partitioned/hits_93.parquet | 2022-07-03T15:44:07 | 127987774       | "751100bf0dac7d489b9836abf3108b99-7"  | NULL    | 278318              | 0    | page_index=false |
| .                                                                                                                                                                                            |
+----------------------------------------------------+---------------------+-----------------+---------------------------------------+---------+---------------------+------+------------------+
```

Since `metadata_cache` is a normal table function, you can use it in most places you can use
a table reference.

For example, to get the total size consumed by the cached entries:

```sql
> select sum(metadata_size_bytes) from metadata_cache();
+-------------------------------------------+
| sum(metadata_cache().metadata_size_bytes) |
+-------------------------------------------+
| 22972345                                  |
+-------------------------------------------+
```

The columns of the returned table are:

| column_name         | data_type | Description                                                                               |
| ------------------- | --------- | ----------------------------------------------------------------------------------------- |
| path                | Utf8      | File path relative to the object store / filesystem root                                  |
| file_modified       | Timestamp | Last modified time of the file                                                            |
| file_size_bytes     | UInt64    | Size of the file in bytes                                                                 |
| e_tag               | Utf8      | [Entity Tag] (ETag) of the file if available                                              |
| version             | Utf8      | Version of the file if available (for object stores that support versioning)              |
| metadata_size_bytes | UInt64    | Size of the cached metadata in memory (not its thrift encoded form)                       |
| hits                | UInt64    | Number of times the cached metadata has been accessed                                     |
| extra               | Utf8      | Extra information about the cached metadata (e.g., if page index information is included) |

[`listingtable`]: https://docs.rs/datafusion/latest/datafusion/datasource/listing/struct.ListingTable.html
[entity tag]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag