commit caa895047fbd41e99aebaf5847d3de6ee6b81e21
author Andrew Lamb <andrew@nerdnetworks.org> Thu Apr 01 07:24:57 2021 -0400
committer Andrew Lamb <andrew@nerdnetworks.org> Thu Apr 01 07:24:57 2021 -0400
tree 43ea36ebffcac2faf2829bdeaa3a478e85015281
parent cc3bcf8f83afc868836cbdb7ccc946a0c83ebf40

ARROW-12107: [Rust][DataFusion] Support `SELECT * from information_schema.columns`

Builds on the code in #9818

# Rationale

Provide schema metadata access (so a user can see what columns exist and their type).

See the doc for background: https://docs.google.com/document/d/12cpZUSNPqVH9Z0BBx6O8REu7TFqL-NPPAYCUPpDls1k/edit#

I plan to add support for `SHOW COLUMNS` possibly as a follow on PR (though I have found out that `SHOW COLUMNS` and `SHOW TABLES` are not supported by either MySQL or by Postgres 🤔 )

# Changes
I chose to add the first 15 columns from `information_schema.columns` You can see the full list in Postgres [here](https://www.postgresql.org/docs/9.5/infoschema-columns.html) and SQL Server [here](https://docs.microsoft.com/en-us/sql/relational-databases/system-information-schema-views/columns-transact-sql?view=sql-server-ver15).

There are a bunch more columns that say "Applies to features not available in PostgreSQL" and that don't apply to DataFusion either-- since my usecase is to get the basic schema information out I chose not to add a bunch of columns that are always null at this time.

I feel the use of column builders here is somewhat awkward (as it requires many calls to `unwrap`). I am thinking of a follow on PR to refactor this code to use `Vec<String>` and `Vec<u64>` and then create `StringArray` and `UInt64Array` directly from them but for now I just want the functionality

# Example use

Setup:
```
echo "1,Foo,44.9" > /tmp/table.csv
echo "2,Bar,22.1" >> /tmp/table.csv
cargo run --bin datafusion-cli
```

Then run :

```
> CREATE EXTERNAL TABLE t(a int, b varchar, c float)
STORED AS CSV
LOCATION '/tmp/table.csv';
0 rows in set. Query took 0 seconds.

>   select table_name, column_name, ordinal_position, is_nullable, data_type from information_schema.columns;
+------------+-------------+------------------+-------------+-----------+
| table_name | column_name | ordinal_position | is_nullable | data_type |
+------------+-------------+------------------+-------------+-----------+
| t          | a           | 0                | NO          | Int32     |
| t          | b           | 1                | NO          | Utf8      |
| t          | c           | 2                | NO          | Float32   |
+------------+-------------+------------------+-------------+-----------+
3 row in set. Query took 0 seconds.
```

Closes #9840 from alamb/alamn/information_schema_columns

Authored-by: Andrew Lamb <andrew@nerdnetworks.org>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>