This commit exposes available SQL functions in Drill and also detects UDFs that
have been dynamically loaded into Drill.
An example is shown below for 2 UDFs dynamically loaded into the cluster, along
side the existing built-in functions that come with Drill.
```
0: jdbc:drill:schema=sys> select source, count(*) as functionCount from
sys.functions group by source;
+-----------------------------------------+----------------+
| source | functionCount |
+-----------------------------------------+----------------+
| built-in | 2704 |
| simple-drill-function-1.0-SNAPSHOT.jar | 12 |
| drill-url-tools-1.0.jar | 1 |
+-----------------------------------------+----------------+
1 row selected (0.211 seconds)
```
The system table exposes information as shown. Since UDFs are lazily
initialized (i.e. only when a SQL query needs it), the `returnType` is not
available and listed as `n/a`.
Once the UDF is initialized, the `returnType` is also available.
The `random(FLOAT8-REQUIRED,FLOAT8-REQUIRED)` function is an example of a
lazily loaded UDF (see `returnType`).
The `url_parse(VARCHAR-REQUIRED)` function is an example of an initialized UDF
(see `returnType`).
Rest are built-in functions that meet the query's filter criteria.
```
0: jdbc:drill:schema=sys>select * from sys.functions where name like 'random'
or `name` like '%url%';
+-------------+----------------------------------+-------------+-----------------------------------------+
| name | signature | returnType |
source |
+-------------+----------------------------------+-------------+-----------------------------------------+
| parse_url | VARCHAR-REQUIRED | LATE | built-in
|
| random | | FLOAT8 | built-in
|
| random | FLOAT8-REQUIRED,FLOAT8-REQUIRED | n/a |
simple-drill-function-1.0-SNAPSHOT.jar |
| url_decode | VARCHAR-REQUIRED | VARCHAR | built-in
|
| url_encode | VARCHAR-REQUIRED | VARCHAR | built-in
|
| url_parse | VARCHAR-REQUIRED | LATE |
drill-url-tools-1.0.jar |
+-------------+----------------------------------+-------------+-----------------------------------------+
6 rows selected (0.221 seconds)
```
[ Full content available at: https://github.com/apache/drill/pull/1483 ]
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