Charles Givre created DRILL-8204:
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Summary: Allow Provided Schema for HTTP Plugin in JSON Mode
Key: DRILL-8204
URL: https://issues.apache.org/jira/browse/DRILL-8204
Project: Apache Drill
Issue Type: Improvement
Components: Storage - Other
Affects Versions: 1.20.0
Reporter: Charles Givre
Assignee: Charles Givre
Fix For: 2.0.0
One of the challenges of querying APIs is inconsistent data. Drill allows you
to provide a schema for individual endpoints. You can do this in one of two
ways: either by
providing a serialized TupleMetadata of the desired schema. This is an advanced
functionality and should only be used by advanced Drill users.
The schema provisioning currently supports complex types of Arrays and Maps at
any nesting level.
### Example Schema Provisioning:
```json
"jsonOptions": {
"providedSchema": [
{
"fieldName": "int_field",
"fieldType": "bigint"
}, {
"fieldName": "jsonField",
"fieldType": "varchar",
"properties": {
"drill.json-mode":"json"
}
},{
// Array field
"fieldName": "stringField",
"fieldType": "varchar",
"isArray": true
}, {
// Map field
"fieldName": "mapField",
"fieldType": "map",
"fields": [
{
"fieldName": "nestedField",
"fieldType": "int"
},{
"fieldName": "nestedField2",
"fieldType": "varchar"
}
]
}
]
}
```
### Example Provisioning the Schema with a JSON String
```json
"jsonOptions": {
"jsonSchema":
"\{\"type\":\"tuple_schema\",\"columns\":[{\"name\":\"outer_map\",\"type\":\"STRUCT<`int_field`
BIGINT, `int_array` ARRAY<BIGINT>>\",\"mode\":\"REQUIRED\"}]}"
}
```
You can print out a JSON string of a schema with the Java code below.
```java
TupleMetadata schema = new SchemaBuilder()
.addNullable("a", MinorType.BIGINT)
.addNullable("m", MinorType.VARCHAR)
.build();
ColumnMetadata m = schema.metadata("m");
m.setProperty(JsonLoader.JSON_MODE, JsonLoader.JSON_LITERAL_MODE);
System.out.println(schema.jsonString());
```
This will generate something like the JSON string below:
```json
{
"type":"tuple_schema",
"columns":[
{"name":"a","type":"BIGINT","mode":"OPTIONAL"},
{"name":"m","type":"VARCHAR","mode":"OPTIONAL","properties":\{"drill.json-mode":"json"}
}
]
}
```
## Dealing With Inconsistent Schemas
One of the major challenges of interacting with JSON data is when the schema is
inconsistent. Drill has a `UNION` data type which is marked as experimental. At
the time of
writing, the HTTP plugin does not support the `UNION`, however supplying a
schema can solve a lot of those issues.
### Json Mode
Drill offers the option of reading all JSON values as a string. While this can
complicate downstream analytics, it can also be a more memory-efficient way of
reading data with
inconsistent schema. Unfortunately, at the time of writing, JSON-mode is only
available with a provided schema. However, future work will allow this mode to
be enabled for
any JSON data.
#### Enabling JSON Mode:
You can enable JSON mode simply by adding the `drill.json-mode` property with a
value of `json` to a field, as shown below:
```json
{
"fieldName": "jsonField",
"fieldType": "varchar",
"properties": {
"drill.json-mode": "json"
}
}
```
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