milvus/docs/design_docs/20230405-default_value.md

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# MEP: Default Value
Current state: Under Discussion
ISSUE: [[Feature]: Support Default Value #23337](https://github.com/milvus-io/milvus/issues/23337)
Keywords: Default, Insert, Upsert
Released: v2.3.1
## Summary
Support Default Value when input data.
## Motivation
For now, Milvus don't support Default function. If the user pass in the same data under a certain field schema, the data can only be passed in repeatedly, which is not so flexible and user-friendly。
We need a way to support Default function, which is more efficient.
## Public Interfaces
Add new field `default_value` in `FieldSchema`
```proto
message FieldSchema {
...
ScalarField default_value = 11; // default_value only support scalars for now
}
```
## Design Details
1. Add the default_value in the field schema as an optional field.
```proto
message FieldSchema {
...
ScalarField default_value = 11; // default_value only support scalars for now
}
```
2. Will use the default_value if no data pass(the field get nil when insert and upsert).
```proto
message FieldData {
...
oneof field {
ScalarField scalars = 3;
VectorField vectors = 4;
}
}
```
```python
# create collection
nb = 3000
fields = [
FieldSchema(name="int64", dtype=DataType.INT64, is_primary=True),
# restrict at most one value to be passed in as the default value
FieldSchema(name="float", dtype=DataType.FLOAT, default_value=1.0)
]
schema = CollectionSchema(
fields=fields, description="collection")
collection = Collection(name="hello_milvus", schema=default_schema)
# insert data
collection.insert(
[
[i for i in range(nb)],
# will use the default_value
[],
]
)
```
## Compatibility, Deprecation, and Migration Plan
| Test Cases | ExpectedBehavior |
|:-----------------------------------------: | :---------------------------------------: |
| schema built in 2.2.x | can be used normally in the new version |
## Test Plan
### Unit Tests
- Test for using default value in proxy
### E2E Tests
| Test Cases | Expected Behavior |
| :------------------------------------------: | :--------------------------------------: |
| set illegal default value | report error |
| set legal default value | use default value as fields data |
| schema built in 2.2.x | can be used normally in the new version |
| don't set default value | the same |
## Rejected Alternatives
Default value is set by column, and the writing method of [1,2,3, {default}, {default}, 4, 5] is not supported.
## References
None