liamphmurphy opened a new issue, #15338:
URL: https://github.com/apache/datafusion/issues/15338
### Describe the bug
This bug for me originated when encountering schema evolutions on Delta
tables using the `delta-rs` library. Whenever a schema evolution occurred on my
table that contains a field with a list of structs, Datafusion is returning
this error:
```
This feature is not implemented: Unsupported CAST from Struct([Field { name:
"properties", data_type: Struct([Field { name: "someNewField", data_type: Utf8,
nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }, Field {
name: "fields", data_type: List(Field { name: "item", data_type: Struct([Field
{ name: "messageId", data_type: Utf8, nullable: true, dict_id: 0,
dict_is_ordered: false, metadata: {} }]), nullable: true, dict_id: 0,
dict_is_ordered: false, metadata: {} }), nullable: true, dict_id: 0,
dict_is_ordered: false, metadata: {} }]), nullable: true, dict_id: 0,
dict_is_ordered: false, metadata: {} }]) to Struct([Field { name: "properties",
data_type: Struct([Field { name: "fields", data_type: List(Field { name:
"element", data_type: Struct([Field { name: "messageId", data_type: Utf8,
nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }]), nullable:
true, dict_id: 0, dict_is_ordered: false, metadata: {} }), nullable: true,
dict_id: 0, dict_is_ordere
d: false, metadata: {} }, Field { name: "someNewField", data_type: Utf8,
nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }]), nullable:
true, dict_id: 0, dict_is_ordered: false, metadata: {} }])
```
### To Reproduce
Below is the python code using delta-rs (which is currently on Datafusion
46) that shows this error
```python
import pyarrow as pa
from deltalake import write_deltalake
# Define the path for the Delta table
delta_table_path = "./datafusion-repro-test-table"
# Define the data for the first write
data_first_write = [
{
"uid": "ws_2",
"event": {
"properties": {
"fields": [
{
"messageId": "veniam sed et elit adipisicing"
}
],
},
}
}
]
schema = pa.schema([
pa.field("uid", pa.string()),
pa.field("event", pa.struct([
pa.field("properties", pa.struct([
pa.field("fields", pa.list_(pa.struct([
pa.field("messageId", pa.string()),
]))),
])),
])),
])
print(schema)
first_write = pa.Table.from_pylist(data_first_write, schema=schema)
# Write data to Delta table for the first write
write_deltalake(delta_table_path, first_write, mode="append", engine="rust",
schema_mode="merge")
#### NOW FOR THE SECOND WRITE THAT BREAKS ####
data_second_write = [
{
"uid": "ws_2",
"event": {
"properties": {
"someNewField": "test-value", # New field
"fields": [
{
"messageId": "veniam sed et elit adipisicing"
}
],
},
}
}
]
second_schema = pa.schema([
pa.field("uid", pa.string()),
pa.field("event", pa.struct([
pa.field("properties", pa.struct([
pa.field("someNewField", pa.string()), # New field
pa.field("fields", pa.list_(pa.struct([
pa.field("messageId", pa.string()),
]))),
])),
])),
])
second_write = pa.Table.from_pylist(data_second_write, schema=second_schema)
# Write data to Delta table for the second write
write_deltalake(delta_table_path, second_write, mode="append",
engine="rust", schema_mode="merge")
```
### Expected behavior
Datafusion would support casting a schema when said schema contains a list
of structs.
### Additional context
Originating bug report in delta-rs:
https://github.com/delta-io/delta-rs/issues/3339
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