xinlifoobar commented on issue #8698:
URL: https://github.com/apache/datafusion/issues/8698#issuecomment-2182112425
Hi @alamb
I am looking into this issue and have a proposal for the following PR. The
support for substrait statistics is quite basic compared to that of DataFusion.
For example, Substrait only supports table-level statistics without precision
and does not support column-level statistics. Here is the current Substrait
`Stats` message:
```protobuf
message Stats {
double row_count = 1;
double record_size = 2;
substrait.extensions.AdvancedExtension advanced_extension = 10;
}
```
In contrast, DataFusion's statistics are more detailed:
```rust
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Statistics {
/// The number of table rows.
pub num_rows: Precision,
/// Total bytes of the table rows.
pub total_byte_size: Precision,
/// Statistics on a column level. It contains a [`ColumnStatistics`] for
/// each field in the schema of the table to which the [`Statistics`]
refer.
pub column_statistics: Vec<ColumnStatistics>,
}
```
To enhance the support of statistics in Substrait, I propose adding an
`AdvancedExtension` to the `Stats` message. This extension is defined as
follows:
```protobuf
// A generic object that can be used to embed additional extension
information
// into the serialized Substrait plan.
message AdvancedExtension {
// An optimization is helpful information that doesn't influence
semantics. May
// be ignored by a consumer.
google.protobuf.Any optimization = 1;
// An enhancement alters semantics. Cannot be ignored by a consumer.
google.protobuf.Any enhancement = 2;
}
```
I would add a new type in the `datafusion-proto` to define the new message
with all the necessary fields. The new message is defined as:
```protobuf
message DatafusionStatsExtension {
// The version of the extension.
int32 version = 1;
// The statistics.
datafusion_common.Statistics statistics = 6;
}
```
On the producer side, it will try to encode the `DatafusionStatsExtension`
message and attach it to the `AdvancedExtension` as an optimization.
On the consumer side, it will try to parse the `AdvancedExtension` message,
extract the `DatafusionStatsExtension` message, and update the `Stats` message
accordingly. If the `DatafusionStatsExtension` message is not present, it will
treat the `num_rows` and `total_byte_size` as table-level statistics with
`Precision::EXACT`.
What do you think about this idea? I'd like to hear your thoughts on this.
Thanks.
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