GitHub user web3creator closed a discussion: Questions about catalog design
```rust
/// Represent a list of named catalogs
pub trait CatalogList: Sync + Send {
/// Returns the catalog list as [`Any`]
/// so that it can be downcast to a specific implementation.
fn as_any(&self) -> &dyn Any;
/// Adds a new catalog to this catalog list
/// If a catalog of the same name existed before, it is replaced in the
list and returned.
fn register_catalog(
&self,
name: String,
catalog: Arc<dyn CatalogProvider>,
) -> Option<Arc<dyn CatalogProvider>>;
/// Retrieves the list of available catalog names
fn catalog_names(&self) -> Vec<String>;
/// Retrieves a specific catalog by name, provided it exists.
fn catalog(&self, name: &str) -> Option<Arc<dyn CatalogProvider>>;
}
/// Represents a catalog, comprising a number of named schemas.
pub trait CatalogProvider: Sync + Send {
/// Returns the catalog provider as [`Any`]
/// so that it can be downcast to a specific implementation.
fn as_any(&self) -> &dyn Any;
/// Retrieves the list of available schema names in this catalog.
fn schema_names(&self) -> Vec<String>;
/// Retrieves a specific schema from the catalog by name, provided it
exists.
fn schema(&self, name: &str) -> Option<Arc<dyn SchemaProvider>>;
/// Adds a new schema to this catalog.
///
/// If a schema of the same name existed before, it is replaced in
/// the catalog and returned.
///
/// By default returns a "Not Implemented" error
fn register_schema(
&self,
name: &str,
schema: Arc<dyn SchemaProvider>,
) -> Result<Option<Arc<dyn SchemaProvider>>> {
// use variables to avoid unused variable warnings
let _ = name;
let _ = schema;
not_impl_err!("Registering new schemas is not supported")
}
/// Removes a schema from this catalog. Implementations of this method
should return
/// errors if the schema exists but cannot be dropped. For example, in
DataFusion's
/// default in-memory catalog, [`MemoryCatalogProvider`], a non-empty schema
/// will only be successfully dropped when `cascade` is true.
/// This is equivalent to how DROP SCHEMA works in PostgreSQL.
///
/// Implementations of this method should return None if schema with `name`
/// does not exist.
///
/// By default returns a "Not Implemented" error
fn deregister_schema(
&self,
_name: &str,
_cascade: bool,
) -> Result<Option<Arc<dyn SchemaProvider>>> {
not_impl_err!("Deregistering new schemas is not supported")
}
}
/// Represents a schema, comprising a number of named tables.
#[async_trait]
pub trait SchemaProvider: Sync + Send {
/// Returns the schema provider as [`Any`](std::any::Any)
/// so that it can be downcast to a specific implementation.
fn as_any(&self) -> &dyn Any;
/// Retrieves the list of available table names in this schema.
fn table_names(&self) -> Vec<String>;
/// Retrieves a specific table from the schema by name, provided it exists.
async fn table(&self, name: &str) -> Option<Arc<dyn TableProvider>>;
/// If supported by the implementation, adds a new table to this schema.
/// If a table of the same name existed before, it returns "Table already
exists" error.
#[allow(unused_variables)]
fn register_table(
&self,
name: String,
table: Arc<dyn TableProvider>,
) -> Result<Option<Arc<dyn TableProvider>>> {
exec_err!("schema provider does not support registering tables")
}
/// If supported by the implementation, removes an existing table from this
schema and returns it.
/// If no table of that name exists, returns Ok(None).
#[allow(unused_variables)]
fn deregister_table(&self, name: &str) -> Result<Option<Arc<dyn
TableProvider>>> {
exec_err!("schema provider does not support deregistering tables")
}
/// If supported by the implementation, checks the table exist in the
schema provider or not.
/// If no matched table in the schema provider, return false.
/// Otherwise, return true.
fn table_exist(&self, name: &str) -> bool;
}
```
1、Why CatalogList doesn’t provide delete function
2、Why is only SchemaProvider's table function asynchronous and
SchemaProvider.table_names 、CatalogList.catalog_names not allowed to get data
from remote?
3、Why is only the table function of SchemaProvider asynchronous, and the rest
of the functions are not asynchronous? Can the catalog and schema be obtained
remotely?
GitHub link: https://github.com/apache/datafusion/discussions/8887
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