Convert `SqlToRel::statement_to_pan` to take ownership rather than references.

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
While working on some other code, I noticed that SqlToRel::statement_to_plan takes a reference to the parse tree -- this results in having to clone() many values (identifiers, etc) while converting a parsed SQL query to a LogicalPlan

The only callsite I could find throws away the parsed query immediately; Doing this would likely save lots of cloning internal to SqlToRel

Describe the solution you'd like

If someone wanted a fun project, they could convert SqlToRel::statement_to_pan to take ownership rather than references.
https://github.com/apache/arrow-datafusion/blob/0052667afae33ba9e549256d0d5d47e2f45e6ffb/datafusion/src/sql/planner.rs#L121

Specifically, change:

    pub fn statement_to_plan(&self, statement: &DFStatement) -> Result<LogicalPlan> {

To

    pub fn statement_to_plan(&self, statement: DFStatement) -> Result<LogicalPlan> {

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
This would be a software engineering exercise -- it wouldn't really require much in the way of knowledge of the guts of DataFusion

[xudong963]

@alamb good catch!

Here I give a specific example to omit clone

    pub fn external_table_to_plan(
        &self,
        statement: &CreateExternalTable,
    ) -> Result<LogicalPlan> {
        let CreateExternalTable {
            name,
            columns,
            file_type,
            has_header,
            location,
        } = statement;

        ...

        Ok(LogicalPlan::CreateExternalTable(PlanCreateExternalTable {
            schema: schema.to_dfschema_ref()?,
            name: name.clone(),
            location: location.clone(),
            file_type: *file_type,
            has_header: *has_header,
        }))
    }

to

    pub fn external_table_to_plan(
        &self,
        statement: CreateExternalTable,
    ) -> Result<LogicalPlan> {
        let CreateExternalTable {
            name,
            columns,
            file_type,
            has_header,
            location,
        } = statement;

       ...

        Ok(LogicalPlan::CreateExternalTable(PlanCreateExternalTable {
            schema: schema.to_dfschema_ref()?,
            name,
            location,
            file_type: *file_type,
            has_header: *has_header,
        }))
    }

BTW, I think it's better to add a good first issue

[doki23]

I wanna take this issue, can I? @alamb

[alamb]

Please do @doki23 !