francis-du commented on a change in pull request #923:
URL: https://github.com/apache/arrow-datafusion/pull/923#discussion_r693898063
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File path: datafusion/src/dataframe.rs
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@@ -223,6 +223,21 @@ pub trait DataFrame: Send + Sync {
/// ```
async fn collect(&self) -> Result<Vec<RecordBatch>>;
+ /// Print results.
+ ///
+ /// ```
+ /// # use datafusion::prelude::*;
+ /// # use datafusion::error::Result;
+ /// # #[tokio::main]
+ /// # async fn main() -> Result<()> {
+ /// let mut ctx = ExecutionContext::new();
+ /// let df = ctx.read_csv("tests/example.csv", CsvReadOptions::new())?;
+ /// df.show().await?;
+ /// # Ok(())
+ /// # }
+ /// ```
+ async fn show(&self) -> Result<()>;
Review comment:
I think it can be inferred from the logical plan whether there is a
limit number. If there is no limit, set it to the default value of 20. If there
is, then the default limit value is not set.
This is an implementation:
```rust
async fn show(&self) -> Result<()> {
let mut num = 20;
if let LogicalPlan::Limit { n, input: _ } = self.to_logical_plan() {
num = n;
}
let results = self.limit(num)?.collect().await?;
Ok(pretty::print_batches(&results)?)
}
```
The user can call the limit function to pass in the number of lines, if
limit is not used, only the default 20 lines will be printed.
eg:
```rust
df.limit(10)?.show().await?; // limit 10
df.show().await?; // default limit 20
```
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