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Posted to github@datafusion.apache.org by "Weijun-H (via GitHub)" <gi...@apache.org> on 2024/04/27 01:36:25 UTC
[PR] fix: Correct null_count in describe() [datafusion]
Weijun-H opened a new pull request, #10260:
URL: https://github.com/apache/datafusion/pull/10260
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Closes #10231
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Re: [PR] fix: Correct null_count in describe() [datafusion]
Posted by "alamb (via GitHub)" <gi...@apache.org>.
alamb commented on code in PR #10260:
URL: https://github.com/apache/datafusion/pull/10260#discussion_r1583694523
##########
datafusion/expr/src/expr_fn.rs:
##########
@@ -215,6 +215,18 @@ pub fn count(expr: Expr) -> Expr {
))
}
+/// Create an expression to represent the COUNT(IS NULL x) aggregate function
Review Comment:
I found this confusing as first because it isn't `COUNT(x IS NULL)` -- I think a more accurate description is `COUNT(x) FILTER (WHERE x IS NULL)` or something like that
```suggestion
/// Create an aggregate that returns the number `NULL` values in a column
```
##########
datafusion/core/src/dataframe/mod.rs:
##########
@@ -534,7 +534,7 @@ impl DataFrame {
vec![],
original_schema_fields
.clone()
- .map(|f| count(is_null(col(f.name()))).alias(f.name()))
+ .map(|f| count_null(col(f.name())).alias(f.name()))
Review Comment:
Got it. Make sense to me. Another classic trick is to use a CASE expression, something like
```sql
SELECT SUM(CASE WHEN x IS NULL THEN 1 ELSE 0 END) as null_count ...
```
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Re: [PR] fix: Correct null_count in describe() [datafusion]
Posted by "Weijun-H (via GitHub)" <gi...@apache.org>.
Weijun-H commented on code in PR #10260:
URL: https://github.com/apache/datafusion/pull/10260#discussion_r1581828836
##########
datafusion/core/src/dataframe/mod.rs:
##########
@@ -534,7 +534,7 @@ impl DataFrame {
vec![],
original_schema_fields
.clone()
- .map(|f| count(is_null(col(f.name()))).alias(f.name()))
+ .map(|f| count_null(col(f.name())).alias(f.name()))
Review Comment:
When using `count`, it sums all `non-null` Expr while `is_null` converts `Expr` to a boolean expression, which is why it doesn't work.
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Re: [PR] fix: Correct null_count in describe() [datafusion]
Posted by "alamb (via GitHub)" <gi...@apache.org>.
alamb commented on code in PR #10260:
URL: https://github.com/apache/datafusion/pull/10260#discussion_r1584900962
##########
datafusion/core/tests/dataframe/describe.rs:
##########
@@ -81,6 +81,36 @@ async fn describe_boolean_binary() -> Result<()> {
Ok(())
}
+#[tokio::test]
+async fn describe_null() -> Result<()> {
+ let ctx = parquet_context().await;
+
+ //add test case for only boolean boolean/binary column
+ let result = ctx
+ .sql("select 'a' as a, null as b")
+ .await?
+ .describe()
+ .await?
+ .collect()
+ .await?;
+ #[rustfmt::skip]
+ let expected = [
+ "+------------+------+------+",
+ "| describe | a | b |",
+ "+------------+------+------+",
+ "| count | 1 | 0 |",
+ "| null_count | 0 | 1 |",
Review Comment:
❤️ -- thank you @Weijun-H
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Re: [PR] fix: Correct null_count in describe() [datafusion]
Posted by "alamb (via GitHub)" <gi...@apache.org>.
alamb merged PR #10260:
URL: https://github.com/apache/datafusion/pull/10260
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Re: [PR] fix: Correct null_count in describe() [datafusion]
Posted by "alamb (via GitHub)" <gi...@apache.org>.
alamb commented on code in PR #10260:
URL: https://github.com/apache/datafusion/pull/10260#discussion_r1581819093
##########
datafusion/expr/src/expr_fn.rs:
##########
@@ -215,6 +215,18 @@ pub fn count(expr: Expr) -> Expr {
))
}
+/// Create an expression to represent the count_null() aggregate function
Review Comment:
I think this function is creating a function like `COUNT(IS NULL x)` rather than a `count_null` function
##########
datafusion/core/tests/dataframe/describe.rs:
##########
@@ -39,7 +39,7 @@ async fn describe() -> Result<()> {
"| describe | id | bool_col | tinyint_col | smallint_col | int_col | bigint_col | float_col | double_col | date_string_col | string_col | timestamp_col | year | month |",
"+------------+-------------------+----------+--------------------+--------------------+--------------------+--------------------+--------------------+--------------------+-----------------+------------+-------------------------+--------------------+-------------------+",
"| count | 7300.0 | 7300 | 7300.0 | 7300.0 | 7300.0 | 7300.0 | 7300.0 | 7300.0 | 7300 | 7300 | 7300 | 7300.0 | 7300.0 |",
- "| null_count | 7300.0 | 7300 | 7300.0 | 7300.0 | 7300.0 | 7300.0 | 7300.0 | 7300.0 | 7300 | 7300 | 7300 | 7300.0 | 7300.0 |",
+ "| null_count | 0.0 | 0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0 | 0 | 0 | 0.0 | 0.0 |",
Review Comment:
that certainly looks better
##########
datafusion/core/src/dataframe/mod.rs:
##########
@@ -534,7 +534,7 @@ impl DataFrame {
vec![],
original_schema_fields
.clone()
- .map(|f| count(is_null(col(f.name()))).alias(f.name()))
+ .map(|f| count_null(col(f.name())).alias(f.name()))
Review Comment:
Instead of a `count_null()` new function, what do you think about using the existing [`count`](https://docs.rs/datafusion/latest/datafusion/logical_expr/fn.count.html) function?
Something like `count(expr.is_null())`
```suggestion
.map(|f| count(col(f.name()).is_null()).alias(f.name())))
```
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