[GitHub] [arrow] vertexclique commented on a change in pull request #8673: ARROW-10609: [Rust] Optimize min/max of non null strings

[GitBox <gi...@apache.org>]

vertexclique commented on a change in pull request #8673:
URL: https://github.com/apache/arrow/pull/8673#discussion_r524238466



##########
File path: rust/arrow/benches/aggregate_kernels.rs
##########
@@ -43,6 +43,25 @@ fn create_array(size: usize, with_nulls: bool) -> ArrayRef {
     Arc::new(builder.finish())
 }
 
+fn create_string_array(size: usize, with_nulls: bool) -> ArrayRef {
+    // use random numbers to avoid spurious compiler optimizations wrt to branching
+    let mut rng = seedable_rng();
+    let mut builder = StringBuilder::new(size);
+
+    for _ in 0..size {
+        if with_nulls && rng.gen::<f32>() > 0.5 {
+            builder.append_null().unwrap();
+        } else {
+            let string = seedable_rng()
+                .sample_iter(&Alphanumeric)
+                .take(10)
+                .collect::<String>();
+            builder.append_value(&string).unwrap();
+        }
+    }
+    Arc::new(builder.finish())
+}

Review comment:
       Can you make the downcasting inside the array create method? That will shave off some excess work.

##########
File path: rust/arrow/src/compute/kernels/aggregate.rs
##########
@@ -32,19 +32,20 @@ fn min_max_string<T: StringOffsetSizeTrait, F: Fn(&str, &str) -> bool>(
     if null_count == array.len() {
         return None;
     }
-    let mut n = "";
-    let mut has_value = false;
     let data = array.data();
-
+    let mut n;
     if null_count == 0 {
-        for i in 0..data.len() {
+        n = array.value(0);
+        for i in 1..data.len() {
             let item = array.value(i);
-            if !has_value || cmp(&n, item) {
-                has_value = true;
+            if cmp(&n, item) {
                 n = item;
             }
         }

Review comment:
       Feels like this code can use fold with your changes now.

##########
File path: rust/arrow/benches/aggregate_kernels.rs
##########
@@ -43,6 +43,25 @@ fn create_array(size: usize, with_nulls: bool) -> ArrayRef {
     Arc::new(builder.finish())
 }
 
+fn create_string_array(size: usize, with_nulls: bool) -> ArrayRef {
+    // use random numbers to avoid spurious compiler optimizations wrt to branching
+    let mut rng = seedable_rng();
+    let mut builder = StringBuilder::new(size);
+
+    for _ in 0..size {
+        if with_nulls && rng.gen::<f32>() > 0.5 {
+            builder.append_null().unwrap();
+        } else {
+            let string = seedable_rng()
+                .sample_iter(&Alphanumeric)
+                .take(10)
+                .collect::<String>();
+            builder.append_value(&string).unwrap();
+        }
+    }
+    Arc::new(builder.finish())
+}

Review comment:
       Would be also nice to change all of these benchmarks to do the same.




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