[GitHub] [arrow-datafusion] alamb commented on a change in pull request #812: Implement vectorized hashing for DictionaryArray types

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

alamb commented on a change in pull request #812:
URL: https://github.com/apache/arrow-datafusion/pull/812#discussion_r682098857



##########
File path: datafusion/src/physical_plan/hash_utils.rs
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@@ -245,9 +249,54 @@ macro_rules! hash_array_float {
     };
 }
 
-/// Creates hash values for every row, based on the values in the columns
+/// Hash the values in a dictionary array
+fn create_hashes_dictionary<K: ArrowDictionaryKeyType>(
+    array: &ArrayRef,
+    random_state: &RandomState,
+    hashes_buffer: &mut Vec<u64>,
+    multi_col: bool,
+) -> Result<()> {
+    let dict_array = array.as_any().downcast_ref::<DictionaryArray<K>>().unwrap();
+
+    // Hash each dictionary value once, and then use that computed
+    // hash for each key value to avoid a potentially expensive
+    // redundant hashing for large dictionary elements (e.g. strings)
+    let dict_values = Arc::clone(dict_array.values());
+    let mut dict_hashes = vec![0; dict_values.len()];
+    create_hashes(&[dict_values], random_state, &mut dict_hashes)?;
+
+    // combine hash for each index in values
+    for (hash, key) in hashes_buffer.iter_mut().zip(dict_array.keys().iter()) {
+        match key {
+            Some(key) => {
+                let idx = key
+                    .to_usize()
+                    .ok_or_else(|| {
+                        DataFusionError::Internal(format!(
+                            "Can not convert key value {:?} to usize in dictionary of type {:?}",
+                            key, dict_array.data_type()
+                        ))
+                    })?;
+
+                *hash = if multi_col {

Review comment:
       good idea -- done ✅  




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