advancedxy commented on code in PR #424:
URL: https://github.com/apache/datafusion-comet/pull/424#discussion_r1600892577
##########
core/src/execution/datafusion/spark_hash.rs:
##########
@@ -193,27 +241,67 @@ macro_rules! hash_array_decimal {
fn create_hashes_dictionary<K: ArrowDictionaryKeyType>(
array: &ArrayRef,
hashes_buffer: &mut [u32],
+ multi_col: bool,
) -> Result<()> {
let dict_array =
array.as_any().downcast_ref::<DictionaryArray<K>>().unwrap();
+ if multi_col {
+ // unpack the dictionary array as each row may have a different hash
input
+ let unpacked = take(dict_array.values().as_ref(), dict_array.keys(),
None)?;
+ create_hashes(&[unpacked], hashes_buffer)?;
+ } else {
+ // 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());
+ // same initial seed as Spark
+ let mut dict_hashes = vec![42; dict_values.len()];
+ create_hashes(&[dict_values], &mut dict_hashes)?;
Review Comment:
> so
```
row_id = 0, dict_key = 1, dict_hashes[1] = hash(1, dict_hashes[0]) but it
should be hash(1, init_seed)
row_id = 1, dict_key = 0, dict_hashes[0] = hash(2, init_seed) but it should
be hash(2, hash(1, init_seed))
row_id = 2, dict_key = 0 dict_hashes[0] = hash(2, init_seed) but it should
be hash(2, hash(2, hash(1, init_seed)))
...
```
Hmm, I think you may have confused with row order and column order, which
indeed is quite confusing.
For function call like `hash(col_a, col_b)`, the actual hash is
`murmur3(col_b, murmur3(col_a, 42))` where 42 is the initial seed, the previous
hash is feed as seed, the order is determined the **input cols**(parameters),
but not related to the _row order_.
For rows in the RecordBatch/ArrowArray, each hash is computed from scratch,
there's no dep between values in different rows.
So the following compute step is not how it currently work
> dict_hashes[0] = hash(2, init_seed)
dict_hashes[1] = hash(1, dict_hashes[0])
dict_hashes[2] = hash(4, dict_hashes[1])
dict_hashes[3] = hash(3, dict_hashes[2])
dict_hashes[4] = hash(5, dict_hashes[3])
Instead, currently it works as
```
dict_hashes[0] = hash(2, init_seed)
dict_hashes[1] = hash(1, init_seed)
dict_hashes[2] = hash(4, init_seed)
dict_hashes[3] = hash(3, init_seed)
dict_hashes[4] = hash(5, init_seed)
```
The hashes of the first col are then feed as seed for the next cols in the
rows.
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