korowa commented on code in PR #11806:
URL: https://github.com/apache/datafusion/pull/11806#discussion_r1703600132
##########
datafusion/physical-plan/src/joins/hash_join.rs:
##########
@@ -1964,12 +1960,21 @@ mod tests {
assert_eq!(columns, vec!["a1", "b2", "c1", "a1", "b2", "c2"]);
- // expected joined records = 3
- // in case batch_size is 1 - additional empty batch for remaining 3-2
row
- let mut expected_batch_count = div_ceil(3, batch_size);
- if batch_size == 1 {
- expected_batch_count += 1;
- }
+ let expected_batch_count = if cfg!(not(feature =
"force_hash_collisions")) {
+ // Expected number of hash table matches = 3
+ // in case batch_size is 1 - additional empty batch for remaining
3-2 row
+ let mut expected_batch_count = div_ceil(3, batch_size);
+ if batch_size == 1 {
+ expected_batch_count += 1;
+ }
+ expected_batch_count
+ } else {
+ // With hash collisions enabled, all records will match each other
+ // and filtered later.
+ // Expected number of matches = 9
+ div_ceil(9, batch_size)
Review Comment:
Yes, since
[create_hashes](https://github.com/apache/datafusion/blob/1d3bdbe14fd01bbd03abeb57d3ec9bc155a36d71/datafusion/common/src/hash_utils.rs#L355)
always sets 0 as hash values, with force-collisions feature enabled, the total
number of hash table matches will be equal to `left.row_count() *
right.row_count()` for each batch size. The result of the output batch count
calculation depends on the batch size though, and it's based on the number of
matches.
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