LiaCastaneda commented on code in PR #21589:
URL: https://github.com/apache/datafusion/pull/21589#discussion_r3109671277
##########
datafusion/physical-plan/src/aggregates/group_values/single_group_by/dictionary.rs:
##########
Review Comment:
Do you think it's worth adding a diagram explaining how the two-phase
grouping works? It took me a while to understand it and a diagram helped a lot
```
Input batch: DictionaryArray
values = ["ap-south", "us-east"] (d=2)
keys = [0, 1, 0, 0, 1] (n=5)
Pre-step: hash all d values once
value_hashes = [h("ap-south"), h("us-east")]
Pass 1 — d iterations (build key_to_group from persistent cache)
unique_dict_value_mapping: {h("ap-south") -> (group_id=0), ...}
│
for each value in values array ─────┘
lookup in map → key_to_group[0] = Some(0)
key_to_group[1] = Some(1)
Pass 2 — n iterations (assign group ids to every row)
for each key in keys array:
key=0 → key_to_group[0] = Some(0) → groups.push(0)
key=1 → key_to_group[1] = Some(1) → groups.push(1)
key=0 → key_to_group[0] = Some(0) → groups.push(0)
key=0 → key_to_group[0] = Some(0) → groups.push(0)
key=1 → key_to_group[1] = Some(1) → groups.push(1)
Output: groups = [0, 1, 0, 0, 1]
```
##########
datafusion/physical-plan/src/aggregates/group_values/single_group_by/dictionary.rs:
##########
Review Comment:
Do you think it's worth adding a diagram explaining how the two-phase
grouping works? It took me a while to understand it and a diagram claude made
helped a lot
```
Input batch: DictionaryArray
values = ["ap-south", "us-east"] (d=2)
keys = [0, 1, 0, 0, 1] (n=5)
Pre-step: hash all d values once
value_hashes = [h("ap-south"), h("us-east")]
Pass 1 — d iterations (build key_to_group from persistent cache)
unique_dict_value_mapping: {h("ap-south") -> (group_id=0), ...}
│
for each value in values array ─────┘
lookup in map → key_to_group[0] = Some(0)
key_to_group[1] = Some(1)
Pass 2 — n iterations (assign group ids to every row)
for each key in keys array:
key=0 → key_to_group[0] = Some(0) → groups.push(0)
key=1 → key_to_group[1] = Some(1) → groups.push(1)
key=0 → key_to_group[0] = Some(0) → groups.push(0)
key=0 → key_to_group[0] = Some(0) → groups.push(0)
key=1 → key_to_group[1] = Some(1) → groups.push(1)
Output: groups = [0, 1, 0, 0, 1]
```
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