andygrove commented on code in PR #23460:
URL: https://github.com/apache/datafusion/pull/23460#discussion_r3562165921
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datafusion/functions/src/unicode/find_in_set.rs:
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@@ -329,16 +334,34 @@ where
let nulls = string_array.nulls().cloned();
let zero = T::Native::from_usize(0).unwrap();
+ // The set (`str_list`) is constant across all rows. For a large set, the
+ // per-row `position` linear scan is O(set_len). Building a lookup from
each
+ // distinct entry to its 1-based position once turns each row into an O(1)
+ // probe (first occurrence wins, exactly matching `position`). Below the
+ // threshold the linear scan's small constant factor is faster, so the map
is
+ // built at most once here rather than per row.
+ let map: Option<HashMap<&str, usize>> =
+ (str_list.len() >= FIND_IN_SET_LOOKUP_THRESHOLD).then(|| {
+ let mut map = HashMap::with_capacity(str_list.len());
Review Comment:
I looked into this, and it doesn't seem like it would improve performance
much, and would be a significant change.
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