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improve nested pagination from O(n log n) to O(n) #563

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merged 1 commit into from Mar 6, 2020
Merged

improve nested pagination from O(n log n) to O(n) #563

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19 changes: 12 additions & 7 deletions query-engine/core/src/interpreter/query_interpreters/nested_pagination.rs
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@@ -1,6 +1,5 @@
use connector::QueryArguments;
use prisma_models::{ManyRecords, RecordProjection};
use std::collections::HashMap;

pub struct NestedPagination {
skip: Option<i64>,
Expand All @@ -21,7 +20,6 @@ impl NestedPagination {
if !self.must_apply_pagination() {
return;
}
let mut count_by_parent_id: HashMap<Option<RecordProjection>, i64> = HashMap::new();
// replacement for SQL order by
// TODO: this must also handle secondary order bys
many_records.records.sort_by_key(|r| {
Expand All @@ -38,18 +36,25 @@ impl NestedPagination {
many_records.records.reverse();
}

// The records are sorted by their parent id. Hence we just need to remember the count for the last parent id to apply pagination.
let mut current_count: i64 = 0;
let mut last_parent_id: Option<RecordProjection> = None;

many_records.records.retain(|record| {
let current_count = count_by_parent_id.get(&record.parent_id).unwrap_or(&0);
let new_count = current_count + 1;
count_by_parent_id.insert(record.parent_id.clone(), new_count);
if last_parent_id == record.parent_id {
current_count = current_count + 1;
} else {
last_parent_id = record.parent_id.clone();
current_count = 1; // this is the first record we see for this parent id
};

let is_beyond_skip_range = match self.skip {
None => true,
Some(skip) => new_count > skip,
Some(skip) => current_count > skip,
};
let is_within_take_range = match self.take {
None => true,
Some(take) => new_count <= take + self.skip.unwrap_or(0),
Some(take) => current_count <= take + self.skip.unwrap_or(0),
};

is_beyond_skip_range && is_within_take_range
Expand Down