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Fix #328 - Dont do limit pushdown during parallel execution #330

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Fix #328 - Dont do limit pushdown during parallel execution #330

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eed717b
reproduced parallel limit problem
noahisaksen May 29, 2025
b055cd1
only push limit down if 1 thread and can use main
noahisaksen May 29, 2025
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5 changes: 5 additions & 0 deletions src/storage/postgres_optimizer.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -54,6 +54,11 @@ static void OptimizePostgresScanLimitPushdown(unique_ptr<LogicalOperator> &op) {
}

auto &bind_data = get.bind_data->Cast<PostgresBindData>();
if (bind_data.max_threads != 1 || !bind_data.can_use_main_thread) {
// cannot push down limit/offset if we are not using the main thread
OptimizePostgresScanLimitPushdown(op->children[0]);
return;
}

string generated_limit_clause = "";
if (limit.limit_val.Type() != LimitNodeType::UNSET) {
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32 changes: 32 additions & 0 deletions test/sql/storage/limit.test
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Expand Up @@ -39,3 +39,35 @@ query II
EXPLAIN FROM s.large_tbl LIMIT 5;
----
logical_opt <!REGEX>:.*LIMIT.*


statement ok
set pg_pages_per_task=1

query I
FROM s.large_tbl LIMIT 5
----
0
1
2
3
4

query I
FROM s.large_tbl LIMIT 5 OFFSET 5
----
5
6
7
8
9

statement ok
set explain_output='optimized_only'

# limit is still in plan as we were not able to push down due to parallel execution

query II
EXPLAIN FROM s.large_tbl LIMIT 5;
----
logical_opt <REGEX>:.*LIMIT.*
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