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[fix](be) Finish pending scanners after shared limit #65814

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[fix](be) Finish pending scanners after shared limit #65814

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9b5ff49
[fix](be) Finish pending scanners after shared limit
Mryange Jul 20, 2026
0d18091
[fix](be) Handle exhausted shared scan limits
Mryange Jul 20, 2026
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11 changes: 5 additions & 6 deletions be/src/exec/scan/scanner_context.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -404,7 +404,7 @@ Status ScannerContext::get_block_from_queue(RuntimeState* state, Block* block, b

if (_completed_tasks.empty() &&
(_num_finished_scanners == _all_scanners.size() ||
(_shared_scan_limit->load(std::memory_order_acquire) == 0 && _in_flight_tasks_num == 0))) {
(_is_shared_scan_limit_exhausted() && _in_flight_tasks_num == 0))) {
_set_scanner_done();
_is_finished = true;
}
Expand Down Expand Up @@ -545,6 +545,10 @@ void ScannerContext::_set_scanner_done() {
_dependency->set_always_ready();
}

bool ScannerContext::_is_shared_scan_limit_exhausted() const {
return limit >= 0 && _shared_scan_limit->load(std::memory_order_acquire) <= 0;
}

void ScannerContext::update_peak_running_scanner(int num) {
#ifndef BE_TEST
_local_state->_peak_running_scanner->add(num);
Expand Down Expand Up @@ -735,11 +739,6 @@ std::shared_ptr<ScanTask> ScannerContext::_pull_next_scan_task(
}

if (!_pending_tasks.empty()) {
// Skip submitting more pending scanners once the LIMIT budget is
// exhausted; they would only open and immediately EOF.
if (_shared_scan_limit->load(std::memory_order_acquire) == 0) {
return nullptr;
}

@github-actions github-actions Bot Jul 20, 2026

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[P1] Avoid opening pending scanners after the LIMIT is satisfied

After this deletion, a context that did not produce the limiting row repeatedly schedules its remaining tasks: every consumed EOS calls schedule_scan_task() before the local finish check, and _scanner_scan() runs prepare()/open() before Scanner::get_block() sees the shared counter <= 0. With many tablet/file ranges this does O(remaining scanners) initialization/eager-read work; worse, an otherwise-unneeded submit/prepare/open error is propagated through _process_status, so a LIMIT query that already has enough rows can now fail.

The old guard can strand a late context whose first schedule already sees zero because its dependency starts blocked; a separate zero-to-negative window exists after EOS. Please keep a no-I/O finite-exhausted path (limit >= 0 && shared_remaining <= 0), but have the scheduling path itself finish/wake the context when it will submit no task and no completed/in-flight work remains (or synthesize a cheap EOS). If work remains in flight, let its callbacks return without starting new pending readers. Do not rely only on a later consumer check, which can be unreachable.

std::shared_ptr<ScanTask> next_scan_task;
next_scan_task = _pending_tasks.top();
_pending_tasks.pop();
Expand Down
1 change: 1 addition & 0 deletions be/src/exec/scan/scanner_context.h
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Expand Up @@ -267,6 +267,7 @@ class ScannerContext : public std::enable_shared_from_this<ScannerContext>,
/// 3. `_free_blocks_memory_usage` < `_max_bytes_in_queue`, remains enough memory to scale up
/// 4. At most scale up `MAX_SCALE_UP_RATIO` times to `_max_thread_num`
void _set_scanner_done();
bool _is_shared_scan_limit_exhausted() const;

RuntimeState* _state = nullptr;
ScanLocalStateBase* _local_state = nullptr;
Expand Down
7 changes: 7 additions & 0 deletions regression-test/data/correctness_p0/test_shared_scan_limit_pending_tasks.out
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@@ -0,0 +1,7 @@
-- This file is automatically generated. You should know what you did if you want to edit this
-- !shared_scan_limit_parallel_2 --

-- !shared_scan_limit_parallel_6 --

-- !shared_scan_limit_parallel_8 --

175 changes: 175 additions & 0 deletions regression-test/suites/correctness_p0/test_shared_scan_limit_pending_tasks.groovy
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@@ -0,0 +1,175 @@
// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements. See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership. The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License. You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied. See the License for the
// specific language governing permissions and limitations
// under the License.

suite("test_shared_scan_limit_pending_tasks") {
sql "DROP TABLE IF EXISTS shared_scan_limit_t1"
sql "DROP TABLE IF EXISTS shared_scan_limit_t3"
sql "DROP TABLE IF EXISTS shared_scan_limit_t4"
sql "DROP TABLE IF EXISTS shared_scan_limit_t5"

sql """
CREATE TABLE shared_scan_limit_t1 (
pk INT,
v1 BIGINT,
v2 BIGINT
) ENGINE=OLAP
DUPLICATE KEY(pk)
DISTRIBUTED BY HASH(pk) BUCKETS 10
PROPERTIES ("replication_num" = "1")
"""
sql """
CREATE TABLE shared_scan_limit_t3 (
pk INT,
v1 BIGINT,
v2 BIGINT
) ENGINE=OLAP
DUPLICATE KEY(pk)
DISTRIBUTED BY HASH(pk) BUCKETS 10
PROPERTIES ("replication_num" = "1")
"""
sql """
CREATE TABLE shared_scan_limit_t4 (
pk INT,
v1 BIGINT,
v2 BIGINT
) ENGINE=OLAP
DUPLICATE KEY(pk)
DISTRIBUTED BY HASH(pk) BUCKETS 10
PROPERTIES ("replication_num" = "1")
"""
sql """
CREATE TABLE shared_scan_limit_t5 (
pk INT,
v1 BIGINT,
v2 BIGINT
) ENGINE=OLAP
DUPLICATE KEY(pk)
DISTRIBUTED BY HASH(pk) BUCKETS 10
PROPERTIES ("replication_num" = "1")
"""

sql """
INSERT INTO shared_scan_limit_t1 VALUES
(0,70,32163), (1,-71,NULL), (2,-85,-222125), (3,26,22867),
(4,17,NULL), (5,NULL,6236439), (6,-16907,-24720), (7,26877,1893),
(8,30405,9), (9,5873343,66), (10,-17,-47), (11,NULL,20987),
(12,NULL,-4672), (13,NULL,3551614), (14,28521,-1372), (15,NULL,NULL),
(16,38,NULL), (17,NULL,-54), (18,NULL,-4686447), (19,NULL,-8420)
"""
sql """
INSERT INTO shared_scan_limit_t3 VALUES
(0,28862,-25225), (1,5516858,-4609390), (2,-7815300,NULL),
(3,NULL,-7685824), (4,22293,26373), (5,NULL,NULL), (6,1237,-31),
(7,1606,3318374), (8,NULL,-21355), (9,NULL,-20127), (10,5413995,-3),
(11,-4718995,NULL), (12,3179854,3733421), (13,18867,76),
(14,15517,-4932405), (15,-30778,NULL), (16,NULL,-7258),
(17,NULL,7520313), (18,6936629,NULL), (19,-45,6231596)
"""
sql """
INSERT INTO shared_scan_limit_t4 VALUES
(0,5649134,4812327), (1,8249417,4670054), (2,106,6714155),
(3,NULL,NULL), (4,-52850,-5113499), (5,-23911,-9637), (6,-26168,9),
(7,-47,-28398), (8,-29518,-2365317), (9,20685,22956), (10,97,25099),
(11,-32617,6143808), (12,NULL,NULL), (13,-52,NULL), (14,7680925,NULL),
(15,10848,NULL), (16,64,-3394), (17,113,-12488), (18,-87,-12093),
(19,NULL,22)
"""
sql """
INSERT INTO shared_scan_limit_t5 VALUES
(0,-3018453,-5763927), (1,-60,-6233027), (2,78,-5304), (3,63,NULL),
(4,23287,NULL), (5,98,7989209), (6,-61,-30493), (7,-6781665,-22321),
(8,-5165806,-75), (9,NULL,31290), (10,1311226,7754904), (11,-72,54),
(12,114,3741372), (13,NULL,NULL), (14,-40,2014555), (15,69,-4555977),
(16,118,-8708), (17,NULL,NULL), (18,-123,NULL), (19,NULL,19843)
"""
sql "sync"

sql "SET enable_sql_cache = false"
sql "SET experimental_enable_local_shuffle = false"
sql "SET query_timeout = 60"

// A shared LIMIT can be exhausted while another ScannerContext still owns pending tasks.
// Every context must still reach EOS instead of leaving its pipeline task blocked forever.
sql "SET parallel_pipeline_task_num = 2"
qt_shared_scan_limit_parallel_2 """
SELECT v1, pk
FROM shared_scan_limit_t1 AS t1
WHERE v2 > (
SELECT SUM(v1)
FROM shared_scan_limit_t5 AS t3
WHERE EXISTS (
SELECT pk
FROM shared_scan_limit_t3 AS t4
WHERE NOT EXISTS (
SELECT pk, v1, v1, v2, v1
FROM shared_scan_limit_t4 AS t5
WHERE v2 NOT IN (
SELECT pk FROM shared_scan_limit_t1 AS t6 ORDER BY pk
)
) AND v1 < 7
)
)
ORDER BY v1, pk DESC
LIMIT 6
"""

sql "SET parallel_pipeline_task_num = 6"
qt_shared_scan_limit_parallel_6 """
SELECT v1, pk
FROM shared_scan_limit_t1 AS t1
WHERE v2 > (
SELECT SUM(v1)
FROM shared_scan_limit_t5 AS t3
WHERE EXISTS (
SELECT pk
FROM shared_scan_limit_t3 AS t4
WHERE NOT EXISTS (
SELECT pk, v1, v1, v2, v1
FROM shared_scan_limit_t4 AS t5
WHERE v2 NOT IN (
SELECT pk FROM shared_scan_limit_t1 AS t6 ORDER BY pk
)
) AND v1 < 7
)
)
ORDER BY v1, pk DESC
LIMIT 6
"""

sql "SET parallel_pipeline_task_num = 8"
qt_shared_scan_limit_parallel_8 """
SELECT v1, pk
FROM shared_scan_limit_t1 AS t1
WHERE v2 > (
SELECT SUM(v1)
FROM shared_scan_limit_t5 AS t3
WHERE EXISTS (
SELECT pk
FROM shared_scan_limit_t3 AS t4
WHERE NOT EXISTS (
SELECT pk, v1, v1, v2, v1
FROM shared_scan_limit_t4 AS t5
WHERE v2 NOT IN (
SELECT pk FROM shared_scan_limit_t1 AS t6 ORDER BY pk
)
) AND v1 < 7
)
)
ORDER BY v1, pk DESC
LIMIT 6
"""
}
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