Bug Report

1. Minimal reproduce step (Required)

Run this only on an isolated test cluster. The configuration changes below are cluster-wide.

1. Save the current Load Base Split configuration:

SHOW CONFIG
WHERE TYPE = 'tikv'
  AND NAME IN (
    'split.qps-threshold',
    'split.byte-threshold',
    'split.region-cpu-overload-threshold-ratio'
  );

1. Lower only the QPS threshold and disable the other triggering conditions:

SET CONFIG tikv `split.qps-threshold` = 100;
SET CONFIG tikv `split.byte-threshold` = 107374182400;
SET CONFIG tikv `split.region-cpu-overload-threshold-ratio` = 0;

1. Create a table whose physical rows occupy only the high end of a sparse clustered-primary-key keyspace:

CREATE DATABASE IF NOT EXISTS lbs_repro;

DROP TABLE IF EXISTS lbs_repro.sparse_pk;

CREATE TABLE lbs_repro.sparse_pk (
    id VARBINARY(64) NOT NULL,
    payload VARBINARY(128),
    PRIMARY KEY (id) CLUSTERED
);

INSERT INTO lbs_repro.sparse_pk
VALUES
    ('z-real-0001', REPEAT('x', 64)),
    ('z-real-0002', REPEAT('x', 64));

The following hot keys are intentionally absent and sort before all physical rows.

1. Record the initial Region information:

SELECT
    REGION_ID,
    START_KEY,
    END_KEY,
    APPROXIMATE_SIZE,
    APPROXIMATE_KEYS
FROM INFORMATION_SCHEMA.TIKV_REGION_STATUS
WHERE DB_NAME = 'lbs_repro'
  AND TABLE_NAME = 'sparse_pk'
  AND IS_INDEX = 0
ORDER BY START_KEY;

1. Through persistent MySQL connections, repeatedly execute two missing-key point reads at more than 100 QPS for at least 30 seconds:

SELECT id
FROM lbs_repro.sparse_pk
WHERE id = 'a-hot-missing';

SELECT id
FROM lbs_repro.sparse_pk
WHERE id = 'b-hot-missing';

For example:

mysqlslap \
  --defaults-extra-file=/path/to/test-only-client.cnf \
  --create-schema=lbs_repro \
  --concurrency=16 \
  --iterations=100 \
  --number-of-queries=20000 \
  --query="SELECT id FROM sparse_pk WHERE id='a-hot-missing'; SELECT id FROM sparse_pk WHERE id='b-hot-missing';"

1. Check the following signals:

• TiKV logs contain load base split region for the target Region, with split reason Load.
• tikv_load_base_split_event{type="ready_to_split"} increases.
• A new record Region appears.
• One resulting Region has APPROXIMATE_KEYS=0 or a very small value.

1. Confirm that the affected logical range contains no rows:

SELECT COUNT(*)
FROM lbs_repro.sparse_pk
WHERE id < 'z-real-0001';

The result should be 0.

1. To reproduce Region accumulation, change the two absent hot keys every 15–30 seconds:

phase 1: a-hot-1, b-hot-1
phase 2: c-hot-2, d-hot-2
phase 3: e-hot-3, f-hot-3
...

Record Region ID sets after each phase. Empty or near-empty Regions continue to accumulate as the hot missing-key range moves.

1. Restore the original TiKV configuration recorded in step 1 and drop the test database.

2. What did you expect to see? (Required)

Load Base Split should avoid a split that leaves one child Region with no live physical keys, or it should merge the cold empty child promptly after the access pattern moves.

A candidate split key could be moved to a nearby physical key boundary, rejected when either child has insufficient keys or bytes, or protected by an empty-child cooldown.

The number of empty Regions should not continue growing from moving negative-read hotspots.

3. What did you see instead (Required)

Production impact observed

Our production workload performs high-frequency historical lookups on a sparse archive table created using IMPORT INTO ... SELECT The primary query pattern is:

SELECT ...
FROM archive_tmp
WHERE original_id = ?
  AND block_height <= ?
ORDER BY block_height DESC
LIMIT 1;

Under this workload, approximately 851 empty or near-empty Regions were generated within a short period. Additional empty Regions continued to appear as long as the workload remained active.

After manually merging the confirmed empty Regions, we observed a significant and immediate performance improvement:

Release Version: v8.5.6
Edition: Community
Git Commit Hash: ae18096e023780bb56bfce33698abec0d4640d0a
Git Branch: HEAD
UTC Build Time: 2026-06-09 03:39:19
GoVersion: go1.25.8
Race Enabled: false
Check Table Before Drop: false
Store: tikv