Performance degradation between 20.1 and 20.3 for large number of very short queries with LowCardinality fields #13864
Description
Schema
ATTACH TABLE default.test (`col1` UUID, `col2` LowCardinality(String)) ENGINE = MergeTree ORDER BY (col1, col2) SETTINGS index_granularity = 128;
Query:
echo "SELECT col1, col2 FROM test PREWHERE col1 = '042fecec-ea68-1b26-00a4-a4f3b3987574'" | clickhouse-benchmark -c 1 -i 2000 -d 0
Results - look on QPS metric:
### 20.1.16.120
Loaded 1 queries.
Queries executed: 2000.
localhost:9000, queries 2000, QPS: 597.941, RPS: 76536.485, MiB/s: 1.168, result RPS: 0.000, result MiB/s: 0.000.
0.000% 0.001 sec.
10.000% 0.001 sec.
20.000% 0.002 sec.
30.000% 0.002 sec.
40.000% 0.002 sec.
50.000% 0.002 sec.
60.000% 0.002 sec.
70.000% 0.002 sec.
80.000% 0.002 sec.
90.000% 0.002 sec.
95.000% 0.002 sec.
99.000% 0.002 sec.
99.900% 0.003 sec.
99.990% 0.015 sec.
### 20.3.2.1
Loaded 1 queries.
Queries executed: 2000.
localhost:9000, queries 2000, QPS: 322.157, RPS: 41236.084, MiB/s: 0.629, result RPS: 0.000, result MiB/s: 0.000.
0.000% 0.001 sec.
10.000% 0.003 sec.
20.000% 0.003 sec.
30.000% 0.003 sec.
40.000% 0.003 sec.
50.000% 0.003 sec.
60.000% 0.003 sec.
70.000% 0.003 sec.
80.000% 0.003 sec.
90.000% 0.003 sec.
95.000% 0.004 sec.
99.000% 0.004 sec.
99.900% 0.016 sec.
99.990% 0.017 sec.
## 20.6.3.28
Loaded 1 queries.
Queries executed: 2000.
localhost:9000, queries 2000, QPS: 308.133, RPS: 39441.007, MiB/s: 0.602, result RPS: 0.000, result MiB/s: 0.000.
0.000% 0.001 sec.
10.000% 0.002 sec.
20.000% 0.003 sec.
30.000% 0.003 sec.
40.000% 0.003 sec.
50.000% 0.003 sec.
60.000% 0.003 sec.
70.000% 0.003 sec.
80.000% 0.004 sec.
90.000% 0.004 sec.
95.000% 0.004 sec.
99.000% 0.004 sec.
99.900% 0.016 sec.
99.990% 0.017 sec.
Bisecting on releases:
v20.2.1.2442-testing (21-Feb-2020 18:15) - last good
v20.3.1.2495-testing (26-Feb-2020 18:27) - first bad
ps. experimental_use_processors don't change the situation.