clickhouse的稀疏索引

CREATE TABLE hits_UserID_URL
(
`UserID` UInt32,
`URL` String,
`EventTime` DateTime
)
ENGINE = MergeTree
PRIMARY KEY (UserID, URL)
ORDER BY (UserID, URL, EventTime)
SETTINGS index_granularity = 8192, index_granularity_bytes = 0;

-- 查看索引的行数marks
SELECT
    part_type,
    path,
    formatReadableQuantity(rows) AS rows,
    formatReadableSize(data_uncompressed_bytes) AS data_uncompressed_bytes,
    formatReadableSize(data_compressed_bytes) AS data_compressed_bytes,
    formatReadableSize(primary_key_bytes_in_memory) AS primary_key_bytes_in_memory,
    marks,
    formatReadableSize(bytes_on_disk) AS bytes_on_disk
FROM system.parts
WHERE (table = 'hits_UserID_URL') AND (active = 1)
FORMAT Vertical;

part_type:                   Wide
path:                        ./store/d9f/d9f36a1a-d2e6-46d4-8fb5-ffe9ad0d5aed/all_1_9_2/
rows:                        8.87 million
data_uncompressed_bytes:     733.28 MiB
data_compressed_bytes:       206.94 MiB
primary_key_bytes_in_memory: 96.93 KiB
marks:                       1083
bytes_on_disk:               207.07 MiB


1 rows in set. Elapsed: 0.003 sec.

SELECT URL, count(URL) AS Count
FROM hits_UserID_URL
WHERE UserID = 749927693
GROUP BY URL
ORDER BY Count DESC
LIMIT 10;

结果：

┌─URL────────────────────────────┬─Count─┐
│ http://auto.ru/chatay-barana.. │ 170 │
│ http://auto.ru/chatay-id=371...│ 52 │
│ http://public_search │ 45 │
│ http://kovrik-medvedevushku-...│ 36 │
│ http://forumal │ 33 │
│ http://korablitz.ru/L_1OFFER...│ 14 │
│ http://auto.ru/chatay-id=371...│ 14 │
│ http://auto.ru/chatay-john-D...│ 13 │
│ http://auto.ru/chatay-john-D...│ 10 │
│ http://wot/html?page/23600_m...│ 9 │
└────────────────────────────────┴───────┘

10 rows in set. Elapsed: 0.005 sec.
Processed 8.19 thousand rows,
740.18 KB (1.53 million rows/s., 138.59 MB/s.)

ClickHouse客户端的输出现在显示，只有8190行流到ClickHouse，而不是进行完整的表扫描。执行计划：

┌─explain───────────────────────────────────────────────────────────────────────────────┐
│ Expression (Projection) │
│ Limit (preliminary LIMIT (without OFFSET)) │
│ Sorting (Sorting for ORDER BY) │
│ Expression (Before ORDER BY) │
│ Aggregating │
│ Expression (Before GROUP BY) │
│ Filter (WHERE) │
│ SettingQuotaAndLimits (Set limits and quota after reading from storage) │
│ ReadFromMergeTree │
│ Indexes: │
│ PrimaryKey │
│ Keys: │
│ UserID │
│ Condition: (UserID in [749927693, 749927693]) │
│ Parts: 1/1 │
│ Granules: 1/1083 │
└───────────────────────────────────────────────────────────────────────────────────────┘

16 rows in set. Elapsed: 0.003 sec.

SELECT UserID, count(UserID) AS Count
FROM hits_UserID_URL
WHERE URL = 'http://public_search'
GROUP BY UserID
ORDER BY Count DESC
LIMIT 10;


┌─────UserID─┬─Count─┐
│ 2459550954 │  3741 │
│ 1084649151 │  2484 │
│  723361875 │   729 │
│ 3087145896 │   695 │
│ 2754931092 │   672 │
│ 1509037307 │   582 │
│ 3085460200 │   573 │
│ 2454360090 │   556 │
│ 3884990840 │   539 │
│  765730816 │   536 │
└────────────┴───────┘

10 rows in set. Elapsed: 0.086 sec.
Processed 8.81 million rows,
799.69 MB (102.11 million rows/s., 9.27 GB/s.)

-- trace_logging 
...Executor): Key condition: (column 1 in ['http://public_search',
                                           'http://public_search'])
...Executor): Used generic exclusion search over index for part all_1_9_2
              with 1537 steps
...Executor): Selected 1/1 parts by partition key, 1 parts by primary key,
              1076/1083 marks by primary key, 1076 marks to read from 5 ranges
...Executor): Reading approx. 8814592 rows with 10 streams