技術標籤：clickhousegroupBitmapclickhouse

用途

官方點就是計算基數,直白點就是統計不重複的個數

引數為Uint型別,就是不是負數的整數
返回值為Uint64型別

案例

測試資料

UserID
1
1
2
3

SELECT groupBitmap(UserID) as num FROM t;

結果

num
3

再舉個例子,比如我們有使用者id uid, string型別,相要統計其個數,則

SELECT groupBitmap(toUInt64(uid)) as num from usertable;

+----+
|num |
+----+
|2399
 
|
+----+

等價於如下查詢

select count(distinct uid) from usertable;

那麼要bitmap做什麼呢?就是建立表時如果選擇bitmap儲存,就會比較節約空間

比如

create table testbit(
    label String,
    name String,
    uv AggregateFunction(groupBitmap,UInt64) comment 'bitmap儲存使用者'
)engine=AggregatingMergeTree()
partition by label
order by (label,name)
 
;

groupBitmapState

可以看出返回值型別是AggregateFunction(groupBitmap,UInt64)

SELECT groupBitmapState(toUInt64(uid)) as num,
       toTypeName(num)
from usertable;

想要知道bitmap儲存的是什麼東西,直接查是看不出來的,需要轉為陣列才可以,比如

select bitmapToArray(groupBitmapState(toUInt64(uid))) from usertable;

如果想求基數,如下

select bitmapCardinality(
 
groupBitmapState(toUInt64(uid))) from usertable;

+--------------------------------------------------+
|bitmapCardinality(groupBitmapState(toUInt64(uid)))|
+--------------------------------------------------+
|2399                                              |
+--------------------------------------------------+

參考

groupBitmap | ClickHouse Documentation