- hll_empty()
描述：创建一个空的hll。
返回值类型：hll
示例：
gbase=# select hll_empty();
hll_empty
------------------------------------------------------------
\x484c4c00000000002b05000000000000000000000000000000000000
(1 row)
- hll_empty(int32 log2m)
描述：创建空的hll并指定参数log2m，取值范围是10到16。若输入-1，则采用内置默认值。
返回值类型：hll
示例：
gbase=# select hll_empty(10);
hll_empty
------------------------------------------------------------
\x484c4c00000000002b04000000000000000000000000000000000000
(1 row)
gbase=# select hll_empty(-1);
hll_empty
------------------------------------------------------------
\x484c4c00000000002b05000000000000000000000000000000000000
(1 row)
- hll_empty(int32 log2m, int32 log2explicit)
描述：创建空的hll并依次指定参数log2m、log2explicit。log2explicit取值范围是0到12，0表示直接跳过Explicit模式。该参数可以用来设置Explicit模式的阈值大小，在数据段长度达到2log2explicit后切换为Sparse模式或者Full模式。若输入-1，则log2explicit采用内置默认值。
返回值类型: hll
示例：
gbase=# select hll_empty(10, 4);
hll_empty
------------------------------------------------------------
\x484c4c00000000001304000000000000000000000000000000000000
(1 row)
gbase=# select hll_empty(10, -1);
hll_empty
------------------------------------------------------------
\x484c4c00000000002b04000000000000000000000000000000000000
(1 row)
- hll_empty(int32 log2m, int32 log2explicit, int64 log2sparse)
描述：创建空的hll并依次指定参数log2m、log2explicit、log2sparse。log2sparse取值范围是0到14，0表示直接跳过Sparse模式。该参数可以用来设置Sparse模式的阈值大小，在数据段长度达到2log2sparse后切换为Full模式。若输入-1，则log2sparse采用内置默认值。
返回值类型：hll
示例：
gbase=# select hll_empty(10, 4, 8);
hll_empty
------------------------------------------------------------
\x484c4c00000000001204000000000000000000000000000000000000
(1 row)
gbase=# select hll_empty(10, 4, -1);
hll_empty
------------------------------------------------------------
\x484c4c00000000001304000000000000000000000000000000000000
(1 row)
- hll_empty(int32 log2m, int32 log2explicit, int64 log2sparse, int32 duplicatecheck)
描述：创建空的hll并依次指定参数log2m、log2explicit、log2sparse、duplicatecheck。duplicatecheck取0或者1，表示是否开启该模式，默认情况下该模式会关闭。若输入-1，则duplicatecheck采用内置默认值。
返回值类型：hll
示例：
gbase=# select hll_empty(10, 4, 8, 0);
hll_empty
------------------------------------------------------------
\x484c4c00000000001204000000000000000000000000000000000000
(1 row)
gbase=# select hll_empty(10, 4, 8, -1);
hll_empty
------------------------------------------------------------
\x484c4c00000000001204000000000000000000000000000000000000
(1 row)
- hll_add(hll, hll_hashval)
描述：把hll_hashval加入到hll中。
返回值类型：hll
示例：
gbase=# select hll_add(hll_empty(), hll_hash_integer(1));
hll_add
----------------------------------------------------------------------------
\x484c4c08000002002b0900000000000000f03f3e2921ff133fbaed3e2921ff133fbaed00
(1 row)
- hll_add_rev(hll_hashval, hll)
描述：把hll_hashval加入到hll中，和hll_add功能一样，只是参数位置进行了交换。
返回值类型：hll
示例：
gbase=# select hll_add_rev(hll_hash_integer(1), hll_empty());
hll_add_rev
----------------------------------------------------------------------------
\x484c4c08000002002b0900000000000000f03f3e2921ff133fbaed3e2921ff133fbaed00
(1 row)
- hll_eq(hll, hll)
描述：比较两个hll是否相等。
返回值类型：bool
示例：
gbase=# select hll_eq(hll_add(hll_empty(), hll_hash_integer(1)), hll_add(hll_empty(), hll_hash_integer(2)));
hll_eq
--------
f
(1 row)
- hll_ne(hll, hll)
描述：比较两个hll是否不相等。
返回值类型：bool
示例：
gbase=# select hll_ne(hll_add(hll_empty(), hll_hash_integer(1)), hll_add(hll_empty(), hll_hash_integer(2)));
hll_ne
--------
t
(1 row)
- hll_cardinality(hll)
描述：计算hll的distinct值。
返回值类型：int
示例：
gbase=# select hll_cardinality(hll_empty() || hll_hash_integer(1));
hll_cardinality
-----------------
1
(1 row)
- hll_union(hll, hll)
描述：把两个hll数据结构union成一个。
返回值类型：hll
示例：
gbase=# select hll_union(hll_add(hll_empty(), hll_hash_integer(1)), hll_add(hll_empty(), hll_hash_integer(2)));
hll_union
--------------------------------------------------------------------------------------------
\x484c4c10002000002b090000000000000000400000000000000000b3ccc49320cca1ae3e2921ff133fbaed00
(1 row)
